Nikon builds a short 3x scope specifically for AR platforms.

I'd likewise encourage you to look at the Redfield 2x7x33 Revolution with their Accu-Range reticle. Built in Oregon by parent company Leupold, lifetime warranty. I just put one on my 45-70 Guidegun, it's a GREAT compact scope, good American made quality & at $175-$200 it's easy on the wallet!