Check your extractor is it marked 7.62 on it's face ? compare it to the .303 extractor ,if they are the same in up and down length ,the one marked 7.62 is not high enough to extract .308 ammo . See if you can find a 8mm extractor and see if that cures the failure to extract the shell and ripping the rim off the case . My SADF Bren barrel in .308 installed in my .303 MkII will not work with the extractor marked 7.62 , works 100% with the taller 8mm one . BRP had the 8mm extractors see if they still stock them .