Ok, get your self comfy, and I'll try to explain as best I can, as short as I can
The basis of any accurate airsoft gun is a combination of consistent output power, and hop consistency. All the barrel needs to be is highly polished, straight and tight enough to hold the air propellant to give the BB its motive power. I'm going to assume you know and understand, basically at least, how a hop up works. When the BB is fired through the hop it begins its path down the barrel, the first couple of inches of which it vibrates violently in the barrel until the hop effect stabilizes it, when it has become stable the BB from that point on runs along the top of the barrel, but to help maintain that stability the BB needs an even cushion of air traveling around the remaining bit that isn't running on the barrel. Now imagine, if you will a 5.95(average) BB traveling down a 6.08, a 6.04 and now your 6.00 barrel in section with the BB touching the top and a thin crescent of air around it, on the 6.08 it would be well cushioned but we would expect a fair amount of leakage, and therefore waisted power, on the 6.04 the cushion would still be there maintaining stability, but a little bit more air is remaining behind the BB to help push it along, this is all good, now we have your 6.00 barrel and a roughly 5.95 BB, so as you can see not alot of cushioning going on, all it would take is a slight defect in the BB to cause it to drop out of its cushion, not a problem in the other barrels as there is some slack to help get the BB back on course, on the 6.00 though if the BB wavers a bit the next thing it hits into is the barrel side/ bottom, because there is next to no cushion, causing it to start rattling in the barrel at a point where it should actually be at its most stable. Most people are aware of the TK Twist barrels for AEGs, these take the air cushion a step further and put the cushion all round the BB this then provides the BB with the best possible flight path down the centre of the barrel meaning that the hop effect isn't scrubbed off by contact with the barrel roof, therefore improving both range and accuracy, the only down side is they are only designed for 1 Joule rifles. There is a theory in Japan at the moment that an 6.08 is infact the most accurate and you should only use less if you are struggling to get the required FPS. And just so you know I half agree, as I am currently using 1 in my SPR and it is as straight as my VSR and EBR which has a Systema 6.04 and Madbull black python 6.03 in it respectively, out to 50 metres, after that distance your BB is then in the lap of the gods, with all the variables that entails, and all the uber gear in the world is not going to change.
So is there a best size to get I here you cry, well no, is the honest answer the best average would be a 6.03/6.04, a 6.08 will get your BB down range but with less power behind it, and a 6.00 will improve your FPS and your short range groupings.
Just a quick word on ammo. The heavier your ammo the faster it will stabilize post hop and, as every player worth his salt will know, on through its flight path.
I think I've covered everything, but if your unclear about or need more info on it, just ask and I'll see if I can explain.
VA
EDIT - Just something I remembered reading and worth a look
http://infectedairsoft.wordpress.com/tech-info/how-to-choose-a-new-inner-barrel/
As he mentions, don't forget that these people that offer all this uber gear are firstly in it to make money and secondly to satisfy customer needs. Just because they write a load of BS saying it will do wonders for your accuracy/constancy doesn't mean its true, and only with the proper knowledge can you then make an informed choice. It sadly looks like you have been stung my friend, put it down to experience and move on
2nd EDIT something else to read from guys that know their shizzle
http://forums.airsoftmechanics.com/index.php?topic=891.0
