The MKII Magazine Tweak
The top of a rimfire magazine has four "lips." The forward lips provide a feed ramp for the rear of the cartridge. The slanted vertical edge lifts the rim of the cartridge up and into alignment with the chamber. Do not mess with it.
The rear lips perform the function of restraining the next round and releasing it at the proper time. In new un-tweaked Ruger rimfire magazines, it can also cause delayed release of the feeding cartridge, and premature ejection of the spent shell casing. These two forces combine to bring about a stove pipe jam.
There is a tweak to be done on the Ruger MKII magazines, that will make every one of them reliable. After a bit of inspection, I realized that I had a "new" un-tweaked magazine in the mix. The rim of the spent casing was striking the left rear lip of this magazine and causing the stove pipes. I took the guilty magazine out of the rotation.
Tonight, while cleaning the pistol, I tweaked the mag. The forward corners of each rear magazine lip required rounding. I used a Dremel, and was careful to keep the rounded corners equal on each side, and the inward curvature of the lip itself unchanged. I cut the final curve with a jeweler's file. When finished, I burnished them.
This performed two functions. The feeding cartridge would have it's rear portion released earlier, allowing for a straighter feed as the bullet approaches the chamber. More importantly, the corner of the left rear feed lip would no longer eject the casing. The spent casing had to remain in the extractor's grip until the ejector itself dislodged it. I removed the firing pin from the pistol, and checked the feed, extraction and ejection with live ammo. Perfect.
Often these stove pipe jams are blamed on the Ruger MKII pistol. It certainly appears the pistol is at fault when every magazine, even brand new ones from Ruger exhibit the same problem. By tweaking the magazines, they perform flawlessly in the pistol with closer tolerances, and they still work reliably in other Ruger MKIIs too. Why Ruger has not addressed this issue and made the rear lips rounded, I do not know. All I know is that the tweaking works.