I have a magazine for my M52-2 that suddenly is producing jams on the 4th round. The cartridge goes halfway into the chamber and sticks. I've taken the magazine apart and cleaned it, oiled liberally, and replaced the spring with a spare, still in the package, that I bought with the pistol several years ago. It still jams, and my other four magazines still function flawlessly. The offending mag has not been dropped, doesn't look bent, looks the same as the rest. What to check now?