I wouldn't off centre grind it I'd stone a larger drill bit on a taper if I were to do it but then I wouldn't I'd go through with an 1/8" or if you wanted to be closer an m4 tapping drill
what sort of pressure do you think is in a pumper valve
the op asked how to get it up to like the older American classics and they were a low volume high pressure valve un like the newer easier to pump lower pressure guns
when they brought out the 3 variants with the generic plastic breech they swapped to the larger tp's of 3.6mm as a cost saving cause lets face it they churn thousands and in many US states are classed as cheap toys out and it was easier to standardise them