Dye barrels, Furious Barrels, Lucky Barrels, Lurker Barrels, CP barrels are all good choices. Ive been using the DYE boomstick for a while and its very nice but it cost alot more than the others. Lucky, Lurker and CP can all be had for about $40 and work great. Ive used all of them and still own a lucky and lurker barrel. they are different bore sizes than my dye just incase i run smaller or bigger paint.
your best bet is to pick up a .685 lucky or cp 1pc barrel (or lurker .684). Chances are it will fit your paint much better and it wont hurt your wallet much.
link to lurker barrels
Be sure to grab some of his grease for only $7! Its good stuff and your g4 will love it. make sure to grease your bolt before you start playing and maybe even half way through the day if you plan on shooting more than a case of paint.