I recently purchased an HK53 SBR built by a reputable HK smith. When I purchased it I was told that the locking piece in it was for the 62 grain bullets (I believe a #3) Seller said that I may have to change to a #15 but it still may function fine with the existing locking piece. I took it out and the gun wont cycle. I was shooting 55 grain WWB. Wont even eject the spent casings. Any thoughts?