I recently picked up a Colt 6920 that on both the outside packaging and on the lower receiver it says M4 Carbine. Civillians can't walk into a gun shop and buy a true M4, only M4 style...flat top upper, collapsable stock, barrel cut for mounting a M203 launcher. M4 is really just a marketing thing.
Back when I got into shooting it was explained to me like this...they are all AR15's, some are rifles, some are carbines. The lower receivers internally are all the same, the only difference between them is the buttstocks/buffer tube. When you buy a civillian parts kit for the lower, there is no difference between AR-15 or M4 style, they are all the same. Once upon a time it did matter if the lower was preban or postban as that affected your choice of buttstock and upper receiver features. For a while Colt was using a different size for the pins that held the upper and lower together. Some are mil-spec, some are not.
Functionally they are the same, one shot per trigger pull, they take the same magazines. For some folks having one rollmarked as M4, LE Only, or whatever is desirable and they are willing to pay extra for that.
Last edited by cryogenic419; February 2, 2013 at 10:18 AM.