I built mine with an 18" barrel and it works pretty damn well. But since I have it set up with only a red dot on it right now, I wouldn't even bother to try 500. It's quite accurate, easily sub MOA, but not a long range round because of the velocity.

Here's the barrel I used http://www.precisionreflex.com/Detai...99636&CAT=4293 but it's been on BO for months now.