Funny thing about an M-16/M4... It uses 5.56 mm rounds, which are smaller than the 7.62 coming out of the AK or sniper rifle, but they're going much faster. Because of this it tends to be better for armor penetration. Likes to tumble through the body, rather than dealing crush damage and creating a big temporary wound cavity. (Bullet goes in, dumps it's kinetic energy like setting off a firecracker under water, but the projectile usually has enough energy to come out the other side of a human target. Human shields aren't great.)
The smaller, faster projectile (5.56 or 556) will punch through undead fleshy targets as it's really meant to tumble and rip and tear blood vessels and perforate organs, rather than impart a great deal of kinetic energy. (So the 556 ALSO has greater over penetration.) They don't really bleed though, so while the bullets will zip through, it won't really do a lot.
However... A headshot is still a headshot. Hitting bone with that much kinetic energy can do 'wonderful' things and might make delicious scrambled brains.
So I might want a 5.56 platform against bandits and players, not so much against zeds. AK is better against zeds, bu tis also very effective against NPC's/Bandits/Players.
Pros for the M-16/M4/AR:
- Lighter. Ammo weighs less (bigger stacks)
- Higher accuracy (Due to a number of factors)
- Over penetration (can go through more than one target, or punch through armor better?)
- Spawling (bleed chance or higher bleed chance?)
- Very common design, MANY attachments and parts
Cons for the M-16/M4:
- Tighter tolerances, requiring more frequent maintenance (faster degradation)
- Less kinetic energy delivery (less effective against zombies)
Having trouble coming up with too many cons. At the end of the day, an M-16/M4 or similar wouldn't be bad to have. I could see it being added to alter the gameplay dynamic a bit. Though right now the SMG has the highest DPS of any weapon, IIRC. Least last I checked the wiki.
It's only a 9mm round, but with good, accurate shooting it's pretty nasty. (Of course that goes for any decent firearm)
Anyway, that's a small bit of analysis on the subject.