16" vs. 20"

[woodybrighton]

20 inch barrel was what standard military ammo was designed to be shot through.
running and weights should be your priority.
unless you get a good marine coach you may pick up bad habits (i.e non usmc approved habits) and that will lead to pain
out of basic you get some latitude in basic you won't

 

[Limeyfellow]

I personally prefer an 18 or 20 inch barrel over the 16 inch. The sights just fit me so much better in their position. I wouldn't have thought they would until I tested them all, but the 18 inch barrel is my preferred length on an ar-15.