Upscaling d20 bullet damage

In a previous post tied bullet damage to the diameter of a round multiplied by the ratio of length to caliber in millimeters. This works fine for bullets which rarely go over 1 inch in diameter, and rarely have a ratio greater than 10:1 (which would max out at 10d12). I randomly mentioned this blog, and that got me pondering how the math would work out. Having looked into NATO Fox Calls for fighter planes, I came to realize that a large amount of weapon specs are available for US Munitions on wikipedia. I'm sure missiles are a bit more complicated than what is presented here, but I thought it would be worth the thought exercise. That being said, one note to mention is that in the previous post, the maximum size was d12 for 1". Most missiles are going to be more than one inch, and the inches should probably be multiplied separately from the ratio, but it seems easier to add the inches to the length ratio and multiply by that number. Both methods are shown here so anyone who wants to use them can use either method

Missile                     Dimensions (ratio) Damage

AIM-7 Sparrow (Fox-1)       ~8x144" (~x18)     d12x26 or d12x144

AIM-9 Sidewinder (Fox-2)    ~5x119" (~x24)     d12x29 or d12x120

AIM-120 AMRAAM (Fox-3)      ~7x144" (~x21)     d12x27 or d12x147

AIM-54 Phoenix (Fox-3)      ~15x154" (~x10)    d12x25 or d12x150

AGM-114 Hellfire (Fox-2)    ~7x64" (~x9)       d12x16 or d12x63

RPG-7 Round (~75x950mm*)    ~3x37" (~x13)      d12x16 or d12x39

AT-4 Round (~84x245mm*)     ~3x9" (~x3)        d12x6  or d12x9

Minuteman ICBM (~5.5x60 ft) ~5x60' (x11)       d12x785 or d12x46,800

Percentile Damage

You may have noticed that multiplying caliber and length separately produces damage in excess of 100 in all cases. It may be helpful to roll fewer dice and get bigger damage values. In that case, round the maximum damage to 100 points in whichever direction makes the most sense, and use that number of d100s. The following is the result of that.

Missile                     Damage Maximums    Percentile Roll

AIM-7 Sparrow (Fox-1)       1728               18d%

AIM-9 Sidewinder (Fox-2)    1440               14d% 

AIM-120 AMRAAM (Fox-3)      1764               18d%

AIM-54 Phoenix (Fox-3)      1800               18d%

AGM-114 Hellfire (Fox-2)    756                8d%

RPG-7 Round (~75x950mm*)    468                4d%

AT-4 Round (~84x245mm*)     108                1d%

Minuteman ICBM (~5.5x60 ft) 561,600            d%x5616**

Musings on Firearms in d20

A long time ago, way back before there were two kinds of Dungeons and Dragons, in an era where the first kind had only three editions, there was a modern version, with guns. It was sort of ok for generic purposes, but they ended up having to put out further supplements on additional firearms, both fact and fiction. That's all well and good, but ya know... it's probably out of print or something, and also you might not want to encourage piracy even if there aren't going to be particularly strong consequences for it. With that in mind, I came up with my own system for gun damage.

Damage Dice

Guns (and everything) deal(s) damage based on F=MV, but we don't need to complicate this. We can replace F with Damage, M with the size of the bullet, and V with a typical load of propellant. We don't have to even do it directly, we can just sort of provide a logical basis.

In this case, the M of the bullet is tied to its caliber, we'll assume that the following is true:

Caliber     mm           Dice
up to.25    up to 6mm    d4s
.26-.40     7-10mm       d6s
.41-.55     11-14mm      d8s
.56-.75     15-19mm      d10s
.76-1"      20-25.4mm    d12s

The V of the bullet is tied to its case length inasmuch as the case length defines what safe amounts of powder can be used to fire projectiles from it (actually thickness and material is more important than actual length, but it's a close enough abstraction for our purposes). To determine the number of dice, use (Case Length/Caliber in mm, rounding to the nearest whole ratio e.g. 9x19 Parabellum is 2d6 with 19 being 2.1x the length of its 9mm caliber. AK rounds (7.62x39) deal 5d6, and 5.56 NATO rounds deal 8d4. A .50 BMG being 12.7x99 makes it 8d8.  Likewise, a slug in 12 gauge is typically 18.5x70mm, which amounts to a ratio of 3.75, or 4d10.

Shotgun Shells

Instead of going into excruciating detail about how many different levels of buckshot and birdshot there are, we can just assume that buckshot reduces dice size by 1, but adds a 2 dice (5d8 instead of 4d10 for 12 gauge) birdshot reduces dice size by 2, but adds 3 dice (7d6 instead of 4d10 for 12 gauge), and Frangible reduces dice size by 3, but adds 4 dice (8d4 instead of 4d10 for 12 gauge).

Oh and one final thought: That original document may have propagated a myth that shotguns fire in a cone. They sort of do, but even at 150 feet you're only going to get about a 3" spread or so.

Weapon Actions

We will categorize weapon actions more broadly here for the sake of game rules--and because if we didn't a character with an M-16 can easily fire off their entire magazine in under a full round.

Single Shot

Covers breach action shotguns, flare guns, bolt action rifles, and single action revolvers.

• Each attack with this requires a single action (in PF2e) to bring a new round into battery

Semi-auto

Covers magazine fed auto-loader pistols, double action revolvers, lever action rifles, and pump action shotguns. 

• You can fire off as many shots as you can pull the trigger. It's a two action activity to multiply damage by 1d3, and a three action activity to multiply damage by 1d6. This costs 3 and 6 ammo respectively

Full-auto

Covers anything where one press of the trigger equals multiple bullets, including modified trigger assemblies that fire a second round when the trigger is released.

• With selector fire you may treat the weapon as a semi-auto
• If it's full auto only--or you've selected burst or full auto--you can still limit your trigger pulls to multiply damage by 1d6 with a single action activity (costing 6 ammo)
• You can multiply damage by 2d6 as a 3 action activity.(costing 12 ammo)