They call me Tom

Well, I'm sure I'm not the only one, I saw the FAR and had to build my own. Unlike most people, I have access to a lathe, which makes it all the easier. I have run into a rather large problem, I'm not getting anywhere near the ranges I should be.
I have a brass barrel, and brass inserts in my shells, the caliber is micro. I'm not losing air pressure or volume, I've filled the cylinder with water to see leaks around the piston seal, there is none. All these lead to one part - the spring.
I bought my AR15 action spring online, but I cannot believe it would be so defective; I'm getting less then 20' of range.
I've tried loading rounds in the barrel directly, in case it was getting hung up on the shell to barrel lip,there was no change. I've tried thinning the rounds, to reduce friction, no change.
Any help?