Forum adverts like this one are shown to any user who is not logged in. Join us by filling out a tiny 3 field form and you will get your own, free, dakka user account which gives a good range of benefits to you: No adverts like this in the forums anymore. Times and dates in your local timezone. Full tracking of what you have read so you can skip to your first unread post, easily see what has changed since you last logged in, and easily see what is new at a glance. Email notifications for threads you want to watch closely. Being a part of the oldest wargaming community on the net. If you are already a member then feel free to login now.

The more I play around with points and use these weapons in game I get the feeling that the gravity cannon is very comparable to a melta gun even versus vehicles.

Does anyone have some existing math comparing the two weapons versus hull values?

Assuming all shots are BS4, grav cannon hasn't moved, and melta is at half range*:

Grav Cannon v AV10 = 0.5 chance to take a Hull Point
Grav Cannon v AV11 = 0.5 chance to take a Hull Point
Grav Cannon v AV12 = 0.5 chance to take a Hull Point
Grav Cannon v AV13 = 0.5 chance to take a Hull Point
Grav Cannon v AV14 = 0.5 chance to take a Hull Point

Meltagun v AV10 = 0.66 chance to glance, 0.64 chance to pen
Meltagun v AV11 = 0.64 chance to glance, 0.61 chance to pen
Meltagun v AV12 = 0.61 chance to glance, 0.54 chance to pen
Meltagun v AV13 = 0.54 chance to glance, 0.48 chance to pen
Meltagun v AV14 = 0.48 chance to glance, 0.38 chance to pen

Meltaguns are almost always better, however you have to be within 6" for this to be accurate. A multi-melta, however, has the same probabilities and a bigger range. grav-cannons are helped byu the number of shots they produce, after having moved, the grav-cannons only have a 0.3 chance to do anything. Grav also immobilises, however, which means all future hull points cause 2. Meltaguns have a chance to one-shot kill with an AP1 pen.

*I am 90% sure my maths is correct.

Sorry can't help on the maths, but thought it worth mentioning that some vehicles that are immune to immobilisation, they probably require a melta esque solution.

My inclination is that the Grav Cannon + Amps is pretty stupid good vs most target types, including vehicles bar one or two special cases.

Overload on Grav though, and one void shield is going to make those vehicles much safer!

The maths is pretty interesting, as the grav-cannon is decent against all vehicles and heavily armoured infantry. While melta decreases in effectiveness as armour goes up (duh), and is no where near as effective against infantry.

If you take into account the potential ways of avoiding the melta rule, then grav looks even better.

 Sledgio wrote:
Assuming all shots are BS4, grav cannon hasn't moved, and melta is at half range*:

Grav Cannon v AV10 = 0.5 chance to take a Hull Point
Grav Cannon v AV11 = 0.5 chance to take a Hull Point
Grav Cannon v AV12 = 0.5 chance to take a Hull Point
Grav Cannon v AV13 = 0.5 chance to take a Hull Point
Grav Cannon v AV14 = 0.5 chance to take a Hull Point

Meltagun v AV10 = 0.66 chance to glance, 0.64 chance to pen
Meltagun v AV11 = 0.64 chance to glance, 0.61 chance to pen
Meltagun v AV12 = 0.61 chance to glance, 0.54 chance to pen
Meltagun v AV13 = 0.54 chance to glance, 0.48 chance to pen
Meltagun v AV14 = 0.48 chance to glance, 0.38 chance to pen

Meltaguns are almost always better, however you have to be within 6" for this to be accurate. A multi-melta, however, has the same probabilities and a bigger range. grav-cannons are helped byu the number of shots they produce, after having moved, the grav-cannons only have a 0.3 chance to do anything. Grav also immobilises, however, which means all future hull points cause 2. Meltaguns have a chance to one-shot kill with an AP1 pen.

*I am 90% sure my maths is correct.

I think your math is off for the grav-cannon.

The probability of taking off a hull point with a single grav-cannon shot is 1 - (1/3 + 2/3 × 5/6 × 5/6) = 11/54 = 20.37%.

The probability of taking off at least 1 hull point with 5 grav-cannon shots is 1 - (1 - 11/54)^5 = 67.98%.

The expected number of hull points lost from 5 grav-cannon shots is 5 x 11/54 = 55/54 = 1.0185.

I'm not sure where you are getting your numbers from but a single grav shot at bs 4 has a 1/9 chance to cause a hull point (2/3 x 1/6) so 5 shots would indeed be roughly 0.5 chance of a hull point.
And my 2 cents, they are two totally different animals. Grav works out brilliantly for heavy armour troops and immobilising transports, but it pays a hefty points premium to do it. Grav guns/cannons are crazy powerful on relentless platforms, but suffer otherwise.
Melta on the other hand is far more accessible to most of the factions, will terrify Pretty much everything with an armour value; and can ID the vast majority of ICs in the game. Granted they have LOS, but it's still a good tool to make your opponent sweat. And cherry on top, they are very nicely costed.
So as far as I'm concerned they both have their place in all armies.
If we were talking grav vs plasma, that would be a totally different ball game.

MonumentOfRibs wrote:
I'm not sure where you are getting your numbers from but a single grav shot at bs 4 has a 1/9 chance to cause a hull point (2/3 x 1/6) so 5 shots would indeed be roughly 0.5 chance of a hull point.

The OP was talking about grav-cannons which always come with a grav-amp allowing re-rolls. The probability of not causing a hull point with a BS4 grav-cannon shot is 1/3 ( = chance of missing) + 2/3 × (5/6 × 5/6) ( = chance of hitting then failing the first D6 and the re-roll). This is 43/54. The chance of causing a hull point is 1 - 43/54 = 11/54.

The probability of causing at least 1 hull point is 1 - (probability of 0 hull points). Probability of 0 hull points = probability all 5 shots fail = (43/54)^5. Probability of at least 1 = 1 - (43/54)^5 = 67.98%.

Without the grav amp, the probability of 5 shots causing at least 1 hull point = 1 - (8/9)^5 = 55.51%.

Do grav amps let you re roll armour pens??

Edit. Never mind I've just re read the rule. No idea how I missed that

MonumentOfRibs wrote:
Do grav amps let you re roll armour pens??

Edit. Never mind I've just re read the rule. No idea how I missed that

It does, So basically every hit with grav cannon which has an amp by default, it has a 11/36 chances to get a six and cause a hull point and cause immobilize. And lets not forget one thing: two or more 6s will cause +1 hull points, that means two grav cannon with amp will kill a 3 hp Leman Russ, three will kill a wave serpant with Halo Field (if it do not choose to jink, and if it jink, it needs four) or a Land Raider.

It's also worth noting the quality of the hits; any hull point from a grav cannon will immobilize a vehicle while the melta needs to further roll a 4+ to achieve the same results.

I thought the grav-amp said only re-roll to wound.


Edit: Double checked, it's re-roll to wound and against vehicles.

Grav does have the advantage of immobilising too, which is no small thing! Plus it means a second grav roll of 6 will mean it takes 2 HP off due to it causing immobilised again.

Plus for the melta math, it requires the target to be within 6 inches, which isn't as straightforward to achieve...

 Sledgio wrote:
Assuming all shots are BS4, grav cannon hasn't moved, and melta is at half range*:

Grav Cannon v AV10 = 0.5 chance to take a Hull Point
Grav Cannon v AV11 = 0.5 chance to take a Hull Point
Grav Cannon v AV12 = 0.5 chance to take a Hull Point
Grav Cannon v AV13 = 0.5 chance to take a Hull Point
Grav Cannon v AV14 = 0.5 chance to take a Hull Point

Meltagun v AV10 = 0.66 chance to glance, 0.64 chance to pen
Meltagun v AV11 = 0.64 chance to glance, 0.61 chance to pen
Meltagun v AV12 = 0.61 chance to glance, 0.54 chance to pen
Meltagun v AV13 = 0.54 chance to glance, 0.48 chance to pen
Meltagun v AV14 = 0.48 chance to glance, 0.38 chance to pen

I've got a few questions about your grav cannon shots. Are you assuming all shots hit? Normally a grav cannon has a 1/6 chance of immobilizing its target. Did you multiply 1/6 * 3 to count the chances of doing an effect? The math is actually quite a bit more complex, as you can't just multiply 1/6 times three. That is the same as saying if you flip a coin twice you will get a heads on one flip. You might also get two rolls of a 6, taking off three hull points.

Also you need to consider grav amps -- which many grav guns have. A grav amp increases the chances of getting that immoblile result from 16.667% to ~30%. (1/6 + (1/6 * 5/6)) -- since you are not rerolling a successful six. Grav cannons also have five shots each, yet cost much more than a melta gun. There are just so many factors to consider!

Finally, remember that often an immobilization result will work just fine! Hey look it's a soulgrinder without a phlegm or torrent. Now it's immobile and it's an extremely overcosted IG autocannon crew. That's a nice Rhino you have there, it would be a shame if something were to happen to it before you got to that objective.

Personally, I think grav > metla given it's flexability and the way it damages units. Melta is still good, and many armies rely upon it, but if grav is available as a choice -- grav it right up!