Read through this thread, with attention to russc's posts in particular: Best FSR Barrels, Barrel Lengths and Bores?
to be frank, given its history of use as part of a larger marketing strategy in the paintball market (among others), I tend to find research/reports endorsed/involving/commissioned by companies that benefit from them to be suspect. not saying the work by hammerhead and CarmatechEng is b.s., just that as has been said, until it can be repeated and wider research by multiple parties has been done I'm just not inclined to support such a blanket conclusion. not saying I'm not excited about the potential of using a 20" long barrel that will end up being more effective than a 16" barrel, that would be fricking awesome and meet my needs perfectly. the conclusion about what barrel works best with fsr are far from concrete at this time, that's all