remember however, if the gun you have is nice already, why alter it?

If you must experiment drill the TP to 4.2mm and tap to m5, get some 6mm long m5 grubscrews and have them drilled from 3 to 4mm in .1mm steps, use a little blue loctite to hold them in and seal the threads, you can now play with TP dia and see what works for you

A little heat and an allen key gets them out, run the tap back in to clean the threads and screw a new one in to resume testing.