I also ventured into Promag with a 10rd mag for the Taurus PT709. Upon reading as much as I could about it, many people say that they don't load/cycle with 10rds in the mag and sometimes even anything over 7rds (in the 10rd mag).
Upon receiving mine, I plugged it in and man it looks funny (not quite the same as the OP as the PT709 10rd mag is just straight plastic, no grip or anything like that, but still just funny looking). I filled them both up and then attempted to take the rounds back out of the mag by "simulating" the slide pushing them out and the cases catch on the lip of the mag until about 8ish rounds are left. Then the spring/follower is able to push all of the bullets tips up. At 10 rounds, the last couple bullets almost lay horizontal. Which would explain the feeding issue. I have left them loaded to compress the springs a little bit and see if that helps. If not, I plan to file the front lip of the mag to [hopefully] allow the brass cases to slide a little better. If that STILL doesn't work, I'll have to file down the front lip of the mag to make it so the brass has no where to catch. The feed ramp should be able to push the nose of the brass up into the chamber from there.
Just some food for thought to anyone who is looking at Promag. I'm not sure if it's the overall design, or poor engineering on Promag's part as to why these 10rd mags don't work off the bat, but to be able to have a few more rounds to shoot at the range without reloading, I'll settle for a little tweaking to make em work.
