I wouldn't blame it on the barrel because that has nothing to with it changing from semi to auto firing. I would say your tanks reg doesn't have a good recharge rate. Go find another marker that shoots at a high ROF and try it. If it's a spacegun it may not do it because the operating pressure is so low. May I suggest replacing it with a Myth reg? They are very cheap, small, light, high quality regs. Best reg out there IMHO.