This was a bolt conversion. The receiver is registered but instead of altering any dimensions one the receiver, he milled the bolt down 1/4 down and made a custom buffer pilot in order to avoid drilling and welding the rear of the receiver. The advantage is no change to the receiver. The downside is that assembling it can be a bit difficult at times. It uses a 1928 bolt, but does use the blish lock systems. So it operates more like an M1 gun. The straight blow back also prevents issues that west hurleys have of misalignment. The gun has had about 500 rounds and no malfunctions. Extremely fast, probably 900 RPM but I prefer fast shooting guns If interested I can show how the conversion was done with some photos and videos.