Been too long

If heat doesn't work, use an impact screwdriver.

Thanks speedy, that worked. I have an old manual impact, the spring loaded type you hit with a hammer. No corrosion, but it had a dab of blue loctite on it so the soldering iron probably would've worked as well.
Some ape screwed the buffer tube on so tight I had to struggle with that, too. After a search for that ape I found him in the bathroom mirror.