Read the AR Primer sticky, it should be already typed up there. Briefly it's the gas system length, midlength is about 2" longer than carbine length and is ideal for a 16" barrel.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek
Last edited by Quentin; 04-22-2013 at 02:11 PM.