Thanks 500 Nitro. The rifle is a pre-war Watson Bros. double which Kynoch ( and youself) believe was originally chambered 400/360P (Purdey). There are some 375 magnum flanged rounds in the case with the gun, however, the rifle really doesn't appear to want to close quite all the way with them in place and I really didn't want to slam it. I notice that Cartridges of the World specs. the 375 flanged magnum at 2.94 case length and 3.80 overall. Is it possible the the bullets need to be seated a bit deeper to create the 3.60 oal? I have not yet measured the bore but think that it's .375 .