Due to the lag in servers and netcode and the game, the difference between the guns are huge when using either the three burst shot or full auto.

The triple burst is by far more accurate when used properly in a preferred firing position (tactical), and breath held. However its accuracy decreases dramatically much like the full auto if used repeatedly.

The full auto gives you a lot less control but makes up in the spray and pray. However it simply does not have the balance that the 3 round burst gives. Therefore you should be picking your gun depending on the map and position.

Since both guns can fire at semi-auto, both have similar (if not the same due to lag and what not) single fire distance shots when firing one bullet per click.

Since both a spray into a enemy's chest or a three round burst into them will incap most people if all shots land in the center mass, even at close quarters either firing mode will put someone down. The question at that point is whether you can aim or not. So if you cant aim well in CQB, use the M4. Use the M16 if you've got the aims. Chances are you wont be shooting someone at 300m simply because the map does not allow you to (fog or distance). But it does when it comes down to hitting targets that are leaning off a wall or window, or prone.

The problem with the 3 round burst is that the lag and bullet registration makes it less effective than it should be. I find that most of the time I use single shot to engage long distance targets. Conversely, the lag also makes the full automatic spray very ineffective as you will often find yourself shooting 20 bullets at an enemy and get taken down as if the enemy didn't take a single hit himself.