The build up is from the plastic wads, I have owned about 6 various makes 12 ga. o/u shotguns and they all did it. I currently have a Ruger Red Label O/U 20 Ga that I use for trap. I have the most problem in summer weather. If I use the bottom barrel the tang on the forearm gets so hot it comes close to burning my hand. Like I said before I shoot a lot of trap 50 shots (2 rounds) is a short day with 100 shots (4 rounds) being a normal day. I shoot 2 sometimes 3 days a week. No I' m not a pro, I average around 21 in most of my leagues.[8D]