Fixed weapon damage problem

Hello,

I'm having an odd problem with fixed weapon damage, and it boggles my mind.

SHTG A 0 A_FireBullets (2, 2, 4, 5, "BulletPuff", 18)
SHTG A 0 A_FireBullets (3, 3, 3, 5, "BulletPuff", 16)
SHTG A 0 A_FireBullets (4, 4, 3, 5, "BulletPuff", 16)

This shotgun shoots 10 "bullets" (4+3+3) at 5 damage each. The flags used are USE_AMMO (2) and NO_RANDOM (16), so 16 and 18. The total damage should be 50, but for some reason full blasts can consistently kill a monster with 80 hit points. If I reduce the damage-per-bullet to 4, the monster then survives.

However, for this pistol a similar code works as expected:

PISG A 0 A_FireBullets (0, 0, 1, 10, "BulletPuff", 18)

The monster always takes 8 shots and dies.

It's not a hugely important matter, but I'd like to know what gives. Is there something I'm missing here? All this is taking place in Heretic (not Doom / Doom II), although I don't think that makes any difference.

EDIT:

I can always use this:

SHTG A 0 A_FireBullets (2, 2, 4, 5, "BulletPuff", FBF_NORANDOM | FBF_USEAMMO)
SHTG A 0 A_FireBullets (3, 3, 3, 5, "BulletPuff", FBF_NORANDOM)
SHTG A 0 A_FireBullets (4, 4, 3, 5, "BulletPuff", FBF_NORANDOM)

Now I'm wondering whether it's possible to just sum up the flag numbers like I did above, or not. I think such thing worked in the past, but maybe my memory fails me.