I just measured my p2x, which uses the same body and tailcap as the newer 6px, and while the majority of the battery tube will handle boring-out just fine, the section right underneath the threading would be at around 0,5mm when bored to 18,5mm. Same goes for the 6p in fact. I know that many people successfully bored out their 6Ps and like it just fine, however I am confident that when dropped on the tailcap, the threading would quickly break at this point, it is just too fragile. When bored to a lesser diameter like 18,1 - 18,25mm it might be rugged enough, but would only accept certain unprotected cells. 18650s vary quite widely in thickness. I would not recommend it. Just letting you know.