I didn`t build one, but I bought a Rock River and now have 2 uppers for it. Whit the standard upper I fired some ammo I built with the cheapest varment bullets I could and shot a group that measured exactly .700 at 100 yrds. That was also with the cheapest powder I could find and the first load I tried. The loads are for coyote hunting and will be fired off hand so I never tried to get better. I do like Rock River.