I remembered that you posted about this gun before:
Just bought brand new and it is very inconsistent. I set the first reg to 140 and the second reg to 100, hammer at 7 and it goes from 833 fps down to 642 fps within 4 shots. The gauge shows 100 psi before I take the shot. What could be causing this?
www.airgunnation.com
You changed hammers and reg(s) and seemed to resolve the issue of declining shot speed. To be frank, at that time, it struck me that there was more to it and that the problem might return - and here we are in a slightly different guise. Sometimes, problems can be masked and return when you change a setting. This often happens when you throw a bunch of parts at a problem, without finding out what’s actually wrong. As I stated back then, major changes in speed don’t arise from 5bar changes in reg pressure. Your gun has issues with the valve and you should send it in to FX (UA). If I understand correctly, it’s been having some variant of this problem since you got it. I don’t think brass pistons will fix it if there was no improvement with Huma regs.
FWIW, I also have the WC BT, which I do poke a lot and use as a travel gun, changing it between 177, 22 and 30. It is not as solid as my Panthera running on the descending slope but nowhere near as variable as you’re reporting.