Features include a 13-1/2" long accessory rail on top and hard mount points for additional M1913 rails on right and left sides of front handgrip. Bottom rail is integral. Contained within the upper receiver is a heavy weight bolt assembly with a longer length of travel than the stock M-11A1 upper recevier . This reduces the rate of fire (ROF) from 1,800 RPM with .380 ACP ammo down to around 650 or 700 RPM with 9mm ammo. Due to the reduced ROF and longer bolt travel, muzzle rise is reduced, the gun feels much smoother in operation and easily stays on target. The MAX-31k attaches to the stock M-11A1 lower receiver without any modification to the lower receiver and uses your existing retaining pin.
The picture above shows a MAX-31k with additional Lage Mfg. accessories that are not included with the MAX-31k upper receiver. Registered lower receiver is not included.