My dad was having problems with his B2000 20 guage ejecting the spent shell and having to manually extract it. I figured it needed the gas piston cleaned, so I downloaded an owners manual and stripped it down, cleaned the piston parts, and put it back together. We tried it out over the weekend and it's still not right. It will kick the shell out, but stove-pipes the empty. I took the barrel off and checked the gas ports and they were clear, but haven't taken the piston down again, to check if I didn't get it back together correctly. Any ideas?