You NEED an new barel, as the stock one is not made the same way.
You need a 500mm barrel. I used one from an M14, but the stop screws won't be in the notches on the barrel.
Still works perfectly. For AEG it is a problem, but not for that the PDI hop-up chamber.
I am using a M16 509mm 6.04 tightbore from DeepFire. So far, I get a perfect flat flight up to about 200'.