This is funny, the OP meant this thread to be about guns in real life. Altho, he never clearly explained it AND posted it in the CSS division discussion. It is understandable. But this is lawltastic reading these replies thinking everyone is talking about real life with "Head shots" and "T" and "CT"
.22, .223, and 5.56 are essentially the same calibre. The difference is the casing, obviously the military needs something more powerful than a squirrel hunting rifle so they put more powder in a thicker more tightly packed casing to give the .223 and 5.56 much more velocity. The m16 fires a 5.56 round, as does the m4, and the m249 SAW. I've never ever heard of and couldnt imagine anyone using the m16 to fire a .22 round. The .50 cal is the only upper reciever and barrel kit that I've ever heard of. The .50 cal is useful at checkpoints where the 5.56 is inadequate. Besides all that, if you were to modify it, it would no longer be an m4. Like the m16 with a .50 caliber modification is known as a 'beowolf,' not a '.50 cal m16 beowolf'
Notice my pretty blue star on the left? Its really pretty...and arguing with me is kinda like telling a fireman you know more about how to use a fire extenguisher than him.
But back to the topic, in game as in real life, I want an m4 unless I'm at a very long range where it would be ineffective. The m4 in game isn't quite as devastating as it is in real life, but it is balanced for accuracy/power in the game.
If I'm playing jail I prefer the M4 for defending against CT's, due to the fact that its good for playing when I'm unfocused (usually while playing jail) or multi-tasking. It's easy to spray with if silenced and fairly easy to line up headshots.
If I'm scrimming or competing I usually try and pick up an AK due to the sheer firepower of the weapon. One shot headshot, and I'm usually pretty good at lining up at least two headshots in a row.
So to conclude; as Wind stated, there is no superior weapon, only a superior weapon depending on setting and situation.
In real life, the Ak is better. You can leave it in water, sand, you name it, and pick it up, it will be firing correctly. The m4 requires a little more maintnence, but it must be better, the US army uses it.