The difference between the M4 and the CAR-15.
5.56mm, 1 in 7 twist, 11.5in BBL, 5.5 in Muzzle break/ Expansion chamber. Full Auto, collapsible stock, A1 pattern receiver. It was made for the then issued 55grn M193 Vietnam era bullet.
5.56mm, 1 in 9 twist, 14.5 in Hvy BBL, 1in birdcage Muzzle break, M203 profile bbl, 3rdn burst(Full auto on A1 SOPMOD), Collapsible stock, A2/A3 pattern rifle. Made to fire the heavier 62 to 77grn bullets. Also built as a modular weapons system.
Unfortunately the modern day press and supposed know it alls in the different media's have used the CAR-15 Moniker to much. Kind of like the use of words likeTactical, Assault Weapons Ban, Assault Rifle, Weapons of Mass Destruction, AK-47 for anything that looks like an AK-47, Riot Guns, etc.