Oh how I wish there was such a thing as 0.266" jacketed bullets! One can get them as hard-lead cast bullets, but up to now I have had no success with them.
When I get around to it - my present excuse is that I have 2 competitions with the M1917 in the next two weeks, so let's say July sometime - I'm going to take a few 264s and turn off the base of the jacket in the lathe, to see if this produces any improvement by (so my theory) allowing a touch of obturation of the base.
Patrick