GURPS 4e has rules for figuring out all manner of stuff for shotguns.
The point blank rule has been broken by High Tech 4e giving rules for smaller than 00 Buckshot.
B409 says:
A weapon with a RoF followed by a multiplier (e.g., RoF 3x9) fires shots that release multiple, smaller projectiles. The most common example is a shotgun. The first number is the number of shots the weapon can actually fire; this is how much ammunition is used up. When resolving the attack, however, multiply shots fired by the second number to get the effective RoF.
Example: Father O’Leary’s shotgun has RoF 3x9. He chooses to fire three times at a demon flapping toward him. For the purpose of the Rapid Fire rules, he his three shots are an attack at RoF 3x9 = 27, because each shell releases multiple buckshot pellets.
At extremely close range, multiple projectiles don’t have time to spread. This increases lethality! At ranges less than 10% of 1/2D, don’t apply the RoF multiplier to RoF. Instead, multiply both basic damage dice and the target’s DR by half that value (round down).
Example: Father O’Leary’s shotgun has 1/2D 50, so once that demon flies to within 5 yards, it is close enough that the pellets won’t disperse much. If O’Leary fires three times, his RoF is 3, not 27. But since the attack is a x9 multiple-projectile round, a x4 multiplier applies to both basic damage and the demon’s DR. The shotgun’s basic damage is 1d+1, so O’Leary rolls 4d+4 for each hit (up to three, depending on how well he rolls). However, the demon’s DR 3 becomes DR 12 against the damage.
#8 does 1d-3(0.5) pi- damage, Range 10/200 and the RoF multiplier is 411 in 20ga. There's a shitload of #8 pellets in an ounce, aren't there?
The rules as written take that 1d-3 and change it to 29d+1(0.5) pi- because it doesn't mention changing the armor divisor or wound modifiers. The first thing we notice is that a slug from a 20ga does 5d-1 pi+... An average hit from the #8 at 1 yard does 51 points of damage, assuming no armor (we'll get to that). A slug does a mere 24. Odd, it's the same mass of lead at the same velocity, but birdshot does more?
Armor. Because of that armor divisor, your skin is DR 1. The (0.5) armor divisor makes it 2. The rules as written would multiply that by 205. So an effective DR 410 against #8 birdshot at 1 yard. The maximum damage rolled from 29d+1(0.5) pi- would be 175. So someone wearing "normal clothing" is immune to being shot with a 20ga shotgun at a yard. At two yards there's at least a chance of taking damage because each 1d-3(0.5) pi- pellet has the potential to roll a 6 and have a single point penetrate to become a point of damage.
But... again... The blunt trauma rules. For every 10 points of raw damage, even if they don't penetrate DR, will do a point of cr damage. So the 175 points of raw damage (max) will do 17 to our DR 410 victim.
At two yards how many hits is very dependent on skill. A single shot gets a +9 to hit, the two shots from an 870 (2x411) get a +10. Since this sort of projectile has a Rcl of 1 every point you make it by adds a single 1d-3(0.5) pi- hit. With a skill of 12 at two yards you'd have a 22 or less to hit, so a maximum of 19 pellets can hit and a minimum of 5 (without accounting for critical success and failure). 19 pellets will, on average not hurt someone seriously, but every roll of 5 or 6 per pellet can do 1 point to an unarmored person.
