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 kirotheavenger wrote:

 Haighus wrote:

 kirotheavenger wrote:
You're still making the assumption that pointblank firing with a firearm is not possible - it absolutely is.

An enemy in arms reach in no way precludes you from shooting them, and parrying a rifle is harder than parrying a sword because just putting a sword in the way is enough to stop another sword reaching you, you can also block a sword swing with a rifle. Whereas to parry a rifle you have to physically push the weapon out of line. A move as simple as taking one step backwards will take you out of range of the sword and allow you full use of your rifle. The range in which a melee weapon is more useful than a potent, automatic, and high capacity firearm is really impossibly short it isn't even the norm in a building or trench network.

When I said speed what I was referring to is how an assault intercessor gets, what, 3 attacks a turn? They can swing their chainsword 3 times. A rifle intercessor gets to fire their boltrifle twice. In no real world can an astartes make more good attacks with a chainsword than they can snap off shots/bursts of a rifle.
Old rules like "if you carry a pistol you get +1 attack with your melee weapon" don't even begin to represent the reality of point blank firearms usage.

I'm pretty sure a "shot" in 40k represents a burst for most small arms. I'm not discounting this at all, but a burst from a gun is far less effective if the shots just deflect off a breastplate. CQB shooting aims at centre mass and aimed shooting into less-armoured limbs and joints is just harder and slower. Up-close, a burst into centre-mass is quicker than a melee strike, but only just, and such a burst will not reliably stop power armoured foes, nor will it be likely to constrain their actions. Melee or retreat then becomes inevitable.

A boltrifle dice may well represent a dice - but we're now still talking about artificial buffs of melee attacks. If a single dice is one chainsword swipe, why is it equally lethal to an entire burst of rifle fire? And it's still being generous to say you can get 3 good swipes in but only 2 good bursts.

You're also under no obligation at all to aim a burst at centre-mass. That's usually what they train because it's easier but you can easily aim it at a head or something. This is even today a trained technique called the Mozambique drill and you see it in real combat footage.
It also assumes that a plasmagun to centre-mass is less effective than a chainsword, which it isn't.
Even aiming a melee weapon is hard - medieval armoured combat was more like wrestling/grappling to then get your rondel out, you're not getting a sword strike straight into an armpit just like that.
We also have the very real question of why aren't astartes more regularly equipped with heavy shotguns or similar? Boltguns are already shown as being [barely] capable of penetrating power armour, a dedicated CQB firearm could do it even more effectively. Instead the only people who wield such weapons are the barely trained initiates

Firstly, helmets have traditionally been as heavily armoured as breastplates, so I'd expect a power armour suit to have very thick faceplates too, for obvious reasons of torsos and heads containing vital organs. Just switching to headshots probably isn't enough with power armour, although would make a difference with open-faced helmets such as typically seen in the likes of carapace armour.

Obviously soldiers can be trained to shoot at more vulnerable spots instead of centre mass, but centre mass is used because it is quick and reliable (especially for a moving foe). Forces like marines, stormtroopers etc with higher levels of general marksmanship are going to be better placed to do so, but from what we know of the likes of power armour and even carapace armour it is just unlikely to incapacitate an armoured opponent in such a way as to prevent them fighting. Also, those techniques are mostly used by special forces, and typically against poorly-trained insurgents in clearing operations in most cases. In the peer combat we are seeing in Ukraine those kinds of techniques are not common despite widespread body armour and instead much more cautious approaches with pre-firing, pieing etc. that rely on volume of fire and minimal exposure are the norm. Neither really maps to an ork or marine counter-charging round the corner instead of a human hiding round the corner, but assuming the enemy is waiting I don't think we'd see much of the rapid-entry clearing style against skilled opponents except by armoured/tough soldiers like marines or orks who can take some accurate return fire and probably keep going.

I agree melee attacks also struggle to hit vulnerable spots, but a huge part of that is that the opponent gets a vote. It is actually quite easy to train a typical human to reliably strike very small fixed targets with a tool (including weapons), easier than it is to teach them to shoot them unless you try to jam the barrel into the target (at which point it can be parried etc and is not functionally different from a melee attack in delivery except needing less strength to injure). For direct comparison, I'd wager it would take less time to get the average untrained person accurately stabbing a face with a pike at 3m than hitting the face with a rifle from the same range in a snapshot scenario (say, the human shape pops up for a brief period to simulate a sudden, close encounter). We are not talking about pikes in general, but just used it as a direct comparison for equivalent short range and ease of a task. I'd also suggest most people would be better at learning how to hit a moving nail with a hammer than shooting the moving nail from a hammer-length away. When we look at historical examples, armoured people in melee did aim for eye slits and joints in combat, they usually resorted to grappling if their opponent was sufficiently skilled to protect vulnerable areas and needed to be overpowered.

The key thing about the opening burst is that a human combatant today cannot block it (although modern body armour might allow them to survive it) and modern assault rifles are relatively easy to rapidly and accurately shoot a burst with, so there isn't a likely need for follow up unless they missed or only wounded the enemy (and wounding probably reduced the combat efficacy of the enemy). A melee enemy essentially gets very little chance to reach melee range, and whoever shoots first for ranged enemies probably wins (assuming the enemy has adequate training/skill, see above about insurgents). But if the first burst is unlikely to incapacitate then you have to contend with retaliation, and if that retaliation is melee then now your firearm is an unwieldy stick unless you can line up a shot again. That is why, at very close quarters, a second burst is unlikely against a melee enemy. The gun has to be pointing at the enemy to be useful as a firearm, and the moment it starts being used to parry or block and prevent the opponent from gaining control of the fight it is not going to be pointing at the enemy much at all. An opening might be generated but now the advantage switches to the purpose-built weapon(s) for melee, for obvious reasons.

Again, this is the best case scenario for melee- an extremely close ranged environment where two enemies encounter each other within range for a single lunge into melee. Things like room clearing. Even today, this is very dangerous for the soldier with the rifle even against enemies without firearms. But with good armour? Melee can be forced and therefore purpose-built melee weapons will enter limited use.

The further out the ranges get, the less melee becomes viable. But we have obvious adaptations for this- Land Raiders, jump packs and so on. But outside of the Black Templars and maybe the Space Wolves, every marine chapter recognises melee is more situational and has a predominence of units armed with ranged primary weapons, and the majority of melee units have either side arms or extra thick armour (and sometimes a massive shield).


Adressing side points:

I'd also hold that attack dice also can represent a flurry of attacks in the same way a single "shot" is probably a burst for small arms. Turns in 40k are obviously quite abstract and there has never been a firm explanation of how long one lasts (except Rogue Trader?) nor is it really desirable. But given how far basic infantry move, I think the idea a single attack actually represents just one strike for most melee attacks is... unlikely. The abstraction is a balance mechanic that give flavour as much as anything. Actual human melee combat can include multiple strikes per second, to say nothing of astartes or eldar and so on.

As an aside, for the purposes of this I am assuming chainswords are reasonable melee weapons and not unwieldy bricks where you have to constantly fight the gyroscopic effect. The lore says they are effective and can be used for duelling, so I am working off that. How that is the case? Who knows. I would assume the materials used and tech inside is able to counter the downsides.

I'm also not really sure why plasma is relevant. Sure, it can be used up close and will take out most things it hits. But it is also a support weapon in the same vein as a grenade launcher or MG, and is clearly much more bulky than equivalent rifles. I'd expect them to be unwieldy and not suited to snap-shooting, and the likes of marine chapters choose to only dispense them sparingly even though a marine chapter typically has the resources to give every marine a plasma gun if they wished. The logical conclusion being they are quite limited in their battlefield role compared to a rifle and are a specialist tool instead of a general-purpose one. In game, they have always been a straight upgrade to basic rifles aside from overheating, but the game is a small snapshot of the overall campaign with abstracted rules. As such, comparing plasma guns to chainswords isn't particularly helpful, they just aren't fulfilling the same roles on the battlefield and there is probably a reason only some soldiers are equipped with plasma guns beyond general rarity.

I don't think shotguns change much. They don't offer much over a modern intermediate-cartridge carbine in close quarters, besides being better at breaching doors. Special forces today rarely use them as primary weapons, although I believe some US marines found shotguns more helpful in Iraq than carbines. It is pretty marginal though. A marine can comfortably wield a boltgun as a carbine and even bolt rifles seem to be fine, so shotguns are probably only a marginal improvement up close. Probably a bigger deal for, say, a Chaos cultist if their autogun is firing massive 8mm bullets to switch to a shotgun for cqb though... Autoguns seem to be closer to automatic battle rifles than assault rifles.

 Haighus wrote:

Firstly, helmets have traditionally been as heavily armoured as breastplates, so I'd expect a power armour suit to have very thick faceplates too, for obvious reasons of torsos and heads containing vital organs. Just switching to headshots probably isn't enough with power armour, although would make a difference with open-faced helmets such as typically seen in the likes of carapace armour.

Obviously soldiers can be trained to shoot at more vulnerable spots instead of centre mass, but centre mass is used because it is quick and reliable (especially for a moving foe). Forces like marines, stormtroopers etc with higher levels of general marksmanship are going to be better placed to do so, but from what we know of the likes of power armour and even carapace armour it is just unlikely to incapacitate an armoured opponent in such a way as to prevent them fighting. Also, those techniques are mostly used by special forces, and typically against poorly-trained insurgents in clearing operations in most cases. In the peer combat we are seeing in Ukraine those kinds of techniques are not common despite widespread body armour and instead much more cautious approaches with pre-firing, pieing etc. that rely on volume of fire and minimal exposure are the norm. Neither really maps to an ork or marine counter-charging round the corner instead of a human hiding round the corner, but assuming the enemy is waiting I don't think we'd see much of the rapid-entry clearing style against skilled opponents except by armoured/tough soldiers like marines or orks who can take some accurate return fire and probably keep going.

Things like bolters are routinely shown as being capable of penetrating power armour though, and the helmet is usually described as a weaker spot than the breastplate.
Chainswords have almost always been pretty well as-lethal hit for hit as the wielder's standard smallarm is (eg astartes chainsword = astartes bolter). There's pretty much zero situation that a chainsword can handle that a bolter cannot, and a bolter can handle several hundred feet worth of more situations to boot.
I really don't see why an enemy charging around the corner makes it *harder* for guns to deal with them - if anything it makes it easier as they deprive you of the difficult step of entering the room yourself.
Special forces in particular have spent a lot of time practicing the best way of entering a contested room - even entering a contested room without collateral damage (which rules out prefiring and grenades). Still their solutions are all firearms based. If there was any validity to charging round that corner with a sword they'd be doing that, as it is they don't even bother to fit bayonets these days.
Remember even just the width of a room is actually quite a lot of range where a firearm is effective but a sword is not.

I agree melee attacks also struggle to hit vulnerable spots, but a huge part of that is that the opponent gets a vote. It is actually quite easy to train a typical human to reliably strike very small fixed targets with a tool (including weapons), easier than it is to teach them to shoot them unless you try to jam the barrel into the target (at which point it can be parried etc and is not functionally different from a melee attack in delivery except needing less strength to injure). For direct comparison, I'd wager it would take less time to get the average untrained person accurately stabbing a face with a pike at 3m than hitting the face with a rifle from the same range in a snapshot scenario (say, the human shape pops up for a brief period to simulate a sudden, close encounter).

Why are we talking about untrained opponents as if that's any relevance to an astartes? Who starts training for combat as a teenager and basically lives their entire life between praying for war, practicing for war, and conducting war?
Plus, it's just a minority of targets that cannot be simply shot by a boltgun. 90% of Orks? Cultists? Gaunts? Boltgun will go straight through. Even enemy astartes? Boltgun can do it still.

Again, this is the best case scenario for melee- an extremely close ranged environment where two enemies encounter each other within range for a single lunge into melee. Things like room clearing. Even today, this is very dangerous for the soldier with the rifle even against enemies without firearms. But with good armour? Melee can be forced and therefore purpose-built melee weapons will enter limited use.

The further out the ranges get, the less melee becomes viable. But we have obvious adaptations for this- Land Raiders, jump packs and so on. But outside of the Black Templars and maybe the Space Wolves, every marine chapter recognises melee is more situational and has a predominence of units armed with ranged primary weapons, and the majority of melee units have either side arms or extra thick armour (and sometimes a massive shield).

As I touched on above you're expanding the size of the 'best case scenario' for melee substantially. It's not room clearing - room clearing has vast gulfs of room (boom boom) in which an attacker will be completely exposed and vulnerable to a shooter. Melee is only remotely viable at literal arms length which is basically just not a thing 99.9% of the time. I made the argument before that a marine equipped with a heavy shield and heavy melee weapon could be an effective force. But something like an assault intercessor or choppa wielding Ork just isn't.
At least Orks have the excuse that they don't really care if 95% of them die when closing in. There's still no reason a Marine would logically forgo their rifle for a sword to fight in the dirt with that Ork.
Things like Landraiders don't really help. We already have armoured vehicles like that and firearms are still the best weapons for clearing a building after a transport has delivered you there. Jump Packs similarly - sure they can reduce the time you spend outside of melee range but there's still a large range and time in which you could have just shot the enemy/they can just shoot you. Jump Packs also slightly undermine the whole "melee is important for ultra close confines" because you've now got a giant thruster that can barely fit in said ultra close confines.

Adressing side points:

I'd also hold that attack dice also can represent a flurry of attacks in the same way a single "shot" is probably a burst for small arms. Turns in 40k are obviously quite abstract and there has never been a firm explanation of how long one lasts (except Rogue Trader?) nor is it really desirable. But given how far basic infantry move, I think the idea a single attack actually represents just one strike for most melee attacks is... unlikely. The abstraction is a balance mechanic that give flavour as much as anything. Actual human melee combat can include multiple strikes per second, to say nothing of astartes or eldar and so on.

Sure you can interpret each dice as widely as you like - but the central point is that a Marine with a chainsword is 50% more deadly than a marine with a rifle. Which just isn't the case given the quantity of high-velocity exploding adamantine a boltrifle can throw.

I'm also not really sure why plasma is relevant. Sure, it can be used up close and will take out most things it hits. But it is also a support weapon in the same vein as a grenade launcher or MG, and is clearly much more bulky than equivalent rifles. I'd expect them to be unwieldy and not suited to snap-shooting, and the likes of marine chapters choose to only dispense them sparingly even though a marine chapter typically has the resources to give every marine a plasma gun if they wished. The logical conclusion being they are quite limited in their battlefield role compared to a rifle and are a specialist tool instead of a general-purpose one. In game, they have always been a straight upgrade to basic rifles aside from overheating, but the game is a small snapshot of the overall campaign with abstracted rules. As such, comparing plasma guns to chainswords isn't particularly helpful, they just aren't fulfilling the same roles on the battlefield and there is probably a reason only some soldiers are equipped with plasma guns beyond general rarity.

Plasma is relevant because it shows there are firearms in usage, even common usage, that are plenty capable of blowing through marines.
If you want to compare basic melee to basic rifles - boltgun beats chainsword 99.9% of the time.
If you wanted to add in fancier melee weapons that start to materially surpass bolters in lethality - plasma guns undercut them again.
As I mentioned earlier, there's only really the thunderhammer standing as a melee weapon without an obviously superior firearm adjacent (although meltas arguably would).

I don't think shotguns change much. They don't offer much over a modern intermediate-cartridge carbine in close quarters, besides being better at breaching doors. Special forces today rarely use them as primary weapons, although I believe some US marines found shotguns more helpful in Iraq than carbines. It is pretty marginal though. A marine can comfortably wield a boltgun as a carbine and even bolt rifles seem to be fine, so shotguns are probably only a marginal improvement up close. Probably a bigger deal for, say, a Chaos cultist if their autogun is firing massive 8mm bullets to switch to a shotgun for cqb though... Autoguns seem to be closer to automatic battle rifles than assault rifles.

Part of the reason modern carbines surpass shotguns for CQB is that penetration isn't really that important so the RoF is a big factor.
But for certain scenarios more oomph is important in 40k - such as Tau pulse blasters vs the pulse carbine.

One reason why Space Marines might also favor melee is because of the limited ammunition they can carry. Bolters and bolter shells in background are higher end weapons that are more expensive and maintenance intensive compared to things like stubbers and autoguns, and especially if the bolter shells have additional religious rites done over them. To expend bolter shells on cultists and mutants would be cost ineffective and risk the marines running out of ammunition at a more important time. Unlike in the computer games, spare bolter reloads are not lying around everywhere to pick up. The need to conserve ammo therefore may mean it is better for the marine to go chainsword a cultist or punch their head in with their armored gauntlet rather than expend a more valuable bolter shell, when the marine is likely to survive whatever ramshackle weaponry the cultist might have.

Iracundus wrote:
One reason why Space Marines might also favor melee is because of the limited ammunition they can carry. Bolters and bolter shells in background are higher end weapons that are more expensive and maintenance intensive compared to things like stubbers and autoguns, and especially if the bolter shells have additional religious rites done over them. To expend bolter shells on cultists and mutants would be cost ineffective and risk the marines running out of ammunition at a more important time. Unlike in the computer games, spare bolter reloads are not lying around everywhere to pick up. The need to conserve ammo therefore may mean it is better for the marine to go chainsword a cultist or punch their head in with their armored gauntlet rather than expend a more valuable bolter shell, when the marine is likely to survive whatever ramshackle weaponry the cultist might have.

Which really makes you ask-why don't Marines, with their power packs, use laser weapons?

I think the explanation given that the 30K background now has it that Marines did have access to other energy weapons like volkite, was that the bolter was the balance between stopping power, reliability, and availability. Other more esoteric energy weapons were less reliable, more maintenance intensive, or were in limited production. That is why not every Marine is carrinyg a plasma gun or plasma pistol. Las weapons seem to have a gap between lasguns and the lascannon, and maybe it is the power draw. The Marines with lascannons are shown with heavy cabling to an upgraded backpack power system. So maybe the lasgun is too weak, and anything bigger (but below lascannon) does not provide enough benefit for the additional hassle of maybe needing a bigger power system.

Iracundus wrote:
I think the explanation given that the 30K background now has it that Marines did have access to other energy weapons like volkite, was that the bolter was the balance between stopping power, reliability, and availability. Other more esoteric energy weapons were less reliable, more maintenance intensive, or were in limited production. That is why not every Marine is carrinyg a plasma gun or plasma pistol. Las weapons seem to have a gap between lasguns and the lascannon, and maybe it is the power draw. The Marines with lascannons are shown with heavy cabling to an upgraded backpack power system. So maybe the lasgun is too weak, and anything bigger (but below lascannon) does not provide enough benefit for the additional hassle of maybe needing a bigger power system.

What about Multilasers? Or Volley Guns?

 JNAProductions wrote:

Iracundus wrote:
I think the explanation given that the 30K background now has it that Marines did have access to other energy weapons like volkite, was that the bolter was the balance between stopping power, reliability, and availability. Other more esoteric energy weapons were less reliable, more maintenance intensive, or were in limited production. That is why not every Marine is carrinyg a plasma gun or plasma pistol. Las weapons seem to have a gap between lasguns and the lascannon, and maybe it is the power draw. The Marines with lascannons are shown with heavy cabling to an upgraded backpack power system. So maybe the lasgun is too weak, and anything bigger (but below lascannon) does not provide enough benefit for the additional hassle of maybe needing a bigger power system.

What about Multilasers? Or Volley Guns?

I think the issue of bulkiness and weight for not enough benefit? The multilaser is really more for unarmored infantry so maybe it was decided the Marines didn't need to carry a larger bulkier laser just for that kind of target as they are effective already against that.

There is a bit of a prestige thing going on with Bolters. They are a high end expensive weapon with a lot of in-universe lore around them. Its a status weapon which is why marines use them. Bolters are also very versatile thanks to the ability to switch ammo types.

Bolter ammo is also not THAT uncommon. Sure, its rare and most guardsmen won't ever see any. But enough officers and other higher end soldiers in the Imperium get bolters and bolt pistols that if marines are desperate, they can requisition nearby stocks.

But yes, Marines will always need to be at least mindful of their ammo usage so punching something to death is a good way to do that.

Iracundus wrote:

 JNAProductions wrote:

Iracundus wrote:
I think the explanation given that the 30K background now has it that Marines did have access to other energy weapons like volkite, was that the bolter was the balance between stopping power, reliability, and availability. Other more esoteric energy weapons were less reliable, more maintenance intensive, or were in limited production. That is why not every Marine is carrinyg a plasma gun or plasma pistol. Las weapons seem to have a gap between lasguns and the lascannon, and maybe it is the power draw. The Marines with lascannons are shown with heavy cabling to an upgraded backpack power system. So maybe the lasgun is too weak, and anything bigger (but below lascannon) does not provide enough benefit for the additional hassle of maybe needing a bigger power system.

What about Multilasers? Or Volley Guns?

I think the issue of bulkiness and weight for not enough benefit? The multilaser is really more for unarmored infantry so maybe it was decided the Marines didn't need to carry a larger bulkier laser just for that kind of target as they are effective already against that.

Volleyguns really aren't that much bulkier and combine the benefits of a lot of bolts merged together, longer range, good armor penetration, it would lack some of the fancier ones that can ignore cover or are loaded with acid, but that's a case of mission pre-planning

On Las Weapons?

Confrontation, the immediate predecessor of Necromunda explained that whilst Las weapons all shared the same power packs, the bigger you went, the more it required and the fewer shots you got.

Sadly I don’t have the WD with that part of the rules, but I was something like 20 Lasgun or Pistols shots per power pack. But, a Lascannon required 6 power packs as a literal battery, and would get a single shot from it.

Not completely gospel as I’ve inserted numbers. But the framework is correct.

Of course. That’s in an Underhive, and so wouldn’t include Astartes Grade versions. We could interpret the cabling on Devastators to be a source of power to recharge the Lascannon battery, rather than directly powering the thing. Though could do both in a pinch perhaps.

 kirotheavenger wrote:

Spoiler:

 Haighus wrote:

Firstly, helmets have traditionally been as heavily armoured as breastplates, so I'd expect a power armour suit to have very thick faceplates too, for obvious reasons of torsos and heads containing vital organs. Just switching to headshots probably isn't enough with power armour, although would make a difference with open-faced helmets such as typically seen in the likes of carapace armour.

Obviously soldiers can be trained to shoot at more vulnerable spots instead of centre mass, but centre mass is used because it is quick and reliable (especially for a moving foe). Forces like marines, stormtroopers etc with higher levels of general marksmanship are going to be better placed to do so, but from what we know of the likes of power armour and even carapace armour it is just unlikely to incapacitate an armoured opponent in such a way as to prevent them fighting. Also, those techniques are mostly used by special forces, and typically against poorly-trained insurgents in clearing operations in most cases. In the peer combat we are seeing in Ukraine those kinds of techniques are not common despite widespread body armour and instead much more cautious approaches with pre-firing, pieing etc. that rely on volume of fire and minimal exposure are the norm. Neither really maps to an ork or marine counter-charging round the corner instead of a human hiding round the corner, but assuming the enemy is waiting I don't think we'd see much of the rapid-entry clearing style against skilled opponents except by armoured/tough soldiers like marines or orks who can take some accurate return fire and probably keep going.

Things like bolters are routinely shown as being capable of penetrating power armour though, and the helmet is usually described as a weaker spot than the breastplate.
Chainswords have almost always been pretty well as-lethal hit for hit as the wielder's standard smallarm is (eg astartes chainsword = astartes bolter). There's pretty much zero situation that a chainsword can handle that a bolter cannot, and a bolter can handle several hundred feet worth of more situations to boot.
I really don't see why an enemy charging around the corner makes it *harder* for guns to deal with them - if anything it makes it easier as they deprive you of the difficult step of entering the room yourself.
Special forces in particular have spent a lot of time practicing the best way of entering a contested room - even entering a contested room without collateral damage (which rules out prefiring and grenades). Still their solutions are all firearms based. If there was any validity to charging round that corner with a sword they'd be doing that, as it is they don't even bother to fit bayonets these days.
Remember even just the width of a room is actually quite a lot of range where a firearm is effective but a sword is not.

I agree melee attacks also struggle to hit vulnerable spots, but a huge part of that is that the opponent gets a vote. It is actually quite easy to train a typical human to reliably strike very small fixed targets with a tool (including weapons), easier than it is to teach them to shoot them unless you try to jam the barrel into the target (at which point it can be parried etc and is not functionally different from a melee attack in delivery except needing less strength to injure). For direct comparison, I'd wager it would take less time to get the average untrained person accurately stabbing a face with a pike at 3m than hitting the face with a rifle from the same range in a snapshot scenario (say, the human shape pops up for a brief period to simulate a sudden, close encounter).

Why are we talking about untrained opponents as if that's any relevance to an astartes? Who starts training for combat as a teenager and basically lives their entire life between praying for war, practicing for war, and conducting war?
Plus, it's just a minority of targets that cannot be simply shot by a boltgun. 90% of Orks? Cultists? Gaunts? Boltgun will go straight through. Even enemy astartes? Boltgun can do it still.

Again, this is the best case scenario for melee- an extremely close ranged environment where two enemies encounter each other within range for a single lunge into melee. Things like room clearing. Even today, this is very dangerous for the soldier with the rifle even against enemies without firearms. But with good armour? Melee can be forced and therefore purpose-built melee weapons will enter limited use.

The further out the ranges get, the less melee becomes viable. But we have obvious adaptations for this- Land Raiders, jump packs and so on. But outside of the Black Templars and maybe the Space Wolves, every marine chapter recognises melee is more situational and has a predominence of units armed with ranged primary weapons, and the majority of melee units have either side arms or extra thick armour (and sometimes a massive shield).

As I touched on above you're expanding the size of the 'best case scenario' for melee substantially. It's not room clearing - room clearing has vast gulfs of room (boom boom) in which an attacker will be completely exposed and vulnerable to a shooter. Melee is only remotely viable at literal arms length which is basically just not a thing 99.9% of the time. I made the argument before that a marine equipped with a heavy shield and heavy melee weapon could be an effective force. But something like an assault intercessor or choppa wielding Ork just isn't.
At least Orks have the excuse that they don't really care if 95% of them die when closing in. There's still no reason a Marine would logically forgo their rifle for a sword to fight in the dirt with that Ork.
Things like Landraiders don't really help. We already have armoured vehicles like that and firearms are still the best weapons for clearing a building after a transport has delivered you there. Jump Packs similarly - sure they can reduce the time you spend outside of melee range but there's still a large range and time in which you could have just shot the enemy/they can just shoot you. Jump Packs also slightly undermine the whole "melee is important for ultra close confines" because you've now got a giant thruster that can barely fit in said ultra close confines.

Adressing side points:

I'd also hold that attack dice also can represent a flurry of attacks in the same way a single "shot" is probably a burst for small arms. Turns in 40k are obviously quite abstract and there has never been a firm explanation of how long one lasts (except Rogue Trader?) nor is it really desirable. But given how far basic infantry move, I think the idea a single attack actually represents just one strike for most melee attacks is... unlikely. The abstraction is a balance mechanic that give flavour as much as anything. Actual human melee combat can include multiple strikes per second, to say nothing of astartes or eldar and so on.

Sure you can interpret each dice as widely as you like - but the central point is that a Marine with a chainsword is 50% more deadly than a marine with a rifle. Which just isn't the case given the quantity of high-velocity exploding adamantine a boltrifle can throw.

I'm also not really sure why plasma is relevant. Sure, it can be used up close and will take out most things it hits. But it is also a support weapon in the same vein as a grenade launcher or MG, and is clearly much more bulky than equivalent rifles. I'd expect them to be unwieldy and not suited to snap-shooting, and the likes of marine chapters choose to only dispense them sparingly even though a marine chapter typically has the resources to give every marine a plasma gun if they wished. The logical conclusion being they are quite limited in their battlefield role compared to a rifle and are a specialist tool instead of a general-purpose one. In game, they have always been a straight upgrade to basic rifles aside from overheating, but the game is a small snapshot of the overall campaign with abstracted rules. As such, comparing plasma guns to chainswords isn't particularly helpful, they just aren't fulfilling the same roles on the battlefield and there is probably a reason only some soldiers are equipped with plasma guns beyond general rarity.

Plasma is relevant because it shows there are firearms in usage, even common usage, that are plenty capable of blowing through marines.
If you want to compare basic melee to basic rifles - boltgun beats chainsword 99.9% of the time.
If you wanted to add in fancier melee weapons that start to materially surpass bolters in lethality - plasma guns undercut them again.
As I mentioned earlier, there's only really the thunderhammer standing as a melee weapon without an obviously superior firearm adjacent (although meltas arguably would).

I don't think shotguns change much. They don't offer much over a modern intermediate-cartridge carbine in close quarters, besides being better at breaching doors. Special forces today rarely use them as primary weapons, although I believe some US marines found shotguns more helpful in Iraq than carbines. It is pretty marginal though. A marine can comfortably wield a boltgun as a carbine and even bolt rifles seem to be fine, so shotguns are probably only a marginal improvement up close. Probably a bigger deal for, say, a Chaos cultist if their autogun is firing massive 8mm bullets to switch to a shotgun for cqb though... Autoguns seem to be closer to automatic battle rifles than assault rifles.

Part of the reason modern carbines surpass shotguns for CQB is that penetration isn't really that important so the RoF is a big factor.
But for certain scenarios more oomph is important in 40k - such as Tau pulse blasters vs the pulse carbine.

I'm going to go back to the original point. Melee only works if soldiers can typically reach it. Modern special forces use firearms because firearms reliably stop opponents even at very close range with today's armour. I've pointed this out above too. That is the current balance between protection and ranged lethality, ranged wins.

In 40k, lots of enemies are not reliably stopped by ranged fire, especially small arms. In close quarters, that makes melee inevitable in the way it has become inevitable throughout history because the enemy gets a vote.

Yes, boltguns _can_ penetrate power armour. But not reliably, and it usually takes a lot of fire. I haven't seen any lore indication that typical small arms are likely to stop a marine from crossing a room and reaching melee with the exception of some very high-powered examples like Tau pulse weapons. There is a reason special armour-piercing bolt rounds were developed to fight traitor astartes, with the trade off that they sometimes explode! Clearly a non-trivial problem. Again, if encountering an enemy at close range who cannot easily break your armour with fire, why would you let them continue to pour fire into you and not disrupt their ability to aim in melee? Especially if they fired first and are also armoured.

Do you disagree that once melee is reached, a purpose-built melee weapon + pistol has an advantage over a rifle/carbine?

Re. game rules making chainswords better than boltguns in a given scenario- the rules are abstracted and modified to balance a given situation. Attacks have also been consistently explained as including point-blank small arms fire in the rules blurbs for melee. It isn't worth going too granular.