I had a GB-20 that was used as a test rifle for a large agency. It had somewhere close to 20,000 rounds through it and it still gave close to 2 inch groups (scoped) @ 100. Needless to say, the barrel throat was cooked after up to 1000 rounds in an afternoon. It might have done better with a better mount, but I always thought it shot better than it should have. By the way, it never needed a repair of any kind, just a good cleaning after use. We ended up buying a bunch of them. Those who neglected the piston/gas chamber cleaning, found that they had a single shot! jack