When the action is closed, look to see if you can see light through the joint at the barrel and standing breech. If the joint has a bigger gap at the top than near the middle, you need to remove a small amount of material from the barrel pivot on the underlug. I use an 11/32" drill bit wrapped with fine crocus cloth so the pivot radius doesn't change, just gets deeper. Work slowly keeping the pivot surface square to the barrel and check fit frequently until it locks up tight, but solidly without slamming it closed. Pick up a cheap set of automotive feeler guage blades, open the action and place the .001" blade across the standing breech above the firing pin and close the action, when the action is closed, you should be able to pull the blade out with some friction, a .002" blade should be tight. Last thing to check is the latch engagement on the locking shelf on the back underside of the barrel. Clean, then smoke the shelf with a candle then close the action, then open the action, the latch will leave a mark in the smoke and should be as wide and deep as the latch tip. You can hone the top of the latch shelf to help engagement if the barrel is tight to the breech but still doesn't lock easily. Don't do any work on the latch on the frame or your other barrel won't fit right, just work on the barrel latch shelf itself, I used a ceramic stone to just touch up the shelf surface.
If the barrel to standing breech joint is tight as checked with the feeler guage blades, I would just check the locking shelf engagement and hone the shelf surface to allow easier engagement.
Fred M has a fair graphic of the handi action is you would like to see what parts are what...
http://www.angelfire.com/ma/ZERMEL/tweakultra.htmlI would highly recommend that if you don't feel comfortable fitting a barrel, just take it to a gunsmith to have it fitted, this DIY process isn't for everyone.
