I have a A391 that I have shot hundreds of times without trouble. It has been sitting in the safe for about 4 years. Prior to storage it was cleaned and reassembled. I took it duck hunting this past weekend and it would not close the bolt all the way after shooting. I knew that cleanliness was not the problem, but was not sure of what it was. When I got home, I attempted to break the gun down and found that the o-ring on the piston is either swollen and causing the piston to stick or some how, I installed the wrong o-ring back in the gun. I tend to think that the o-ring possibly swelled from sitting to long. Have any of you had anything like this happen? If so, any advice would be appreciated. I ordered a new o-ring and am waiting on it now.