Quote Originally Posted by tacticalpanda View Post
I was just removing move old barrel nut and all these sand like balls started to come out. Wondering if they are still caught in the action.
Those little balls are the steel ball used to peen the outside surface of the metal prior to blueing. They get caught up in the threads and the bluing salts make them stick there. Go over all your threads, inside and out with some solvent (I used Kroil) and a toothpick and pick them out one by one. They will keep the barrel from threading on if they are in the threads. Compressed air or canned air for cleaning computers may help, but in my case I have had to pick them out manually.

When you clean out the threads in the receiver and the old barrel, see if it will screw back in by hand. Then try the new one.

Also, if you have calipers, measure the outside diameter of the threads just to make sure it is not a large shank barrel. Stranger things have happened.