Historically BSA have a rep for setting reg pressure excessively low, which kills efficiency. Typically I'd expect between 50 and 70% of the manufacturer-quoted shot capacity from an untouched R10; however when properly setup they will do the quoted numbers.
While (in the case of the R10 at least) the regs are shite this is fundamentally a setup issue, rather than a reg problem.
I've no experience of the reg in the R12 so can't comment on them specifically, but it wouldn't surprise me if BSA are continuing down the same path of insufficient reg pressure.. which, assuming the reg's serviceable should be able to be addressed with a good setup.