The 6.01 is not always better, there are a few people who can explain this better than me, but it has to do with the tolerances inside the barrel being too tight. If I were you, unless I needed the barrel right away I would go ahead and wait for the 6.03.
barrel length will also be an issue, if your using a spring gun, because 650mm is a long barrel especially for a spring gun. From what I've heard the 6.01/6.03 debate, depends on which company your buying from. company A is supposed to have better 6.01 barrels than 6.03; while company B has better 6.03 barrels than 6.01 barrels. Though I don't have the money to test this theory myself. Your best bet though is to research your gun.
When an bb travels through a 6.01 barrel, it has to push the air in the barrel forward, thus giving it less fps, also it sucks in air behind the bb creating a sort of turbulence which causes it to be less accurate.
If a bb travels through, let's say, a 6.03 barrel, the air that pushes the bb forward also pushes the air in front of the bb forward, giving the bb less resistance from the air it has to push forward because there is very little left. It won't have to suck in air behind the bb aswell because there is a space between the bb and the barrel (this space should be 0.08).
You should check what size your bb's are, just because it says 6 mm doesn't mean it's 6 mm.
Most companies make bb's that are 5.95-5.97mm. So a 5.95 bb would need a 6.03 barrel to get the best performance. A 5.97 bb would need a 6.05 barrel etc.
I read this somewhere but can't remember where, if there is someone who knows what I'm talking about but can explain it better, please post it to make life easier
What PVT13 is speaking of is called "Barrel Suck" which has already been proven to be a myth. It is most commonly associated with an AEG. (I will provide Documentation if you wish but it has been pretty widely known to be false) So don't even worry about turbulence and pressure in the barrel. The BB moves so quickly out of the barrel then the real forces take hold. The real thing that truly affects Accuracy is Hop-Up.
The reason you may want to use a TBB is to raise your FPS... but that's about it.
Ya, barrel suck doesnt exist. Besides the effect is only worrying to the AEG crowd anyway. And they think that its volume of the barrel, and speed of the piston working backwards to create the suck. Its not possible with a Bolt Action gun.
And besides, This gun that Im working on is my gas gun.
Lastly, I ended up getting the 6.01 madbul ultimate TBB.
Its not a Black Python, but its pretty cool. 45$ out the door, so not a bad price.
The hop up I ended up with untill I can find another firefly, is a Kind Arms purple 80 hard bucking. Additionally I put a H spacer in it, so. . . time will tell. I work days, and that doesnt leave me alot of time to try to figure out the way of the Hop up.
Good to know it's not true. The guy who wrote it was quite detailed about how it worked and wrote all these formulas for effective range, fps and all kinds of other stuff. So it was pretty easy to be convinced that it works like that, specially when your rather new to airsoft.
Actually, from my understanding, the phenomenon pvt13 is talking about is something completely different from barrel suck. Barrel suck was thought to be an issue arising from the piston of an AEG sucking air back in to the cylinder before the bb has time to reach the end of the barrel; what most people had failed to take into account is that with any barrel length available on the market, the bb simply leaves the barrel before barrel suck could become a factor. HOWEVER, in-barrel turbulence is a very real factor that is detrimental to both muzzle velocity and accuracy that arises from imperfections in the surface of the inner barrel.
Basically a combination of what Mojo and pvt13 have posted is true, the turbulence thing very much exists, barrel suck also exists but is purely a small vacuum pressure exerted on the bb when the piston is at rest.
The turbulence created infront of the bb also exists, which is why ported barrels were invented to get around this problem, but as already discussed above can be alleviated by using a larger bore.
A shorter barrel is also more beneficial as it a) is not going to be subject to operator movement for as long as if contained in a longer one, and b) it is more beneficial in regards barrel volume vs cylinder volume vs bb overall weight.... a larger volume of air will shift a heavier bb easier down a shorter barrel