I've disassembled and clEaned my 20ga a400 dozens of times with never a problem. Today the action bar assembly comes to a dead stop half way into the receiver. Everything looks normal with the inside of the receiver and with the bolt. I've fiddled with the bolt release, removed the trigger group. Nothing helps. What's up?

Get the bolt arms properly lined up with the spring tube in the stock. I hold the gun vertically to accomplish this.

Recoil spring is probably hitting the magazine tube.

Yes, this happened to me once. It was just after I replaced the recoil spring. The spring was not properly centered in the plastic sleeve. The fix was as simple as pressing the spring evenly around the plastic sleeve so that the spring passed cleanly over the end of the magazine tube.

Got it fixed! Thanks, guys...you nailed it. The only thing I did differently today was to run a patch wrapped around a bore brush through the spring. Apparently, I disturbed it's normal position somehow, although that wasn't visually apparent. I put the spring assembly onto the magazine tube backwards just to see what would happen, and that seemed to fix the problem. Went right into position after that. Now I know!