I have related this experience before, decades back I had a Westley droplock .375 H&H belted mag, peepsight in the rib, super rifle, cased, mint, extra locks, the works. One of the finest rifles I have ever owned. I and Dick Vogt, who long ago wrote an article for Gun Digest on loading for doubles, tried to get it to shoot. We tried about everything - it did well with Kynoch factory loads, you could still get some, but no luck with American brass, powder, or bullets. Finally halfway decent groups were obtained but with such heavy loads of 4350 that there were severe extraction and ejection problems. I sold it for about $3000 - you can estimate from that how long ago this was. Then to our absolute dumbfounded fury the buyer wrote us some weeks later that he had had no trouble getting it to stay in 3" with handloads !! And perfect ejection. We were so mad we never quizzed him about the load. Turned me off rimless cases for doubles forever.