"I tested 3 to 4.2mm in .1mm steps and settled on 3.9, i know Wonky likes 3.9 also, JB likes 5/32 which is just a nats over 3.9, T20 uses 4mm, BTDT runs 4mm on his 77k....the list is long for rifles that all run very close to the same transfer port dia...now ask yourself why"

The list of many longtime professional "tuners" who kept to the original or even sleeved transfer ports is even longer..... now ask yourself why? from first hand experience I include in that list Cardew, Mike Wade of Power without Powder fame, Venom, Worcestershire Black Powder Supplies, Kestock and many more. They must have done a half decent job to stay in business as long as they did with many repeat customers.

The best piece of advice is.
"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"