I am hoping that you try various .577 BPE bullet weights, all lead, of course; not copper jacketed, as well as paper patched, as most .577 BPE bullets were, so we can learn what results you get with different bullets and charges you try. That rifle is pretty light weight, if I recall correctly, so it probably was regulated for the lighter weight BPE bullets. Hope, too, that you are using a chrono. to get velocity correct. Let us know what results you get, as we all love the .577.