I suppose they are tested to a certain level which they can be shown to be within safe limits. Certainly they are built well and even re-proof o.k. after having 15 thou. skimmed out on occasion. It suppose risking higher power ammo than intended is a bit like jacking up the 5 ton truck with a jack tested safe for up to 3 ton trucks and then crawling underneath. You would probably be safe 'most of the time' but I personally wouldn't risk it. Lagopus.....