Thank you for the help everyone. I figured it out.
It was that tab and slot that were hanging up. I carefully filed each side of the tab two or three times, probably taking off a couple thou at most, cleaned it up, and put a very small amount of oil on it: just a light glaze, being sure to wipe off the inside surface. When I tried to put the barrel back on, it was still tight but didn't feel quite like such a hard metal-to-metal stop as before. A few taps on the end of the barrel with a rubber mallet and everything eventually slid right into place!
Action works great by hand, and everything seems great. Now to take it to the range and break it in!
Thanks, again, everyone. Great community, here.