Couple of thoughts: proof versus service load are two different things. So proof marks are often just indications, not gospel or science. Second, you should slug both barrels at each end. That is, drive a slightly oversize slug through each chamber and out an inch or two. Then run a slug all the way down each bore. You will probably find that the bores are tapered. About 2-4 thousandths. Tapered bore was almost standard in British BPEs. So see how your big bullet is starting out versus what it is exiting as.

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