HK91 Locking Piece for Sound Suppression

In order to use a can on a PTR-91 or other G3/91 clone, apparently it's necessary to use a different locking piece that slows down the bolt carrier with the increased back-pressure.  Rim Country Manufacturing sells a #17, 36° part that fits the bill.  $50 ain't so bad.  They also have US-made trunions, bolt carriers, and bolt heads for MP5- and 53-type actions.