If you have access to the Dark Heresy stuff the math is pretty simple. Add the Min damage and the Max damage and divide the whole thing by 3 (round normal). We just used Pen as AP straight
That makes an auto pistol 5P with SA/FA; a shotgun is 6P, etc.
It gets ugly when you do the advanced weapons, for example:
AutoCannon 18P, AP -4, SA/BF/FA
Heavy Stubber 6P, AP -3, FA
MP Lascannon 25P, AP -10, SS (yes I know lasers are AP-half, but this makes more sense
IG Boltgun 7P, AP -4, SA/BF
That's not even the Astartes stuff.

Their boltgun is like a BF panther cannon (only 1 more damage)
