Just a little bit of info in case anyone stumbles across this down the road. I tried to the factory spring with .04" worth of shims and that was enough to push the regulator wide open and I blew a burst disk. However, .02" shim was stable for a few days that I had the tank set up like that. So anyone wanting to set this up for a low power plinker/pester, you're gonna be lucky to get much over 800-900 psi with the factory spring.