Well the AR15 to my knowledge is just a civilian version of the M16. The M4 is a shortened carbine of the M16...but there is no MG or sniper variation to my knowledge from the M16. Nothing in wide use that I know about. Most of the newer rifles are a family of variations from the start like the G36, the XM8 prototypes (canned apparently), Type 95, etc.
The Ar-15 is missing some things the m16 has. I think it is calssified as rifle so it can be sold legally, while the M16 is an assault rifle so only military and law enforcement can use them. As far as a sniper version, you can Get a M16A4 with a modified barrel, attached scope, 3lb 2 stage trigger, and bipod, is as close to "sniper" as it comes for a M16, and its freakin close and looks sexy. The M16A1 was full auto, but the military realized that after 3 rounds you are missing because of the recoil
The M16 and M4 are both chambered for 5.56. Ballistically speaking the M4s muzzle velocity is slightly slower and it's slightly less accurate. Practically the M4 jams quadruple from the M16 in the field
Less accurate? the only thing is it is harder to maintain a steady aim with a shorter weapon, the distance from the front to rear sight post magnifies small movements, so the longer the distance the more you can see how much your aim is shifting. Personally I scored higher on rifle qual with a M4 than I ever did with a M16
As for jamming more, not really, it has the same upper and lower receiver as a M16, just a different but stock and barrel. Maybe just soldiers not taking care of their weapons. The only jamming problems Ive had were caused by the mag double feeding or failing in some other way.