I have bought mostly 5.52 H&N's for several different airguns, however upon checking with a PelletGage, I find that most are even larger then 'advertised' so I might recommend to you to check the diameter of the experimental pellets that you use to verify the size. There also are smaller diameter pellets within the 5.52 tin, so that would lead you to believe that the gun didn't like 5.52 when that would be a false reading.