I tested six of the new JM 10 rounders Friday morning at the local range. I used my 196 series SS ranch rifle for the test. Only had time to put 20 rounds of Black Hills 55gr FMJ through each, but the mags loaded and fed fine.
My only complaint is that, unlike a Ruger mag, JMs will not drop free when released. They fit so snuggly that postive action is required to remove the mag. A couple were also hard to insert the first couple of times, but they do seem to loosen up a bit with use.
Would be nice if Ruger would make some 10 rounders, especially for us in the PRK, but, until that unlikely event, the JMs will do nicely.
BTW I do believe all the mags now at the John Mason website are "new and improved".