Right, here goes!
The best way to check that the barrel that you are using is ok is to make sure that you have fresh, quality paint. Then make sure that the paint can be blown through the barrel but will not roll out.
Make sure the barrel is really clean and that te o rings on the inside of the freak back are not worn (this will make the insert sit lopsided). Put the insert in so that the bore size can be read through the window and then chrono your gun so that it is nice and consistent.
If there are still problems then you may want to change the barrel.