If your talking about the kits that come from several parts companies, then the big difference is the weight of the barrels. The M4 profile barrel not only has the notch in front of the handgaurd, it is also cut down under the hand gaurs. Most of the CAR kits I've seen come with an HBAR barrel. There is a noticable difference in how heavy they feel.


So basically, not only do the M4 profile barrels look cool, they weigh less. This only applies to 16" carbine kits made with the HBAR.