#4 Buck

#4 Buck does 1d-1(0.5) pi- and gives 27 pellets in 12ga 2-3/4"  00 Buck does 1d pi and you get 9 pellets.  For the record, a .22LR from a pistol does 1d+1 pi-

Coincidentally, the M576 multi-purpose round for the 40mm grenade launcher also uses #4 buck, but just 20 pellets.  40x46mm is a much larger round than 12ga 2-3/4"!

Despite the low damage, it was, apparently, very popular with shotgunners in Vietnam.

The #4 gets a +5 to hit from one shot and +6 for two; 00 gets +2 for one shot and +4 for two due to the number of pellets flying.

More hits could mean more damage despite the halving of the damage after penetration.  For a single shot the same roll means 3 more pellets hit.

Range is kinda pathetic, so let's keep it to 20 yards, -6 to hit.

A skill 11 shooter will have a 7 chance to hit with a single round and 9 with the RoF of 2 of a pump shotgun shooting 00 buck.  On average, they're gonna miss with both.

The same shooter will have a 10 with a single round and 11 with two.  On average, they'll get one pellet to hit with one round and two with two.

Our grenadier gets a +4 to hit with his single M576 round and will likely be missing...

Hits do more damage than misses regardless.  (1 point or 3 points is more than zero).

If we let the enemy get much closer, like say, 7 yards... it's only -3 to hit.

That means rolls to hit of 10 and 12 for the 00 and the average roll will now land 1 or 3 hits.

#4 needs 13 and 14 to hit and will land 4 and 5 hits respectively.

40mm needs 12 so the average roll lands 3 hits.

00 buck will do (on average) 3.5 hits per round; so 3 or 10.

#4 buck will do (on average) 1.5 hits per round; so 6 or 7.

40mm does the same average damage so 4 points of damage from its three hits.

At longer ranges #4 is going to work better because you're going to be hitting more.