The majority of testing has shown that a barrel has little to do with accuracy.
PunkWorks:
(summary link of all their barrel testing)
http://www.youtube.com/watch?v=197yZ859 ... re=related(vault of all their experiments)
http://www.techpb.com/forum/index.php?showforum=541Mech. Engineer on PBNation:
http://www.pbnation.com/showthread.php?t=2218018What is the most important factor in accuracy?
YOU. I can not stress this enough. Being confident and comfortable with your marker setup will provide the most drastic increase of eliminations you are able to make. I believe that paintball is 60% mental. When a player is confident other players pick up on it, and end up downplaying themselves. When a player is comfortable with his or her marker they usually know where they are shooting without really aiming.
Now, with that being said, Flasc has done some tests showing a larger bore is more accurate with an A5 platform:
http://www.youtube.com/watch?v=r7LMQLSfiDYhttp://www.youtube.com/watch?v=Tzum4cAFGDENow, his results couldn't be replicated by PunkWorks.
So you've basically got a few options:
1.) Go by the assumption that all barrels are just as accurate. Save your money for paint.
2.) Go by the assumption that all barrels are just as accurate. However, you wish to increase your air efficiency (ie. underbore). Some of your better options are:
a).
FlascBased out of Ingersoll, he allows local pickup. He sells a 9" control bore of .678 and that allow various tips. A good option
b). Freak Kit (ie. one back, front & insert)
Barrel KitFreak Kit (ie. full kit)
Full KitThe kit I have. I bought used so it was significantly cheaper.
c).
Lurker EigenbarrelConsidered one of the best underbore barrels
d). Freak bore your current barrel and buy an insert:
Super Staunchy Customse). Convince Homey J to get his lathe up and running so he can bore out your barrel for Freak inserts:
I think this is the best option

3). Go by the assumption that an overbore increases accuracy. You realize you will lose air efficiency, but feel that it will be worth it. Some of your better options are:
a).
FlascBased out of Ingersoll, he allows local pickup. He sells a 9" control bore of .696 and that allow various tips. A good option
b). Freak Kit (ie. one back, front & insert)
Barrel KitFreak Kit (ie. full kit)
Full KitThe kit I have. I bought used so it was significantly cheaper.
c).
Lapco Big ShotConsidered one of the best overbore barrels. Apex compatible.
d). Freak bore your current barrel and buy an insert:
Super Staunchy Customse). Convince Homey J to get his lathe up and running so he can bore out your barrel for Freak inserts:
I think this is the best option

Disclaimer:
I will used the word accuracy throughout even though, technically, it's precision we're talking about. Also, we're only discussing vertical precision for the most part. Everybody in the industry does this so I did also for this discussion.
Following some of the links further back will allow you to research ideas and draw your own conclusions. Cheers.