I flew gatwick to aberdeen over summer with a full setup (inc paint).
I had to check it with security over the phone and it was marker on the BA system that i had a sports firearm in my hold bag.
That was it, no extra check and i didn't need to unscrew the reg. i did take batteries out of my marker as good practice. BTW you can carry paint in you hand lugague as its a munition
Also mentioned above is the hold of a plane pressurised (i say yes, mate say no, ensuing argument with no conclusion, anyone help?)
Edit: your also not allowed the tank to be pressurised. The words explosive decompresion are never good when a plane is involved. I don't think its anything t do with the change in pressure, its if the valve gets knocked off and firing the tank through the side of the plane.