In my limited experience of Flatline barrels they are quite fussy about what paint they shoot, I guess the extra strain put on the shell in order to get it to spin has some effect.
You should be OK with most of the winter paints available, such as Draxxus Frostbite, though as the Flatline increases your range the velocity and therefore the force of impact drops off, so the tougher shell may not break on target as easily.
Its hard to predict what paints will be what calibre, different batches of the same paint vary. If your buying paint on a site maybe you can borrow a few balls of different brands/types form others and shoot them over the chrono range to determine what works bes for you. Then go get a box of what's best.