Thanks for the ideas. I do have a piston ring compressor but it's too big. I ended up cutting some thin shim stock and using it like an expanding tube inside the barrel. I butted it right up against where the finished barrel size begins so it acted like a ramp. Went in fairly easy but then I discovered you can't insert the rod into the barrel if the check valve is still attached, duh. So I had to take off the check valve to get it pushed in all the way and back on the machine.
I'm eating lunch before I test it, I don't want to find out it leaks and ruin my lunch.