Rapid fire explosive rules, again

[Ulzgoroth]

So it was said:

Originally Posted by DouglasCole

Like one of the first things we brought out was a player in the test game decided to pack a full-auto grenade launcher. If you know regular GURPS, that's a "OK, everyone get pizza while we resolve all this" sort of thing. We've got a way of handling this that makes it just another Tuesday.

And I immediately had a tangential question.

I like to believe I do know regular GURPS.

But to the best of my knowledge, that isn't a 'we need a bunch of time to execute the rules' problem, that is a 'they never actually published the rules' problem.

So. Am I missing something? Can anyone lay out...using only actual published rules with citation, please...how you resolve the effects of firing off an automatic grenade launcher?



My understanding: If you manage to hit with all shots, you're okay(*), everything explodes on (or in) the target. If any shots miss, though, we start running off the map, because we really need to know where (on the map) those shots go.

The option that's sort of mapped is Hitting the Wrong Target, p389. The problem there is that rule implies your shot is going on a basically flat trajectory - it's got no chance of hitting near the target you aimed at unless they've got a convenient backstop. Maybe not what you expect from a grenade launcher. Also, it makes the scatter cone very narrow (1 hex wide out to unlimited distance). And it can get really screwy if there's a target that totally obstructs that line but you still manage to miss it. Still, you can follow those instructions! So that's better than I implied!

But if you want something that produces a scatter pattern on the ground, instead of one sprayed across the backdrop, well. The obvious place to look is Scatter, p414, but you likely cannot. That only applies "if you fail your attack roll". So it's inapplicable when you hit with 2 of 6 grenades. And if you do fully miss, well, it doesn't say a word about RoF, so at best it implies your entire burst lands in one place.

(*) Okay except for questions like 'what actually are the fragmentation rules when somebody is hit, is it really safer than being 1 yard away?' And the potentially large number of dice to be rolled and calculations scribbled.

[mlangsdorf]

I think the problem that Doug is referencing with an automatic grenade launcher is the amount of die-rolling, distance measuring, and math required to get the damage for 5 people standing in a rough cluster when 3 grenades with explosive and fragmentation damage land near them.

It's 3-15 explosive damage rolls, each divided by a multiple of 3 and up to 15 additional attack rolls for the fragmentation that produce 0-3 additional damage rolls each. Each fragmentation attack has a separate range penalty. It's just too much work.

That's aside from the huge rules gaps that you cite. But if you walk a burst of automatic grenade fire across 5 targets and can actually calculate where the hits and misses land, that's just the start of a mess of die rolling and calculation that most GMs would prefer to avoid.

[cmdicely]

Originally Posted by Ulzgoroth

My understanding: If you manage to hit with all shots, you’re okay(*), everything explodes on (or in) the target. If any shots miss, though, we start running off the map, because we really need to know where (on the map) those shots go.

The option that’s sort of mapped is Hitting the Wrong Target, p389.

I think the more appropriate one is Scatter (p414) which applies “When an area-effect, cone, or explosive attack misses its target,” but the not-explicit part is how you figure out the margin of failure for each shot that missed. But since you would get additional hit for each full multiple of Rcl in Margin of Success, you can invert that to find the margin of failure for each missed shot: the first missed shot (if you hit at least one) will have a margin of failure of Rcl minus 1 for each point the original margin of success was short of the next multiple of Rcl, and that will increase by Rcl for each additional missed shot. (If you miss, the margin of failure for the first shot is the actual margin of failure, and increase that by Rcl for each additional shot.)

[Ulzgoroth]

Originally Posted by cmdicely

I think the more appropriate one is Scatter (p414) which applies “When an area-effect, cone, or explosive attack misses its target,” but the not-explicit part is how you figure out the margin of failure for each shot that missed. But since you would get additional hit for each full multiple of Rcl in Margin of Success, you can invert that to find the margin of failure for each missed shot: the first missed shot (if you hit at least one) will have a margin of failure of Rcl minus 1 for each point the original margin of success was short of the next multiple of Rcl, and that will increase by Rcl for each additional missed shot. (If you miss, the margin of failure for the first shot is the actual margin of failure, and increase that by Rcl for each additional shot.)

Yes, I am aware that's the kind of thing people like to make up. Which is why I specifically asked for actual rules answers.

[Varyon]

Originally Posted by Ulzgoroth

Yes, I am aware that's the kind of thing people like to make up. Which is why I specifically asked for actual rules answers.

Yeah, I don't think there's anything for this RAW, aside from Hitting the Wrong Target - and as you note, that doesn't quite work for a lot of attacks like this. The adapted Scatter rules suggested above are probably the best we've got for now - but hopefully whatever they've come up with for Mission X handles this as well. While that sounds like it's going to be on a pretty heavily-modified version of the base GURPS system, I suspect the relevant bits won't be too difficult to integrate into mainline GURPS.

[David L Pulver]

Originally Posted by Ulzgoroth

Yes, I am aware that's the kind of thing people like to make up. Which is why I specifically asked for actual rules answers.

I ran into this problem yesterday when considering the case of an aircraft autocannon firing near-vertically with HE ammo against a ground target, with several other possible targets nearby. If it fired a 20 or 30 rounds and maybe eight or so hit, the others would realistically scatter about the vicinity. Other possible cases are indirect or similar plunging fire with AGL, automatic mortars like the Vasilik, multi-tube rocket launchers, etc.

I could not find any official rules for it either. My bad!

Interestingly, the Hero System also seems to choke on this one.

[David L Pulver]

Originally Posted by Varyon

Mission X handles this as well. While that sounds like it's going to be on a pretty heavily-modified version of the base GURPS system, I suspect the relevant bits won't be too difficult to integrate into mainline GURPS.

I think it could probably handle it better, but I'm not sure the bits would be easy to integrate into mainstream GURPS.

[Anthony]

Honestly, there should just be a general rule for saturation fire where your hit chance is based on total projectiles per hex; 100 projectiles into a 10x10 area and 10,000 projectiles into a 100x100 area are the same thing.

[fula farbrorn]

The way i have handled it is works as follows

You fire a 10 round burst of 40mm grenades into your target (Hex A) you roll and hit with 3 of them ensuring those all land in the first hex (Realistically scattering a bit inside of that hex) then you roll Scatter from Hex A using your recoil to see how far you scatter, Then the next round scatters from Hex B and then from C and so on and on.

[cmdicely]

Originally Posted by Ulzgoroth

Yes, I am aware that’s the kind of thing people like to make up. Which is why I specifically asked for actual rules answers.

Grenades are, by the actual rules, explosive attacks. Those are the actual rules that apply when explosive attacks miss. The rules on that page don’t handhold you through calculating the margin of failure for each round in the case of rapid fire attacks, but the roll needed for the round to hit and the difference between that roll and the actual roll – the definition of margin of failure – are well-defined in the rules.

So I think it is odd to say that those are somehow less the actual rules for this situation than “Hitting the Wrong Target”. (I would agree that, actual rules or not, they aren’t particularly satisfying for, say, an autocannon firing explosive shells in a direct fire as opposed to high-RoF indirect fire, as they are more optimized for aerial or high-arc attacks targeting a spot on the ground, but “satisfying” is a different question than “actual rules”.)