I would still recommend buying a full freak kit, it will come with a front, a back and 8 inserts from .679 - .695. If you like you can get a shorter front to give you a choice of a longer or shorter overall barrel length. They are also pretty quiet and look good too, I mean who does not like the classic all American front with the helix porting!!
The complete kit can be had for around £150ish new which when you look at other barrel systems is very cheap indeed. To get something similar from DYE for example the UL with a selection of backs would potentially cost you 3X that amount. Also I think that the Empire super freak kits are still about and they come with all of the above an Apex attachment as well which were fairly popular with the scenario boys a few yews back, although I cannot see you ever using it in sup air/walk on play.
As has been previously stated the study that was done on barrels a few years back, to cut a long story short, found paint to barrel match was the defining factor with accuracy for paint markers aside from paint quality, End of story.
Optimum barrel lengths were stated at 12'' - 16'' anything longer and velocity increases were nesessary to maintain the same muzzle velocity as a 12'' - 16'' barrel. That's fine but then your marker uses more air (less efficient) and more stress is placed on the paint ball at point of firing (more likely to break in the barrel)
Ultimately barrel length is a personal thing but using a barrel that's is too long, is unwieldy and ultimately pointless as no discernible advantage can be gained IMHO.