I wouldn’t not want to try that with the ATFE (Which should be a party store, not a FedGov agency) - they normally not noted for their sense of humor.
Welding cannot be easily reversed, firing pins OTOH, not so much...
Well, you could pop the pin and weld over the hole.
I’m just saying that with a piece like that it’d be nice to keep it in a shape so that people could still see how the action works.
Weld the whole thing shut and you could probably never get it open again to even oil it!