Don't reinvent the wheel. Even airsoft mechanics with tremendous experience don't make inner barrels. Those should be very straight lengthwise. If you turn it and work on it that way, it can be precise in diameter but it still can be wavy.
There is a lapping tool someone makes that polishes the barrel lengthwise, but it doesn't make much difference.
The high end inner barrels are very good and I don't think they can be improved in a workshop.
Also bear in mind that inner barrels don't contribute much to the accuracy. Even if you have a normal chinese gun, and change only the inner barrel, there won't be much improvement. If the original wasn't faulty (scratches or bent) then the difference is not noticeable. Precision barrels are one of the worst price/value upgrades.
When you get the gun, check the inner barrel if it's straight, and polish it maybe.