Detailed Shotgun Ammo – House Rule

I alluded to a set of house rules that I came up with for shotguns to deal with small shot sizes in my post on Rapid Fire rules and suppression fire.

I had thought I’d posted about them, but looking, I don’t think I did.

Here’s the deal: it’s commonly known that birdshot stinks as a person-killer. Granted, it’s probably better than harsh language, but not by much. The intimidation and suppression value will be large (see Cool Under Fire, Tactical Shooting, p. 34), and in the real world, that ain’t nuthin’.

Anyway, using my article and revised spreadsheet, it’s possible to turn the usual statistics about shotguns into a pretty detailed table.

A note about the previous version that you may have seen. The data was wrong. The lead pellets were too fast (and thus too-high in penetration, even more than they should have been) and the steel pellets were probably too many. I set the lead shot to 1275fps, mostly, except in a few cases where standard 2.75″ loads were different. The #1 Buckshot was only 1250fps; the 00 and 000 Buckshot were a heavy-kicking 1325. Steel shot tended to range from 1400-1550fps, so I settled on 1450, and 1 and 1/8 oz for steel shot, and 1.25 oz for lead, but I made whole-numbers of pellets. All in all, a lot of fiddling.

OK, that’s unreadable. click on the image (or here) to go to the full Excel file.

Point is, the calculated wound modifiers, the things in the squiggly brackets, drop below 0.5 (the value for pi-) pretty fast.

What I do is to cluster real pellets into “effective” pellets by ensuring every wound modifier rounds up to a full pi-. so everything smaller than #2 Lead Buckshot (1d+1 pi-) has an effective RoF lower than the number of pellets actually thrown. The clustering of pellets (and therefore damage and effective RoF, and therefore Rapid Fire bonus) means that there really are only a few different types of shot that are worth of GURPS’ resolution.

They are:

Every lead or steel example from 1d and lower has a more-or-less equivalent variant in the other material. So if you want to have a lead-free world, or steel shot hasn’t been invented yet, you can swap the materials for equal stats.

As shown in my musings about rapid fire, targeting, and using suppression fire to mimic scatter around a target, the relatively higher hit percentages using the corrected Effective RoF mean the number of pellets in a shot that actually strike home will go up to more sensible values.

Some notes on the table(s)

I tried to give a reasonable selection. A point change in damage was enough to get a row by itself, which explains down to 2 Buck. The rest, I collapsed all the identical rapid fire bonuses into one, but allowed differentiation by damage or, in one case, range. So #3 lead buckshot does 1d pi- but has a 1/2D of nearly 70 yds. Steel F shot also does 1d pi-, but the lighter pellets only have a 1/2 of 40 yards, but you get +1 to hit because there are more of them. So there’s a legit choice there.

+Hans-Christian Vortisch picked on my Max Range numbers, so I fiddled a bit. I set the program to calculate where the average penetration falls to a ludicrously low 0.017 points of damage. That put the actual max range for 00 Buckshot closer to his real-world number, and I just let the math roll from there.

The penetration/damage figure only takes into account projectile energy and cross-section. No mushrooming or funky stuff. It tends to get a bit wonky at low projectile cross-sections, because the low cross-sections force the penetration numbers up to an unrealistically high value. Regardless of what the KE/cross-section numbers look like, these low-mass, low sectional density projectiles won’t penetrate deeply. The wound channel modifiers (values in squirrely brackets) correct for that somewhat.

Still might produce unrealistically exaggerated penetration values, so another way to figure this is that zeros actually count as zero when rolling for damage. So if you roll a 1, 2, or 3 on 1d-3, you do no damage, not the usual “minumum 1 for pi.” This table has zero-penetration hits built into it. If you want to re-convert to the usual “minimum piercing roll is a 1,” then you map it this way:

1d 3.50
1d-1 2.67
1d-2 2.00
1d-3 1.50
1d-4 1.17
1d-5 1.00

So our 12G #9 Birdshot (1.07 points of damage) would be 1d-5 instead of 1d-3. The average damage on 1d-5 (minimum 1) is 1. The average on 1d-3 (minimum 0) is also 1. Either way can work; you probably will sort out different choices than those I picked above.

Parting Shot

This table started life as a super-detailed look at shotguns. It ended with what I think are better estimates of the effective RoF and damages of these loads for those who need them. The condensed table provides the right amount of choices (not stupid-high, but enough to differentiate) without useless detail.

Hope you enjoy!

7 thoughts on “Detailed Shotgun Ammo – House Rule

  1. Hi, Mr Cole:
    First of all, I'm a native Spanish speaker with an average level of written English comprehension, so it's easy to me to get lost with GURPS more complicated rules.
    I'm running a GURPS Zombies campaing, located in my country, Spain. Here in Spain we have a lot of hunting shotguns and hunting ammo, but it's really difficult (and highly illegal) to get access to police or military weapons.
    The fact is that my group of survivors found two hunting shotguns in an empty house and I told to them that they found two types of ammo: buck shot for larger animals (I'm using the standar stats for a 12 shotgun in GURPS High Tech) and birdshot for smaller game.
    I put my head nearly to explode trying to understand the rules for shotgun ammo in GHT and, when I was about to give up I found this article and Excel sheet. And you saved my GMaster life!! But…
    I have still problems with the damage of birdshots at close range. Even with your modified effective RoF I find that, for example, a 26d-130 vs DRx26 (if I made the correct maths for a 9 shot ammo) per shot, a quite silly thing for a RPG that tries to resolve the most of dice rolls with d6.
    Where did I lost myself with rules?
    Or if I'm right with the numbers, how do you resolve this situation (not hypothetical for me in the game, but an every day 'reality' in my apocalypse low cost world campaign)?

    1. The new chart – the "red" one in my post, replaces the rule in High Tech. It collects pellets together until the wound modifier sums to 0.5, the value for pi-.

      The funky calculation for penetration that I use probably overstates the penetration value of the pellets, but the lead 9-shot is 1d-3, and if you roll a zero, it does NO DAMAGE.

      So don't multiply DR and damage and whatnot. Just allow a roll of 1-3 to do no damage, and then let the chips fall where they may.

      The allowable damage, then, is 1d-3 pi-, which will do 1, 1, or 2 points of damage to an unarmored person. Keep it simple. You will have to hit a LOT of times to get appreciable damage, and DR 1 even will vastly cut down wounds even with multiple hits.

      Let me know if that helps, and thanks for reading!

    2. When I say "roll a zero," I mean if 1d-3 rolls 1-3, you get -2 through 0 damage, which (unlike normal GURPS rules) is not rounded up to 1 pi-, but is an actual zero, for no penetrating damage.

    3. Thank you very much for your quick answer.
      I downloaded the Excel file and I have 1d-4 for a 9-shot. It can be a mistake?
      I missed the part where you explained the "roll a zero" rule so, at close range, you don't multiply the base damage for half the effective RoF as the Basic rules says?

  2. Ok, I know I'm becoming an annoying pest and maybe you're thinking about shoot me with the appropriate birdshot, but I just realize that the 'red chart' I've got downloading your Excel file and the one shown on the post as a picture are quite different, and I thought they're the same. In the first one there's no damage type next to damage dice (pi++, pi+, pi, pi-), and the damage dice also differs from the red chart in the picture. Actually, I think the blue chart on the post, the red chart on the post and the red chart I see in the downloaded file, are different among them.
    I think, I'm missing something so, would you mind teaching me how I am wrong?
    On the other hand, if you think I'm being pesky, please tell me and I'll stop bothering. Seriously, no problem.
    Thank you again.

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