When the gun is closed it moves more up and down more than in and out. I can put .003 and .004 (which is a little tight) in the space between the barrels and standing breach.
When I put a .002 thick piece of oiled paper on the hook and put the gun back together it seems to lock up tight with no vertical movement of the barrels.
I originally thought it was the bolt but after doing the above suggestion from SKB it looks like it is the hook. I am a bit unsure about shimming the hook but I don't think it will do any harm.
