· British 303 are head spaced on the rim – really a rim space, which is changed with the bolt head for one with a different (target) length – the rifle action is “failed cartridge safe” by design – the rim space limits are very generous by design and manufacture (with war time relaxations) to allow all grades of ammunition to chamber under all conditions. Tightening and "over head spacing" (rim spacing) is probable and this challenges the bolt achieving “lock”. · Bolt heads w