Really, it's impossible to drive-by with non-SMGs/Pistols because all M4s and AKs are long two-handed weapons and it's very hard to drive and use them at the same time.
In the previous GTAs, it seems that while you drive by, the SMGs were enhanced so that rate of fire and power was increased a lot so that it killed peds in one shot and slightly damaged vehicles quicker. I'm really against this whole realism thing, but no one does a drive-by with assault rifles.