The M-4 is the carbine version of the M-16 in use by the military. It has the telescopic adjustable buttstock, and removable carry handle. A lot of them have the modular handguards to mount a foregrip and lights, etc.... The civilian model is still the AR-15 though.
Yeah, you're right, I've seen them too, but I think somehow they still have to have the AR designation somewhere in the model number to denote that it is non-military. I have a Bushmaster XM-15E2, which is basically a semi-auto M-16A2, but they had to get around calling it what it really is for the Brady Bill. (I know it's expired, but unfortunately I live in Commie NY, and they retained most if not all of the provisions of the Brady Bill).
Yes the M-4s are the carbine versions. They developed them during or after samolia. The soldiers were complaining that the m16 was not ideal for close quaters battle while sweeping houses in the city. So the m-4 was developed to fill this demand. Telescoping stock, shorter barrel, and possibly some picatiney rails also.
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