Wow...New Year's zombies. Problem solved, got 20-rd PMags that have a little extra space for extra compression. I slid the baseplate half off the straight mags to look inside when loaded, and saw that the spring was already fully compressed and couldn't go any more.
Just to clarify, the ONLY mags that I have ever had trouble seating and chambering when fully loaded are the aforementioned ones, and the only jam I've had in more than a year was from a cheesy black follower.