Nice gun Sarg. - Nakihunter has the scoop as I'd go. Firelapping first, then try a variety of bullets, even 220gr. .308's, as well as 150 through 215gr. .312's and possibly .313's and if these fail, only if absolutely necessary, a re-bore/re-cut to 8mm on the .303 case. the ballistics will be quite favourable, just slightly better than the original parent .303, but easy to duplicate .303 to regulate to the sigths. The muzzle is a bit slim for anything much bigger for my tastes, however the .303 case does very well right to .400 cal. I'd stick to 8mm. Might even have it bores to a .321" groove diameter just for shooting 200gr.+ cast bullets of .323" diameter. For me, a 240gr. Cast bullet in .323" at about 1,900fps to 2,000fps just sounds like fun.