Don't listen to Spyder_Dude. I used to own a Flatline Barrel System myself, but after a few times or firing it, it was unmatched in the distance it shot (That's how they make it seem really good), but it was quite inaccurate. I hear otherwise that it is good, but it didn't really work so hot for me. Luckilly I was able to get a money-back refund on it and get a 'Cocker.
If you want to know about what barrel to buy, I heard nothing but good things from almost everyone I know that uses an Armson Stealth Barrel (5 people)