Built a lower from M16 parts and aftermarket receiver. Shot it with a AR15 upper. Today I purchased a 300 Blackout upper from Barnes. Barnes Precision Machine | USA Owned, USA made rifles, parts, and accessories. Pictures and review when it arrives. Also shot a Barnes AR15, slick, has most milspec visiable items. Bolt Carrier Asb. easy to clean due to tough and slick coating. Runs up to 70 grain Boolits! YMMV