I own a .251 FLEX slug only shooter and a SuperFLEX .357 (and a WARP too) and if you have the barrel tension nut snug (not too tight or you will pull the circlip off at the breach) they are very accurate. A CF tube will not do much unless the breach is modified so that the sleeve on the barrel actually sockets into it. Just having the sleeve forward of the barrel index ring will not do much.

On my SuperFLEX I had Jim at WAR convert the circlip system to a two piece collet, then I can put a big tension on the barrel, like 20 foot-pounds. Makes it VERY rigid and upped the accuracy.
 
Thanks for all the info, I have a .308 flex and I'm trying to find a good slug and trying to figure out what's the highest fps I can shoot it, I ordered couple weight slugs from NSA ammo, my flex has a regulator, now that wicked air rifle is closing or sounds like jim is closing, where can i get parts for it, for example if I want to make mine a double bottle