If the m16 is great, upgrade the m4. If you really want to get a new gun... Just find a gun that looks cool...giving it a high ROF and range + accuracy can be done easily afterwards.
So just pick something that appeals to you. Maybe go to a few games and hold a few guns, see what feels comfortable. A lot of people say AKs are the least ergonomic guns they've ever held... I loved mine, super comfy and easy to wield. Everyone has different tastes, figure out what yours are.
Not for glory, nor riches, nor honours, but for freedom alone, which no good man gives up except with his life.