M16A1 doesnt exist in BF3. Hopefully never will. That gun was a pain in the ass for the troops in vietnam.
You are talking about the M16A3 or M16A4 right? Why is it that the M16A3 has full/semi automatic and M16A4 has burst/semi automatic? Both should have all options.
Its a game you fruit cake... Do you think they would program "pain in the ass m16 in Vietnam" into the gun for the game. The 1st ARs were the bad guns (no forward assist, no chrome lining/bolt and so on), the biggest problem with the 16a1 was not cleaning it and "old" stock powder problems.
A3/A4 full auto, 20" barrel, M4 3 round burst and 14" barrel. So with that said, if you want the game to be correct, both guns can't do the same things. Like, A3/A4 should have better shooting range (longer barrel) as well as full auto, but heavier because of the longer barrel (so running in the game should be less), m4 less range (shorter barrel), 3 round burst, but weighs less, you should be able to run longer. Side note, not sure what the game has as troops, but the marines use the A3/A4 and the army uses the M4, I believe.