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How do!

Watched a YouTube video yesterday, where An Actual Doctor discusses 40K weapons. And part of the topic was the impact of a regular human attempting to fire an Astartes Bolter.

He pegged it at 75 Cal, and explained the recoil would do horrific damage. But, what he didn’t include in his comments is how a Bolt round actual works. Rather than just being a particularly large shell with enough charge to propel the Bolt round to the target, it’s a two stage system. The initial charge propels it out the barrel, then the rocket motor kicks in to accelerate the round to the target.

Now, him not mentioning it doesn’t mean he didn’t account for it. And that’s where my question comes in.

Based on real world weapon physics, and that two stage system, can we work out the likely recoil? I appreciate I’ve not included an initial muzzle velocity, so those who know such things may have to guesstimate somewhat.

Oh, and here’s the video I watched for your viewing pleasure.

Without an initial velocity it's impossible to make anything but a wild finger-in-the-air guess.

40mm grenade launchers are very common, and substantially larger than .75cal. So clearly, if the velocity is low enough it can be fired by a human without that much difficulty.

But not only is the initial velocity of the bolter unknown, but so is the weight of the boltshell. Especially as it uses futuristic scifi metals it's basically impossible to guesstimate this either.

All we have to go on is 'the lore'. But the lore is so wildly inconsistent you get dramatically different accounts even at different points of the same book, let alone a consistent picture across the entire 'canon' with the different authors and everything.

So TLR I don't think any attempt that logically concluding how a bolter would work is remotely possible. Nothing short of just making gak up.
We are fairly consistently told in lore though that astartes weapons are too big and powerful and awesome for humans to wield. But then there's also multiple instances of humans picking up and using astartes weaponary, so... it depends on the story the author wants to tell.

Any estimation of recoil is going to require a ton of assumptions on things like muzzle velocity, materials science, fictional chemistry etc that even ball parking it is going to be an exercise in educated guesswork

Having said that I'll take a stab at it after work

For me, this is just one of the long running inconsistencies in the background. Its kind of up there with autoguns being described as using caseless ammo, but there usually an awful lot of brass depicted in artwork

RT doesn't particularly go into the mechanics of a bolter, but the 2nd ed Wargear book explicitly says bolts come out of the barrel at "low velocity" implying minimal recoil.

Wargear describes the heavy bolter as having a more powerful propellant and explosive charge, befitting a support weapon version, but increased recoil from that is likely to be at least partly countered by the additional mass of the weapon.

3rd ed notes the existence of low velocity solid slug types for covert work, but doesn't go into the initial recoil bit.

The Munitorum Manual is somewhat self-contradictory as it explicitly describes bolt guns as generating "enormous recoil", but then goes on to use the older description of the bolt coming out of the muzzle under low velocity. Its also funny that in the Manual, bolt pistols are said to fire exactly the same round as a bolt gun (and they are interchangeable), but the issue of recoil just doesn't come up, even though bolt pistols are clearly lighter weapons.

The only other place I have seen any decent description of bi-propellant ammo in fiction is from Iain Banks' Against a Dark Background with the entertainingly named FrintArms 10mm Hand Cannon. It only gets a few mentions, and I don't have the time just now to dig out the specific references and see if it goes into its operation, but I remember the description as "bang, raaaark, pop"

As kirotheavenger says, we have no way of telling.

Calibre alone is not helpful- .75 calibre was a common calibre for muskets for hundreds of years. These certainly kicked, especially if loaded well with an individually cast ball fitted to the gun and a leather wad to minimise windage, but they were obviously manageable. Now, gunpowder burns a lot slower than modern propellants, so a big part of this is the slower acceleration of the bullet, but it gives an example of how calibre is only part of the story.

The other aspect is we have zero idea how much space magic technology goes into a typical boltgun, or really any Imperial firearm. Oversized calibres compared to modern weapons are common, like autoguns with 8.25mm bullets. We do know that the Imperium has access to gravity-manipulating technology and "inertial dampners", which may also manipulate gravity. It would make sense if similar tech featured in firearms. The Imperium probably doesn't understand it, they may not even know that is what a particular doohickey does on a given firearm, but the STC blueprint says this doohickey must be made this way and fitted here on a given gun and they do just that.

Take this cut-away schematic of a Terminator stormbolter:

What exactly does a "blast compensator" do? Sounds like some kind of recoil management to me. Now, stormbolters are noticeable for apparently being more mobile than standard bolters despite being significantly more bulky, so this could be the reason why, but I would not be at all surprised if Imperial firearms routinely include blast compensators to make recoil more manageable.


Automatically Appended Next Post:

 Flinty wrote:
For me, this is just one of the long running inconsistencies in the background. Its kind of up there with autoguns being described as using caseless ammo, but there usually an awful lot of brass depicted in artwork

RT doesn't particularly go into the mechanics of a bolter, but the 2nd ed Wargear book explicitly says bolts come out of the barrel at "low velocity" implying minimal recoil.

Wargear describes the heavy bolter as having a more powerful propellant and explosive charge, befitting a support weapon version, but increased recoil from that is likely to be at least partly countered by the additional mass of the weapon.

3rd ed notes the existence of low velocity solid slug types for covert work, but doesn't go into the initial recoil bit.

The Munitorum Manual is somewhat self-contradictory as it explicitly describes bolt guns as generating "enormous recoil", but then goes on to use the older description of the bolt coming out of the muzzle under low velocity. Its also funny that in the Manual, bolt pistols are said to fire exactly the same round as a bolt gun (and they are interchangeable), but the issue of recoil just doesn't come up, even though bolt pistols are clearly lighter weapons.

This latter point does actually make sense. A shorter barrel will have less recoil for the same round as the bullet doesn't accelerate for as long before reaching the end and letting the gas dissipate, at least up to the optimum length of barrel where the gas is still accelerating. A sawn-off shotgun, for example, has much less recoil than the full-length original firing the same cartridge.

Low velocity and heavy recoil aren't incompatible with a heavy projectile. Grenade launchers were mentioned above- these are fairly low velocity, but the weight of the projectile can still give them significant recoil. Rifle grenades were notorious for the recoil and could even crack gunstocks.

Interesting stuff!

On the blast compensator, I’m put in mind of the barrel thingy put on Tommy Gun’s to help with their recoil. So perhaps here, it’s a vent to allow some of the gas pressure to push the Stormbolter down with each shot?

Also clarification on the broader topic. I’m not looking to discuss whether a Marine can handle the recoil. We know their enhanced physiques are further aided in strength and stance by their power armour. I think I even read somewhere that power armour auto compensates for recoil by adjusting your stance. Not enough to remove its impact entirely, but enough to aid your aim significantly.

Fair points @Haighus. I loves me a physics lesson

Is it possible to opine about the purpose of the initial kick charge? If its simply to get the round clear of the barrel and to start a bit of spin, then it could be super weedy. If its supposed to give a decent chance of penetration into a target at short range before the rocket motor can properly kick in, then it would need to be beefier.

While one could rely only on the mass-reactive explosive to provide lethality at short range, as a largely unarmoured human, I would want some kind of minimum safe distance before the thing went boom.

Looks like the Neopup (a close current bolter analogue) has a minimum arming range of between 5 and 15m. Anyone who is up on muzzle momentum for lethality at that kind of range willing to share the likely charge needed?

I presume that the initial charge is to give you a decent bit of zero-range velocity, otherwise it would be pointless.

Gyrojets (a close IRL equivalent to a bolter) were entirely rocket propelled through the barrel and hit their peak velocity at about 10m away. So just pure rocket propulsion is basically fine except for very close ranges.

Also artistic depictions of bolt rounds usually show a fairly beefy casing, pretty much the same size as what you'd expect from a regular bullet.
Could this be because artists are just painting on vibes not on technical accuracy, and are simply depicting casings that you and I are familiar with and understand as such? Pff, don't be ridiculous, we all know shotguns fire out of their magazine tubes and GW artists are gun experts.
So this would suggest the boltround gets a pretty substantial kick at the start.

Personally I've kinda started to assume bolters are *primarily* propelled by the initial kick, and that the rocket propulsion is fairly minimal, doing little more than maintaining velocity over long range. This also maximises the explosive payload (as otherwise even a .75cal bullet would have little space for a usable bang at the target).

Interesting thoughts on the kick then rocket thing. And I’m now thinking perhaps I’ve not thought about each correctly.

It’s entirely possible that the initial charge is, well, standard. Enough to speed the round to usual bullet speeds. Then, the rocket motor kicks in to further accelerate it for armour penetration.

If so? Even at point blank range, a bolter will still make an unarmoured roughly human analogous target go splat. But at range, it gets better at penetrating armour thanks to the rocket boost?

Hmmm.

We do have one piece of solid information: their recoil is strong enough to prevent a space marine from effectively wielding it in one hand without special training. That's explicitly stated in the fluff for Grey Hunters. And since we know space marines are strong and bolters aren't all that large (compared to their other weapons) that must be quite a bit. We also know that it's difficult for normal humans to use bolters two-handed unless they've been specially modified to dampen the recoil (but it can still be done).

We also know that bolt pistols are designed to pierce through armor at point-blank range, so that to me implies a fairly high muzzle velocity. Maybe not equal to an autogun or shuriken catapult (or lasgun, ha) but enough to make it fully effective with no time for the rocket to accelerate the bolt. Otherwise assault marines would probably use laspistols or something. And finally, they're just usually depicted as kicking a lot and sending out large brass casings, because that looks cool. Maybe they can fire caseless but usually don't (sniper variants probably still would).

I feel like that all gives a rough picture.

Definitely useful info.

Though I would stress my question isn’t about Bolters in general, but Astartes Bolters in particular.

We know they’re larger and chunkier, and game stats aside? A larger calibre.

Also on Grey Hunters? This is likely shoddy hazy memory, but their training is more than just how to wield a Bolter one handed, but doing so in close quarters, whilst also wielding a melee weapon in your other hand.

The difference between me, arguably, being able to fire a shotgun one handed, and me not braining or causing other mischief to myself, and being able to do when someone is trying to smack me in the face with a frying pan. That gulf of ‘probably maybe could’ and ‘can do so with skill, control and aplomb” skill level.

Hmm I guess all I can say is I don't think a space marine has ever been allowed to wield a bolter in each hand, or shoot their bolter and their bolt pistol in the same phase, even for Grey Hunters. So that makes me think it's more difficult than using them off-hand in melee, though I'm sure it's more game balance than anything.

I mean they're strong enough to technically do it but it would be less effective than shooting one normally. You're probably safer being shot at by a guy with an uzi in each hand than a guy aiming one properly.

This is why I think it boils down to specialised training.

I mean, there are various things I’ve the oomph to pick up one handed, but do little more than carry. Whereas someone with specific combat training (like learning how to counter balance and control the momentum and that) can do so and beat the snot out of someone whilst at it.

Strap in lads, we're Mathin'

EDIT: I have no idea why "r" keeps being changed to "are", it's probably some autocorrect nonsense to prevent L33tsp33k. Whatever it is it's bloody infuriating and I've tried to change it 5 times now!

So first off: Assumptions:
- We are talking about a standard Astartes bolt per the diagram on Page 60 of the 3rd edition rulebook
- Images are approximately to scale
Materials assumptions:
- Diamantine is effectively diamond and has all the physical properties of it
- "Depleted Deuterium" is Depleted Uranium - Deuterium is a gas, and depleted deuterium is equally stupid. It's sci-fi nonsense meant to sound cool. DU makes sense in the context, if you disagree you can do your own calculations

We know the Bolt is 0.75 calibre, but calibre is a measure of diameter in inches, we still need the length, and a more sensible unit of measurement
0.75 inches is 19.05mm
In the reference image the diameter of the bolt is 62 pixels, and the length is 147 pixels
19.05 / 62 = 0.30725806451, which is how much each pixel is in mm, so a length of 147 pixels gives us a length of 45.1669354839mm

So our bolt is approximately 19mm x 45mm

The volume of a cylinder is π(r squared)h (and this is how I discover BBCode doesn't have superscript!)
We know d = 19.05 and d=2r, so are = 9.525mm
are squared is 90.725625
90.725625 x 45.1669354839 = 4097.79845111
π x 4097.79845111 = 12873.6135099

So IF a bolter shell was a perfect cylinder then it would have a volume of 12873.61 mm cubed
But it isn't, it tapers at the end, so we need to account for the tip

As luck would have it, the distance between the base and where the tip begins is exactly 100 pixels, so we know the tip is a cone with a base diameter of 19.05 and a height of (47 x 0.30725806451 =) 14.441129032mm

Volume of a cone is π(r squared)(h/3) but we have all the necessary values now:
are squared = 90.725625
h/3 = 4.81370967733
So pi x are squared x h/3 gives us 1372.0177663352514116180995121611, or approximately 1372mm cubed

And if we recalculate the rest of the bolt with a new height of just 100 pixels we get 8757.56021mm cubed
Add this to our cone and we have a total volume of 10129.5779763 mm cubed for the entire bolt shell
(The bolt tip is obviously not a perfect cone and nor is its body a perfect, featureless cylinder, but it's a good number to go off of for now)

So we have a total approximate volume of 10,129.58mm cubed. How does that help us? Because we need MASS

mass is density x volume. Now if we're taking Deuterium to be an analogue for depleted uranium, DU has a density of 19.1g per cm cubed, so if the entire bolt was a solid block of DU it'd weight 193.47 grams. Bullets are typically weighed in grains rather than grams but a gram is 15.43236 grains, usually rounded to just 15
So a solid DU bolt shell, if it existed, would be 2986 grains
Compared to a .50 BMG bullet topping out at around at 800 grains

But of course, it isn't a solid lump of radioactive material, it's hollowed out and filled with all kinds of fun additions.

But now I'm tired. Tomorrow I will divide our volume of bolt shell into approximate percentages of different components and calculate mass based on those.
Then we can move on to estimating muzzle velocity, which will give us the force required to propel the bolt at that speed
Which will then give us the recoil (of the bullet, not of the weapon, for the weapon recoil we need to subtract the mass of the bolter from the equation and I'm not going through all of that.

TLDR:
BOLTS ARE HUGE AND HEAVY AND PACK A LOT OF FORCE

*reads a lot of maths*

*can’t really remember his GCSE maths because who does outside of careers that need it, but recognises the equations even if he can’t personally verify*

Conclusion?

Oh all that's just geometry, we haven't even gotten to the physics yet.

That's where things get brain-melty

I think you’ll find it’s written jommetry.

Surprised no one's referenced this yet. Of course it's not lore accurate but it's another weapon in the ballpark. Notably, it has some mechanism/s to reduce kickback as well.

 Insectum7 wrote:
Surprised no one's referenced this yet. Of course it's not lore accurate but it's another weapon in the ballpark. Notably, it has some mechanism/s to reduce kickback as well.

Even comes with auto senses and both metal storm and kraken ammunition!

Inconsistencies in the lore can potentially be explained by different makes and models having different recoil profiles. I can also see an inexperienced soldiers occasionally getting a boltgun or bolt pistol, perhaps looted from a fallen more worthy soldier, and breaking his wrist as he uses an unfamiliar weapon. Or the "fact" that using it can potentially hurt you is used to keep the rank and file from screwing around with weaponry meant for elite troops.

Plenty of videos of inexperienced shooters getting handed a magnum revolver or shotgun and it kicks their booty out there. I can imagine the same happening with bolters and a whole in-universe equivalent to "Fudd lore" happening.

 Mad Doc Grotsnik wrote:

Though I would stress my question isn’t about Bolters in general, but Astartes Bolters in particular.

We know they’re larger and chunkier, and game stats aside? A larger calibre.

No we don't. That's just some third party fan fiction. They're the same and normal humans can fire them just fine.



That's the exact same Godwyn pattern bolter marines used to have, before the actually bigger marine bolters, the bolt rifles, were introduced with the primaris.
Those might be difficult for normal humans to fire, but certainly not impossible as there are human-portable heavy bolters and other heavy weapons too.

 Crimson wrote:
That's just some third party fan fiction.

Interesting. I assume by this you mean the various BL books and RPG books like Dark Heresy?

Dark Heresy in particular seems a popular source for the devastating effect of Astartes boltguns on human wielders. Considering its writers and designers featured such names as Alan Bligh and Andy Hall, though, it doesn't exactly strike me as something as outlandish and badly deprived of oversight as some BL contractor's first 40k book. Alan Bligh in particular is my favourite Warhammer background designer (RIP...) and I tend to place his work above just about everything else.

 Ashiraya wrote:

 Crimson wrote:
That's just some third party fan fiction.

Interesting. I assume by this you mean the various BL books and RPG books like Dark Heresy?

Dark Heresy in particular seems a popular source for the devastating effect of Astartes boltguns on human wielders. Considering its writers and designers featured such names as Alan Bligh and Andy Hall, though, it doesn't exactly strike me as something as outlandish and badly deprived of oversight as some BL contractor's first 40k book. Alan Bligh in particular is my favourite Warhammer background designer (RIP...) and I tend to place his work above just about everything else.

Dark Heresy in particular. It is a licenced work. But sure, a lot of BL stuff, especially about marines, is very "fan fictionish." In any case, the models of weapons are either identical or very similar, and they have always had identical rules on the tabletop, nor do codices imply that these would be drastically different weapons. And bigger and more powerful bolter variants do exist, and these look different, have different rules and are identified as such. I think the idea that a weapon called the same, represented by the same weapon model and having the same rules in the game would somehow be a radically different weapon in the lore is just laughable.

Fan Fictiony, or background you don’t agree with? Not having a go at you, but there is a difference.

On the Catachan lad? Looking the same and being the same isn’t the same.

Example? Found on the 40K Wiki.

Godwyn-De'az Pattern - The Godwyn-De'az Pattern Bolter is the standard pattern of bolter used by all of the Orders Militant of the Adepta Sororitas since their formation in the 36th Millennium. This pattern has been in use by the Sisters of Battle since that time because it remains more reliable and more potent than any other pattern of bolter developed for use by the Sororitas. The Godwyn-De'az Pattern is designed to be used by a normal Human and thus is much smaller than an Astartes bolter. This pattern of bolter is designed to make use of the Sarissa as an attachment. The Sarissa is a vicious, curved, bayonet-like blade that can transform the Godwyn-De'az Bolter into an effective close combat weapon even as it retains all of its ranged functionality.

Emphasis mine.

Now, I’ll grant you smaller size doesn’t automatically mean smaller calibre. But, it can be an indicator. After all, if you’re making it embiggened, why not chamber it for a larger, presumably more potent round?

Because logistics. Keeping marine and Guard calibres/round dimensions consistent helps with this. If marines can only be resupplied from their own stocks, then a big benefit of the DM is lost.

 Mad Doc Grotsnik wrote:
Fan Fictiony, or background you don’t agree with? Not having a go at you, but there is a difference.

My initial comment about "fan fiction" was regarding licenced stuff like the RPGs. That's like getting your Star Trek lore form the Star Trek online game. Not canon. Not that 40K has consistent canon to begin with.BL stuff is all over the place depending on the author. Codex lore is the most consistent source of lore.

On the Catachan lad? Looking the same and being the same isn’t the same.

So be honest. Do you honestly think that the designers would use the exact same look, size and rules to represent the weapon they actually thought was not the same weapon? No they would not.

Example? Found on the 40K Wiki.

And what it the source of the information beyond the Wiki? Was this in the codex?. Not in the current one, at least, I don't think. Or was it from the licenced RPG or some such? That's not official.

Now, I’ll grant you smaller size doesn’t automatically mean smaller calibre. But, it can be an indicator. After all, if you’re making it embiggened, why not chamber it for a larger, presumably more potent round?

But there is zero indication of smaller calibre. Bolters use the standard 0.75 calibre bolts.That's in all the lore. Like sure, there can be some small variations between different types of bolters. Like marine ones probably have larger grips for example. But the idea that they would be some drastically different weapon with different capabilities is just wild. Like I said, bigger bolters (bolt rifle, heavy bolt rifle, heavy bolter) do exist and they are modelled differently and have different rules. And we literally have normal humans firing heavy bolters. Do you think that normal marine bolters are somehow bigger and more powerful than human-portable heavy bolters? I guess you then also think that marine heavy bolters fire battle cannon shells?

Codex Witch Hunters. Thanks for playing.

And we know the calibre of the Storm Bolter illustrated above. Specifically an Astartes Model of the same.

Heavy Bolters exist and are a larger calibre. Therefore, Bolt Shells are available in different calibres. So by no means are all infantry scaled Bolters going to be the same calibre.

 Mad Doc Grotsnik wrote:
Codex Witch Hunters. Thanks for playing.

You sure?

This is the text I can find in that book:



No mention of it being smaller.

And we know the calibre of the Storm Bolter illustrated above. Specifically an Astartes Model of the same.

Yes, 0.75 like all bolters have ever been.

Heavy Bolters exist and are a larger calibre. Therefore, Bolt Shells are available in different calibres. So by no means are all infantry scaled Bolters going to be the same calibre.

Heavy bolter shells are not bolter shells. Again, heavy bolters are something that are specifically called to be different in the codices, modelled differently and have different rules.

Christ are we going to have to point to the "everything is canon" sign in every single thread?

GW licensed the RPGs, they had oversight on the contents, they explicitly stated licensed things are canon but noooooo, thet's not good enough because we need to draw made up arbitrary lines over what does and does not count as "real" in made up toy soldier world even after we've been explicitly told what counts. For decades.

To be honest the wiki does have some real groaners:

The Space Marines are not the only warriors of the Imperium to carry boltguns into battle, but the version carried by the Firstborn Adeptus Astartes, the Mark Vb Godwyn Pattern Boltgun, is by far the largest and most devastating. By comparison, the smaller patterns of boltgun carried by the Adeptus Sororitas or the champions of the Astra Militarum are pale reflections. So large is the Godwyn Pattern that no normal man could heft one, let alone survive its unforgiving recoil.

Yes, if a human tries to shoot a regular space marine bolter he will instantly die from the recoil. And the mechanically identical versions used by other armies are only "pale reflections" for some reason.

Other features include a biometric sensor integrated into the weapon's grip that allows the bolter to identify its user's genetic signature so that only Astartes can make use of the bolter.

Why would you want to prevent your enemy from shooting your bolter if the recoil will instantly kill them? The only enemies who could do so are other Astartes (who already have their own bolters).

The bolter is a large .75 calibre semiautomatic assault weapon... ...and can fire in bursts of up to four with a single trigger pull.

That's not semiautomatic...

It is illegal within the Imperium for a non-Astartes to wield an Astartes pattern bolter

You just told me this was physically impossible! But spess mehrines are so uber epic that people would steal their bolters just to die shooting them if it wasn't also illegal. Calling this stuff bad fanfiction is entirely fair, it's the same desperate-to-be-special style that genre is notorious for.


All of that aside, I think it's easy to say that SoBs use .75 bolters that have a smaller frame because they're less ruggedly built. A space marine is expected to bash an ork in the head with his bolter and have it continue to function afterwards, whereas an SoB can't hit with equal force and they apparently favor bayonets instead. That would be consistent with the wargame rules (both shooot at S4 but space marines make melee attacks at S4 even when only armed with their bolter, SoBs are limited to S3). That wouldn't, however, imply more recoil for the space marine, just a heavier gun. (Unless marine bolters have a higher muzzle velocity, which is possible but isn't reflected in the stats of SoB bolt pistols.)

The wiki does contain a list of bolters that don't use .75 and the De'az isn't mentioned, so I'm pretty sure they're still in .75:

The .75 calibre is now the standard for bolts in the 41st Millennium. However, during the early days of the Great Crusade in the late 30th Millennium, there were bolters that made use of variable calibre ammunition. This included the ancient Tigris Pattern Bolter, which fired .60 calibre bolts, and the Phobos Pattern Bolter, which made use of .70 calibre bolts. The Space Marine Legions' archaic Ikanos Pattern Bolt Pistol made use of a .50 calibre bolt.