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The McCormick PowerMags didn't solve the problem.
Discussed the issue with a 1911 guy that works at my local range. He obviously assumed I was doing something wrong (limp-wristing, etc.) and asked if he could shoot the gun. Sure enough, he had the same problem. He referred me to the range's gunsmith.
The gunsmith saw rough spots on the ramp where the bullet nose had been banging into it. He didn't know whether the rough spots were the cause, or the result, of the problem. But he polished things up, and went back to the range with me. Nope. Still had jams on the 8th round, with several different brands of magazine.
Puzzled, he was about to give up. But then he pulled the magazine out of his own 1911 which he carried IWB, and asked me to try it. The mag ran fine. We re-loaded and shot again. Fine. Did it again, fine. After running his mag several times without a problem, we looked to see how it was different from everything else we had tried. It turns out the follower on his mag was a solid base, plastic design where as the other types we had been using were what he called "leaf" designs.
He didn't even know what brand of mag his was. It had no company markings on it. He offered to give it to me for free, but in exchange I gave him the two new McCormick PowerMags I had just purchased. I didn't have any use for them if they didn't fix my problem.
So I went home and googled the images of 1911 mags and scrolled through until I found one that had a plastic, solid base follower that looked like the one he gave me. It was a Mec-Gar. I immediately went to the Mec-Gar website and ordered two of them. I was held up for a while due to the holidays and surgery, but yesterday I went to the range to test them.
I ran each of the two new Mec-Gar mags through several cycles, and there were no malfunctions. My Colt ran like a top! I'm very pleased.
However, I do continue to wonder why these two Colts (the previous one that I struggled with for two years, and this new one) were both having trouble, while my Ruger Commander will eat anything, regardless of mag type. Are they built with a difference in design, or what? This is something of a rhetorical question, don't really expect anybody to have an answer...unless maybe there is one.
Thanks for all the help!