The former: full-power 125gr .357 in that gun—or any pint sized, 23-25 oz gun, IMHO—is brutal. The limit is my hands, not the gun. I mean, I’m sure that a steady diet of old-school 125 SJHP wouldn’t do the K6 any favors over the long haul, but I don’t think I could personally run enough of them to crater the gun.