M16 and AK-47 are equivalent in-game for mostly legacy reasons, since they were the original two automatic weapons and they had the same stats. The other distinctions in accuracy are based on weight and ease of use rather than shooting range accuracy -- I generally assume that all combat in LCS takes place at short enough range that if you point the gun at the other person and pull the trigger, you hit them. It's just a matter of how easy it is to point the gun in the right direction on reflex. With that metric, SMGs and the M4 Carbine get accuracy bonuses over the M16 and AK-47, while the heavier weapons do a bit more damage per hit.
Notably, it doesn't matter too much how much damage you're doing on the high end. Everyone has 100 blood (or hit points), so a weapon that averages more than that each attack is going to kill in one hit, regardless of how much extra damage you pile on. The only reason overkill damage doesn't always kill in one hit is because lethal damage is not permitted on arms and legs. You have to hit the head or torso to kill someone, or just do enough damage to get their health low enough that they can't fight or run, then let them bleed to death.
Where the high-end damage of the weapon may matter is when you're trying to punch through body armor. In that case, the armor penetration value of the weapon will be pretty major (since it lets the attack ignore part of the armor), and any extra damage you pile on will help to offset the damage reduction from what isn't ignored. Lightweight weapons without much overkill damage or armor penetration are susceptible to body shots that do zero damage against police body armor.