It's not the bolt lugs which are the problem: it's the bolt lug recesses in the receiver. When the magazine well on a standard length Model 98 Mauser action, or for that matter on a pre-64 Model 70 Winchester action, is opened up fore and aft enough to accomodate the Rigby cartridge and to adapt the feed ramp to the larger cartridge, metal is taken the receiver off directly behind the lower locking lug recess, to the point that pressure on the lower lug can distort the recess, causing, as Paul Chapman says, headspace problems and stringing.