Is the gun new or used ? I had the same problem in my youth 20 gauge, but I bought it used. After a very thourough cleaning it cleared up. I had to really scrub the chamber to get it to come clean. Seemed that the more I scrubed, the more gunk came out. A lot of guys have polished their chambers to clear this problem up. I haven't done this so I'll let someone else give you directions. Either way good luck, and congradulations on your son becoming a shooter.