The good, the bad, and the ugly - Bushmaster .450 Bushmaster Upper Assembly w/5-Round Magazine

I would like to first address a few technical notes. The two 91700 uppers I received in August 2018 arrived with .750in low profile, dimpled gas blocks, NOT the miniature Picatinny block featured on most websites. As mentioned in other reviews, the handguard is a stout 7 inches long, with light to medium knurl, no heat dissipation slots, and a sling mount installed that would accept any clip on style sling, or a one inch wide sling through the flat loop. I find it frustrating and confusing that retailers- let alone Bushmaster- seem content to let the buyer guess what will actually show up at their doorstep, but I digress.. On to the review.

The first upper receiver I got had an obviously loose handguard. I planned to replace it anyway, but the issue was that the handguard was Loctited to the barrel nut, meaning the barrel nut was not torqued to spec and the barrel was subsequently loose as well. I pulled the front gas block and tube, and the barrel nut was 1/8 turn from even being hand tight. As the wheels in my head began turning, I pulled the BCG out to examine, finding heavy hammer drag marks and bolt face fouling. Having no idea how Bushmaster test fires their uppers and BCGs, and not feeling comfortable moving forward with an upper that clearly missed the final inspection station at the factory, I called Optics Planet.

OP was very sympathetic and helpful in issuing an RMA for exchange, and the waiting began.

The second upper I received was properly assembled, and mated to my Anderson lower without issue. The BCG exhibited the same fouled face and hammer drag marks, so that must be business as usual. Chemical treatments on the receiver and barrel appeared high quality, but there were sharp edges and gritty feeling to some of the metal underneath. Good enough. I installed a Doublestar 12in handguard, Atibal Mirage, grabbed some 250gr Hornady Black and headed to the range.

The included magazine was very bad. The feed lips were so sharp and jagged, they actually carved notches in the brass. Not scuffs or scratches, grooves. Standard buffer and spring combo failed to properly pick up and chamber a round from a full mag down to a single round. It did manage to strip the last round and return to battery. It did eject brass properly, and the action stayed open on an empty mag. The final area of redemption was the accuracy portion of testing. At 100yds, I was able to zero in four shots, and managed to group right at 1 MOA.

I filed the feed lips, and am able to manually cycle a full magazine now. Hopefully live fire will produce the same result. Ultimately I would recommend this upper, with the caveat that the user be somewhat mechanical and exceedingly patient. Hopefully it will run well and smooth out with use. Thanks for reading. Shoot straight, shoot safe!