pressure gauge or not?

[judgment_arms]

Just the fact that this is the second time you’ve commented on my grammar, although this time you were answering a question.

[paaiyan]

What did I say the first time?

[judgment_arms]

OK just a little pet peeve of mine, don't tell someone else they need to learn how to spell when you can't seem to do it yourself. "Bellow were you type." I do believe you might just mean, below where you type. And "I'm assuming sense you can weld." You might want to exchange "sense" for since. Like I said, just a pet peeve of mine.

[Brian the brain]

If you teke out the schrader valve stem, so you have an open channel from the pump, the reading is accurate.You would have to close the filling area via a ballvalve or something before taking the pump off.
Forn instance, filling a piston-gun through the pilot ( a blowgun or ballvalve) would eliminate the need for an onboard gauge.

This would be fine if you would shoot right after filling.

If you want to keep the gun under pressure there would be no way of telling the exact pressure, even if the gun doesn't leak, because of changes in temperature, influencing the pressure.
But that isn't a big thing.