European proofhouses don't test guns only, they are constantly testing ammunition too. Before an ammunition manufacturer or importer releases a new batch of ammo, he has to send samples to the proofhouse. There it is pressure tested. If it is not within the maximum pressure established for that cartridge, it cannot be sold to the public. So some American factory loads are not available in Europe, because they exceed CIP pressure limits. Often this prescribed testing is not worthwhile for the importers, because the small numbers that may be sold don't justify the expenses to have the ammo tested and approved. Further, if an ammo maker changes any component, the new lot has to be reprooved.