Pressure rising- defect or bad execution?

[jester_s1]

I got a used Kingman bottomline regulator for my Imagine. I have it set at 400psi and running to the stock expansion chamber and ASA with the stock low pressure chamber. I've noticed that when I screw the tank in the gauge goes to 400, then creeps up to 430-440 in maybe 10 seconds or so. The pressure seems to hold steady while I am shooting (once I let the extra pressure out with the first shot or two) but it concerns me that I may be getting an extra hot shot now and then. Any ideas as to the cause? I was thinking either a bad regulator or the cool CO2 warming up in the xchamber.
I'm using a 20oz CO2 tank with an anti-siphon tube.

[Tabris17]

Co2 really isn't meant to have a guage read out for it, that's why. Co2 is pressurized to a certain output and that's what you're seeing, the co2 is expanding and raising the pressure as it goes from liquid to gas.
It sounds like that bottomline is meant for Nitrogen, not co2 and it could be damaging the gauge in turn should you ever want to use it for Nitrogen.

[Uziel Gal]

It could be due to the CO2 expanding further, or it could be due to the regulator seat seal being dirty, so that it cannot seal up properly - this will slowly allow air to pass through the regulator, so the pressure will slowly rise.

You can solve the regulator problem by cleaning or replacing the reg seat seal. If the problem is caused by the CO2 continuing to expand, your best bet is to replace the expansion chamber with a gas-through fore grip - it generally isn't a good idea to place an expansion chamber after a regulator, as it can cause exactly the problem you are having. The smaller volume of the foregrip would mean that there is less gas stored so the pressure build up would be reduced, as there is less gas to expand.