Bushmaster is also using these carriers; as are other manufacturers who have lawyers on retainer & who would not risk a massive NFA violation for such installations. Its a non-issue. If you are still worried & think that you know better than Colt, Bushmaster, etc, its possible to pay a machinist enough to modify your carrier to prior Colt specifications & have an AR block installed in the lower. Just to be on the safe side, ya know. Can never be too careful!