The Foxtrot Mike Products .45 ACP Upper Receiver comes fully assembled and ready to drop on any pistol caliber lower receiver that uses Glock(R)magazines. With its full-length Picatinny top rail and M-LOK compatible handguard, the Foxtrot Mike .45 ACP Upper offers plenty of mounting space for optics and other accessories. A unique interlocking connector between the upper receiver and handguard ensures the barrel nut will not work loose over time. Premium Melonite-finished 4150 carbone steel Button-rifled bore, 1-10" twist Ambidextrous, non-reciprocating forward charging handle Bolt/carrier unit with hard Melonite finish, tooDoes NOT work on the BRN-180 lowersEngineered to function reliably, no matter which manufacturers' lower it's mated with, the Foxtrot Mike .45 ACP Upper Receiver is forgiving enough to cycle even the most economical steel or brass cased ammo.Foxtrot Mike Products FM-45 Upper Receivers are not compatible with Korean magazines or aluminum cased ammunition. FM Products also does not support the use of Stern Defense .45ACP Conversion Adapter as it requires adjustment to work properly.