from what I have seen, they will increase the acuracy in single shot, but not in full auto. some "tight bores" are not as tight as others. the average size of a BB will/should be 5.98mm, while a CA stock barrel is 6.08mm.
here are some inner dimentions of some after market barrels
star 6.05
KM 6.04
deep fire 6.04
systema 6.04
matrix 6.035
mad bull 6.03
prometheus 6.03
I thought you could get a 6.02, but I don't recall who made those.