I got a good shotgun reloader (better than my first) and ran more rounds through it and still had some hangups even with the reloader sizing them to "factory fresh" sizes. So I regrouped and re-examined the magazine.
After doing some research, it appears that some of the magazines will draw up a little and become too narrow to feed rounds. This seems to be "the" issue that people face with the Promag magazines.
Inspired by the "cork and fork" method of resizing the follower (by stuffing a cork inside the follower and submerging it in a cup of boiling water), I set about to correct what appears to be too narrow a magazine for my loads. It was REALLY SIMPLE to fix!
Here's how I fixed it:
1. Secure the bottom of the magazine in the vise - carefully - squeezing against the front and back.
2. I dug around in my garage and discovered one of my screwdriver handles is slightly larger than the inside of the magazine.
3. I inserted this into the the magazine and slid it to the point where I could detect the narrow section.
4. I heated up the propane torch and gently, patiently, in rapid passes, heated the plastic material on both sides to soften it. Took probably 5 minutes to heat it up, maybe longer. Seeing as how I was applying flame to plastic, I was VERY CAREFUL to avoid burning or melting the plastic! A heat gun would be infinitely better than using a torch. It is simply too easy to burn the plastic, but I didn't have a heat gun available at the time.
5. After I was satisfied that the plastic softened enough, I let it cool down and removed the screwdriver.
PRESTO! The rounds that previously hung up in the middle now fall through the empty magazine body like it isn't even there! No sanding, no scraping mold flashing, none of that.
After testing it a few times at the range, even in really cold weather (there was snow on the ground) the plastic didn't shrink or narrow at all. I now have 100% consistent feeding out of all of my reloads, even the ones that are slightly puckered (thanks to my unperfected shotshell reloading skills). I have yet to heat the follower in the same way but I'm confident that the higher heat from the propane torch vs. boiling water.
Hopefully this will help someone who has a magazine that isn't feeding properly!
FYI I never got those deformed hulls to round back out - I just shot them all through my SPAS after all