I bought a 20" upper last year directly from BM when they were on sale. I've only shot the Hornady FTX 250 gr. factory ammo, I've never reloaded. I have it mounted with the Leupold Ultimate Slam 2-7x33 scope with the Burris PEPR mount. When the magnification is set at the 7x, the markings on the reticle are accurate for the drop bullet drop (crosshairs zeroed at 100 yds, the first dot below shoots 200yds).
I've only shot it at the range so far and I've been very pleased with it, easily shooting groups touching at 100 yds (it makes big holes!). 200 yds is great too. Haven't shot further out than that, but it looks like the bullet has a significant drop past 250yds. I have yet to take a kill shot

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Check out
http://www.450bushmaster.net they're pretty informative over there.