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Old 02-26-2022, 02:17 PM   #1
Michael Thayne
 
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Default Writing a "massed fire" rule for when the "rapid fire" rules don't make sense

A few GURPS supplements—like Zombies and the Spaceships series—suggest using the rapid fire rules to resolve many attacks at once. Unfortunately, this makes no sense, because the rapid fire rules are designed on the assumption that recoil will prevent you from scoring 10x as many shots just because you used 10x more dakka.

One very simple approach would be to borrow from the multiple close combat rules (B392) and say that to resolve a bunch of identical attacks with a single roll, use best effective skill + 1/5 the effective skill (rounded down) of all other attackers. Treat melee attacks as having Rcl 1. This seems to produce reasonable-ish results. Far from perfect, but surprisingly reasonable for such a simple rule.

Playing around with some numbers in a spreadsheet, another option I came up with is to have each attacker after the first add the lower of [(effective skill - 10), (RoF - 1)]. Again, treat melee attacks as having Rcl 1. Also, treat a +0 bonus as +1/2, i.e. every two attackers with an effective skill of 10, or a higher effective skill but only an RoF of 1, give a +1 bonus. This rule was designed to produce (roughly) the right number of hits on average and is arguably more realistic for highly skilled attackers. OTOH the variance doesn't scale right at all.

The maximally realistic rule would probably do something fancy with standard deviations, but I don't know if such a rule could be made practical for the gaming table. Though I'd be interested if people have their own ideas on how to improve these suggested rules, or if they have their own completely different ideas.
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Old 02-26-2022, 02:48 PM   #2
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Default Re: Writing a "massed fire" rule for when the "rapid fire" rules don't make sense

Quote:
Originally Posted by Michael Thayne View Post
One very simple approach would be to borrow from the multiple close combat rules (B392) and say that to resolve a bunch of identical attacks with a single roll, use best effective skill + 1/5 the effective skill (rounded down) of all other attackers. Treat melee attacks as having Rcl 1. This seems to produce reasonable-ish results. Far from perfect, but surprisingly reasonable for such a simple rule.
It's the right kind of growth but the actual scaling seems broken. Each added skill-10 attacker generates 2 extra hits in melee or with Rcl-1 weapons, when being skill 10 means they really should be generating 1/2 an extra hit.
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Originally Posted by Michael Thayne View Post
The maximally realistic rule would probably do something fancy with standard deviations, but I don't know if such a rule could be made practical for the gaming table. Though I'd be interested if people have their own ideas on how to improve these suggested rules, or if they have their own completely different ideas.
Well, the simple-ish but not so bad rule for really massed fire would be to find the average number of hits per shooter - you probably want a table of skill vs. rcl because I don't think the arithmetic can be made easy - and multiply by number of shooters. And then you could just stop there even! If the fire is sufficiently massed, the law of large numbers pretty much says the aggregate will match the average.

But if you want some random in (and you probably do) you could perturb that number by doing something like multiplying by 6d and dividing by 21. That should be a narrow enough variance not to be too crazy while not having perfectly predictable results. And it's few enough dice you can actually roll it.
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Old 02-26-2022, 02:52 PM   #3
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Default Re: Writing a "massed fire" rule for when the "rapid fire" rules don't make sense

If you know the number of attackers and the effective skill level, you can simply use the success percentage at that skill level and say there are a corresponding number of hits (you can even work out crit success and failure numbers).

So if you are working with, say, an effective skill of 12 and 10 attacks, you know that 74.07% of attacks would hit, which is 7 or 8 out of 10 - call it 7 on the basis of only counting whole numbers and not fractions. Also, 1.85% will be a crit success (3 or 4), so that's 0 crit. On the flip side, 1.85% will also be a crit failure, so that's 0 crit failure.

If you move it to 100, it becomes 74 hits, 1 crit success and 1 crit failure.

This removes the "swings", but with larger numbers would come more consistency. If you wanted some "swing" you could perhaps have a random effective skill modifier up or down by 1 or even 2, with whatever percentage chance you like to do it to reflect "luck". But really, this is very simple to model in table and provides super quick resolution of larger numbers that likely reflect general results the higher your numbers any way.
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Last edited by Farmer; 02-26-2022 at 05:16 PM. Reason: Crit numbers were wrong
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Old 02-26-2022, 03:07 PM   #4
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Default Re: Writing a "massed fire" rule for when the "rapid fire" rules don't make sense

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Originally Posted by Farmer View Post
If you know the number of attackers and the effective skill level, you can simply use the success percentage at that skill level and say there are a corresponding number of hits (you can even work out crit success and failure numbers).

So if you are working with, say, an effective skill of 12 and 10 attacks, you know that 74.07% of attacks would hit, which is 7 or 8 out of 10 - call it 7 on the basis of only counting whole numbers and not fractions. Also, 1.85% will be a crit success (3 or 4), so that's 1 crit. On the flip side, 1.85% will also be a crit failure, so that's 1 crit failure.

If you move it to 100, it becomes 74 hits, 18 crit successes and 18 crit failures.

This removes the "swings", but with larger numbers would come more consistency. If you wanted some "swing" you could perhaps have a random effective skill modifier up or down by 1 or even 2, with whatever percentage chance you like to do it to reflect "luck". But really, this is very simple to model in table and provides super quick resolution of larger numbers that likely reflect general results the higher your numbers any way.
This matches to what I suggested except that it assumes the attackers are using RoF 1 weaponry (or at least assuming they never score multiple hits).
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Old 02-26-2022, 03:18 PM   #5
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Default Re: Writing a "massed fire" rule for when the "rapid fire" rules don't make sense

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Originally Posted by Farmer View Post
Also, 1.85% will be a crit success (3 or 4), so that's 1 crit. On the flip side, 1.85% will also be a crit failure, so that's 1 crit failure.

If you move it to 100, it becomes 74 hits, 18 crit successes and 18 crit failures.
Would you like to correct the number of criticals?
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Old 02-26-2022, 03:49 PM   #6
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Default Re: Writing a "massed fire" rule for when the "rapid fire" rules don't make sense

Took me a bit to find it, but here's something I came up with some time ago. Note it's faster if, instead of rolling 3d6 three times, you just roll 3dF (dF are Fudge dice, d6's that have two +'s, two -'s, and two blank faces; treating 1,2 as +, 3,4 as blank, and 5,6 as -, lets you use a normal d6 in place of dF).

Note that gives you some variability (but with fairly similar spread in results as what you'd see actually rolling each attack separately), and works anytime you'd need to make a lot of rolls against the same target number. If does require a calculator, and a table lookup (to see how the probabilities work out), however, but it supports figuring out the MoS/MoF of each attack as well.
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Old 02-26-2022, 04:36 PM   #7
Anthony
 
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Default Re: Writing a "massed fire" rule for when the "rapid fire" rules don't make sense

I was pondering coming up with a 'horde' trait to allow using the single combat rules for mass combat. Never finished it, but the basic mechanic probably works here. What I have:

Quote:
Horde 50/level (minimum 2 levels)
A horde is a mass of identical creatures -- to determine horde rating, look up the number of creatures on the range/speed chart and add 2, so 10 creatures are a level 6 horde, 100 are a level 12.

When attacking a horde with single target weapons, cap injury (after modifiers) at 2xHP, and then apply IT(DR) corresponding to its level (if it doesn't need all of its levels to reduce injury below 1, there's a chance of 8+(remaining levels) to reduce to 0). If making multiple attacks, using rapid fire, or the like, rather than calculating per hit, you can just cap injury on each hit, sum the injury, and apply horde modifiers at the end. For example, if you shoot a platoon of soldiers (50 people, horde rating 10) with a a tank gun for 6d*30(3) pi++, that 630 damage is reduced to 619 by the target's DR 35 vest with insert, then doubled to 1238 by being pi++, then capped at 22 because the soldier only has 11 hp. Since it has HR 10 and only needs 9 levels to reduce below 1, there is a 9- chance to reduce damage to zero. If the horde takes damage, it suffers normal penalties (a shock penalty for seeing a member blown to bits seems fair enough).

When attacking a horde with explosives, horde rating is capped at (SM of area that contains the horde); when using area fire, it is capped at (SM of area that contains the horde) - (SM of area effect). For example, if the platoon above was in a parade formation 5 yards wide x 10 yards long, it would only have horde rating 4 against explosives, but in a skirmish formation 50 yards wide it would have its full rating of 8. If you're using blind fire to shoot at an area, treat the contents as having a horde rating of (SM of area) - (SM of area), no matter the actual number, and if this exceeds its actual horde rating, there is a (8+remaining levels) chance for a total miss. This can be applied to individuals.

When a horde attacks, it makes a number of attacks equal to its horde rating; these attacks can be rapid fire, multiattack, or whatever. If the horde is using melee attacks it's possible not all members can reach the front line; limit horde rating to the rating for the number that can actually attack (for a single line, this is (SM for length of front line) - (SM of individual combatants)). Thus, if a Roman century is fighting on a bridge that's 5 yards wide, it only gets a horde rating of 4 (instead of 10) unless it can figure out how to involve its back elements (throwing javelins, etc).

When a horde attacks a horde, it may reduce its horde rating to a minimum of 2 to reduce the enemy horde rating by the same amount, with a minimum of 2. Thus, if a platoon (horde rating 10) attacks a company (horde rating 14), it only makes 2 attacks, but it reduces enemy horde rating from 14 to 6.
I suspect the above would be cleaner with conditional injury, as the injury modifiers get replaced by 'cap severity at 2 - horde rating'.
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Old 02-26-2022, 05:15 PM   #8
Farmer
 
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Default Re: Writing a "massed fire" rule for when the "rapid fire" rules don't make sense

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Originally Posted by johndallman View Post
Would you like to correct the number of criticals?
Yep, thanks, will do.
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Old 02-26-2022, 05:18 PM   #9
Farmer
 
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Default Re: Writing a "massed fire" rule for when the "rapid fire" rules don't make sense

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Originally Posted by Ulzgoroth View Post
This matches to what I suggested except that it assumes the attackers are using RoF 1 weaponry (or at least assuming they never score multiple hits).
Yep. You just add in the number of attacks and their effective skill level.
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Old 02-26-2022, 05:30 PM   #10
Michael Thayne
 
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Default Re: Writing a "massed fire" rule for when the "rapid fire" rules don't make sense

Quote:
Originally Posted by Ulzgoroth View Post
It's the right kind of growth but the actual scaling seems broken. Each added skill-10 attacker generates 2 extra hits in melee or with Rcl-1 weapons, when being skill 10 means they really should be generating 1/2 an extra hit.
Aah, yeah, hmmm, nevermind.

Quote:
Well, the simple-ish but not so bad rule for really massed fire would be to find the average number of hits per shooter - you probably want a table of skill vs. rcl because I don't think the arithmetic can be made easy - and multiply by number of shooters.
So I actually made this table for Rcl 1 before making the OP. My second proposed rule was mean't to be a conservative approximation of that rule—it underrates the value of massed fire, but not as much as the standard rapid-fire rules.

Though having just made the tables for Rcl 2 and Rcl 3, I wonder if I should have used skill-9 instead. For skill-11 and RoF 13+, you expect ~2.1 hits at Rcl 1, ~1.2 hits at Rcl 2, and ~0.9 hits at Rcl. Increasing the expected number of hits by (skill-9) / Rcl gets +2 hits, +1 hit, and +2/3 hits, respectively. So that would be fine. The skill-9 rule would actually do pretty well in general. But for Rcl 1 it's over by about +1/2 at high skill levels. IDK, maybe that's fine.

Quote:
And then you could just stop there even! If the fire is sufficiently massed, the law of large numbers pretty much says the aggregate will match the average.
Eh, sort-of. The absolute difference between the aggregate and the average will actually grow as the number of trails grows. It just won't grow as fast as the average itself, so (average - aggregate) / average goes to 0 as the number of trials grows. My second proposed rule keeps the absolute difference between the aggregate and average fixed as number of attacks grows, which is not all that realistic, but realism-wise is strictly superior to just fixing the difference at 0.

Lol. I just had an idea that seems too simple but might be the best simple rule. What if you just summed the skill of all the attackers and rolled (n x 3)d where n is the number of attackers? Not that you'd want to do that for truly huge numbers of attackers but for a single-digit n, D&D players roll that number of dice all the time, at least at high levels.
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