headroom
I've built several guns from OLY kits and everything I've ever gotten was a complete rifle minus bolt carrier. All other components of the bolt carrier group were present, right down to the firing pin retaining pin and the gas rings. All you need is an AR15 bolt carrier and you're in business. If you don't have three or four of these laying around (Like, doesn't everybody?) You can pick one up from any of a number of advertisers in Shotgun News. If you don't have a sub to SGN, check out their website www.shotgunnews.com and you should find one there. starting prices are in the 65 to 70 dollar range.
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Your mind is your primary weapon.