My 1903 tumbles all .264 bullets under 160gr, and is only "acceptable" with .264 160s. Then I read my 1950s vintage NRA "Reloader's Guide" wherein the groove diameter for M/S 6.5 rifles is listed as .268". I got some 160gr Hornady's that measure .268", and my so-so carbine became a tackdriver. Curl, your pristine bore slugging .2675" confirms my experience. BTW, when loading .268" bullets, drop the powder charge 5 gr and work up again.