Rapid fire explosive rules, again
 

So it was said:
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.

Re: Rapid fire explosive rules, again
 

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.

Re: Rapid fire explosive rules, again
 

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.)

Re: Rapid fire explosive rules, again
 

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.

Re: Rapid fire explosive rules, again
 

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.

Re: Rapid fire explosive rules, again
 

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.

Re: Rapid fire explosive rules, again
 

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

Re: Rapid fire explosive rules, again
 

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.

Re: Rapid fire explosive rules, again
 

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.

Re: Rapid fire explosive rules, again
 

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”.)

Re: Rapid fire explosive rules, again
 

Margin of failure for each round is not, in fact, a thing that has any existence under the rules. Margin of failure is a characteristic of a success roll. There is only one success roll in the scenario, and that roll succeeded. Inventing a virtual roll for each shot is just that - pure invention, not anything supported by the text.


Really, the problem is your first sentence: grenades are not explosive attacks. Grenades are grenades. They can be used to make explosive attacks. But when you make a RoF N attack using N grenades, you make 1 attack, not N attacks.

Re: Rapid fire explosive rules, again
 

It's probably not RAI that the extra rounds just vanish, either.

Regardless of whether scatter by Rcl is actually the RAI or not, it's still an unwieldy amount of rolling, first to hit, then for scatter, then for explosive damage for each round, then for fragments to hit for each round and finally fragment damage.

Re: Rapid fire explosive rules, again
 

So this is absolutely not RAW, but here's a way to partially simplify that. You'll roll to hit as normal, and then roll once for each shot that misses to determine scatter. For explosive damage, roll only once and use that for all the explosions (you'll divide each by a value based on distance from the target - typically 3*yards - and add them all together, rounding at the end). For fragments/shrapnel, assume the attack is actually at base skill 9 with RoF 50 (enough for a +6 to match with the actual rules, which give them base skill 15). Starting with the nearest explosion to the character, any within the same range band gives +50 to RoF, any within 1 range band gives +30, within 2 is +20, within 3 is +15, and so forth, following SSR. The total resulting RoF determines the bonus to skill.

So, let's say you're using an RoF 6 Rcl 3 automatic grenade launcher. You hit your target with MoS 5 and they fail to Dodge. So that's two grenades on target, one 1 yard away, one 4 yards away, then one at 7 yards away, and finally one at 10 yards away. You roll your 1d6 4 times to work out scatter. For the target, the first two automatically deal full damage; you just use 1 roll for the rest - if you rolled 30 damage, those are 10, 7.5, 4.3, and 3, respectively, for a total of 24.8 (added to the two max-damage hits for contact explosions), which I think would round down to 24 cr. For fragmentation, let's say all are within range (so it's at least a [2d] attack). The closest are actually the two that directly impacted the target*, so you start with RoF 50 and add another 50. The next one out (at 1 yard) is still at +0 to hit, so +50. The next one out (4 yards) is at -2 to hit, so +20. The next is -3, for +15, and the last is -4, for +10. That's a total RoF of 195, for a +7 to hit. Note in this case that's indistinguishable from the RoF 100 you get from the two direct hits (100-199 is +7). So you roll against 16 to determine how many additional fragments hit the target.

For another target in the same area, the divisors on the various explosions will be different, and you'll need to recalculate sharpnel RoF based on the distance of the explosions from this actual target.

*The rules just say you score one automatic hit with shrapnel on a direct hit. I'd say do it as 1 automatic hit, then also roll against 15 to see if there are additional hits.


Note the above isn't something that is likely to be done quickly absent something like computer assistance, but at least it should be faster than rolling individually for each explosion (both for explosion damage and fragmentation). This does make multiple sources of fragmentation less lethal than doing things the longer way, but that's just the nature of the RoF rules.

If you're a real glutton for complexity, note each explosion would be knocking the character back, but in different directions. This would be a vector sum situation, using the adjusted crushing damage for each explosion.