"I think he means the carbine version of the M16A1. It's basically the same as M4 in shape but has a 1:12" twist non-heavy barrel, A1 pistol, Old style carry handle and rear sites (non-flattop), A1 flash hider and thinner handguards. "
This describes at least five variants, and three subvariants. Call it a 610/XM177. They were the most common.
For sheer trivia, the "CAR-15" was/is the proposed "survival carbine" version. only 10 were made. Had a 10" barrel,conical flashhider (Think M3 SMG hider.) noncollapsing tube buttstock (Basically a buttplate on the buffertube) and a cut down pistolgrip.
Saying "CAR 15" when you mean a "Model 610", "GAU-5" or "XM177" is akin to calling a Model 19 S&W "One-o-dem cowboy gunz".