if you look up the process of the making paintballs its hard to get it the right sizes
I would suggest if your gonna get a barrel save your money
and get a CP barrel (no specific bore size) for $25 and then work your way
to other barrels
My suggestion is CP barrel first, and if you really want into dive into matching your paint
then you can look into barrel kits, but do your research first.
1) J&J Ceramic barrel kits - great barrel kits, never had an issue with any of them, only probelm I find is that there is two kits, Also you can get the Apex 2 barrel adaptor that will allow you to use an apex on your barrel
the elite edge
(With 5 control bores (.676, .679, .682, 685, .688) and 10", 12", and 14" fronts.
http://www.rockstartactical.com/product_p/jj-edge-elt-a5.htm)and the edge kit
(http://www.pbreview.com/products/reviews/1197/
The Edge is a 2 Piece barrel that includes 5 backs: .682 .685 .688 .691 and .693.)
2)FLASC barrel kits, all depends on the package you get but I have heard great things from this kit
(http://www.flasc.com/pages/barrel.html)
3)Super Freak barrel kit - freak inserts and also comes with an apex
(http://www.ansgear.com/Empire_Super_Freak_Barrel_s/3374.htm)
(I dont condone how ansgear runs their business and i suggest staying away from them, hustle paintball will be alot better for your self)
4). CP barrel kit - great barrel cheap and works well
kit is a bit expensive but thats what happens when you get a kit but they are worth it
http://www.punisherspb.com/custom-products-full-bore-paintball-barrel-kit.aspx