I have a 21 here and maybe we can help each other out as I have a question as well, but a different one.

Anyway, when I tore mine down, all I had to do was lock the bolt into battery before I inserted the action bar into the slot in the receiver. It seemed to pick it up just fine for me. Just shoved it in and racked it. I have yet to test fire it, but the hammer drops and it seems to function. I will tear the thing apart again and see if I can get more insight into what the problem might be.

But I have a question for you about your mag tube. I just got this 21 and it was rather rough. The little indents on the mag tube look to have been bubba'd. On yours, are these there to hold the mag spring in place? Or does the spring go all the way to the cap on the mag tube? I was told this train wreck had problems feeding and I am kind of wondering if the spring tension was on the low side thus not popping out shells properly.