I know a bit on this as I have done it, most worn/pitted 303/311-312 barrels will not clean up enough to go to 8mm, I had a light profile (SMLE type) & heavy barrel (MLM/MLE type) bored out to 375 but the 375 is VERY close to the sight holes & has to be keeped to low presure (I dont have the barrels as they can not be send to me now days) we have a poster on this site who rebore to 338/303 (a non Speeder round) & he likes it very much !

I like the look of some of the 9mm rimmed cartridges to !

Oh one barrel to look for & may be found shot/rusted out is a H profile heavy barrel that were made for the SMLE rifle, these have no rear sight holes & can be rebored out to what ever size you like, even up to 40cal, I was thinking of doing a 40/60 Maynard but may well go to 405Win as it will fit in my 1888 type mag .