From what I've read, 6.03 is a safer bet than 6.01. A 6.03 has enough "wiggle room" to safely fire a slightly flawed bb; that same bb will can leave a scratch down the length of your 6.01.
For what it's worth, a 6.03 inner barrel is usually the first (and often the only) upgrade I put in my AEGs. It's never let me down.
I don't know what the stock inner barrel length is for your gun. If you're going beyond the stock length then you may want to put in a different piston. A longer barrel may require more air power than the stock piston provides. I don't know what your needs are; that's just a thought.
If you're looking for range, you may also consider a "R-Hop" or a "Flat Hop" in place of your stock hop up system. You can google those terms to read more.
My understanding is some KWA guns use a proprietary hop up unit. If that's the case then you may need some mods or a different unit altogether. I'll yield the floor to more experienced KWA users . . .
On a sidenote, if the gun is not physically restricted to semiautomatic fire then you may also consider making that modification as well. Some events (such as American MilSim) require DMR replicas to be physically unable to fire fully automatically. It all depends where you want to use it.
You can call So Go. The techs there can give you better guidance than me.
Good luck with your new baby!
