if you want realism, then you better go learn some bullet ballistics
not implying that i have any deep knowledge on the matter,but Bullets arent the only factor that decide's the firepower of a gun, that is, the damege it will do
alot of factors go in to the damege a bullet can inflict, for example, Gas operated weapons, that is automatic weapons, shot much weaker then pump action rifles because there the gas from the bullet is used to reload the gun and not only propel the bullet,which makes for slower propulsion of the bullet in question
then you have the barrel lengh, how long can the expanding gas keep propelling the bullet through the barrel? once outside the barrel, the bullet only slows down, its entire acceleration happens while inside the gun still.
then you have the barrels "rifling" and smoothness and many other smaller factors that i dont know by not being an expert on weapons
