what is the differance

[remshooter]

saw 2 brand new Colt AR-15's at gun store ,both looked just alike ,,one is marked Colt AR-15A3 the other marked Colt M-4 sorry for my ignorance ,carried a M-16-A1 in Army 30 years ago ,just now getting my interest back in Ar's...P.S these are civillian models ,i do know the military M-4 has a 14 inch barrel compared to civ.legel 16 inch barrel

[themighty9mm]

The ar15-a4, will have a 20 inch barrel, rifle length gas system, and the A2 stock. The M4 wil have a 16 inch barrel, carbine length gas system, and collapsible stock

[remshooter]

thank you my fault it should say A3 I think ,they were both carbines

[themighty9mm]

I'll take a guess at the model then. I'll assume one is a 6920, and the other is the 6721. So the difference will be barrel profile if that is the case. The 6920 will have a m203 cut in the barrel, where as the 6721 will not. http://www.colt.com/Catalog/Law-Enforcement/Products/Colt-Tactical-Carbine#98017-features--models.