For critical safety information, on this round, refer to the article: https://support.leeprecision.net/en/knowledgebase/article/7-5-x-55-swiss. Those who find that discussion confusing are welcome to visit the Internet where the discussions are more complete and equally far more confusing. I must admit I remain concerningly confused! The important points are: The older-model guns have a slightly different chamber The older guns also work at significantly lower pressure The modern 7.5×55 Swiss chambering and associated Military loads have a seriously storied history. The standard Swiss WW-II Military load used a bullet that was so efficient the U. S. adopted it for use in its anti-aircraft and armor piercing 30-06 rounds. Then, it had to discontinue that loading after discovering the bullet was so efficient it would shoot off most, if not all, Military rifle ranges! This case was the progenitor of the 284 Winchester. Running 284 cases through the 7.5×55 die results in entirely functional 7.5×55 cases, despite the 0.027-inch difference in rim diameter. Ballistically, in the more modern rifles, the 7.5×55 comes close to duplicating 30-06 performance, which is no faint praise.