Okay folks, YES this IS the scope you're looking for. Simply put, zeroing this optic and then laying in the prone at 500+ yards in my USMC service, I was hitting black every time. Now, yes, that has everything to do with marksmanship, however, this scope gave me the confidence and ability to effectively send rounds down range both quickly and accurately. I've used it on our M16A4's (personal preference) and the M4. So, yes, it works fine on both 16" and 20" barrels. Never forget which rounds you're sending down range, as this ultimately affects bullet drop. Personally, upon looking at other scope, both fixed power and variable, there just isn't anything that can match the quality of this scope, or Trijicon in general. It's small, shorter length profile I think is it's selling feature for me, as most other optics take up 2/3 the length of your rails. I will also second other comments and say YES, it holds its zero just fine after it's been bumped around. I've taken this baby on tours and hopping out of the vix and onto the dirt quick, fast and in a hurry, you definitely need something that's going to withstand a beating. This scope does just that. To summarize, if you're rocking an AR, carbine length or rifle, and are mainly shooting up to 500 yards or so, this scope will go to work for you and them some. Enjoy, Semper Fi!