Not a fan at all - FM Products Glock Style 9mm 5 inch AR Rear Charging Builders Kits

Extremely difficult to swap out muzzle devices. It has non-standard threads and the barrel is lose and just continues to spin, so you have to remove the barrel and clamp it down to swap muzzle devices. Since the threads are out of the norm, muzzle devices are limited unless you get an thread adapter.