I know other people have done this mod by hand, but my opinion is that it should be done accurately with the right equipment (i.e. lathe) and by somebody who's very familiar with their equipment.
In case it helps, here are some measurements from one of my 6PD's that was bored out on a lathe.
Tube ID (after bored out) = 18.35mm (0.0722")
OD inside the O-ring groove on tail threads = 19.56mm (0.0770")
Wall thickness at this thinnest point = 1.21mm (0.0475")
OD from base of threads = 19.7mm (0.7756")
Wall thickness at this point = 1.35mm (0.053) left for all thickness.
OD of thinnest point of body = 23.6mm (0.9291)
Wall thickness at this point = 5.25mm (0.2067").
Length of 18.35mm bore from end of tail cap threads towards front of light = 77mm
This leaves ~4mm of the original bore left towards the front of the light so the negative contact spring (the large spring) has plenty of material to rest against.
The thinnest part of this light is inside the o-ring grooves on the tail threads. The wall thickness at this thin point is a bit more than 1/16 of an inch, which means there's not a lot of margin for error. Even though others have said they've done this mod successfully by hand, I just can't recommend it with this margin of error at this one point in the light.
Throughout the entire body (except for the tail cap threads), you have at least 5.25mm (0.2067") of wall thickness. Boring out the tube to 18.35mm at a length of ~77 so that you're leaving ~4mm of the original bore towards the front of the light would be hard to do by hand.
I'm sorry to hear things did not work out for you with this mod. I wish Surefire would have made the tube for this light compatible with the 18650 because this battery has a lot to offer for its size. The 6PD with an 18650 is a wonderful and versatile light. Lighthound sells a body from Leef for this application, but it's quite expensive. I already owned several Surefire 6PD's for use in my businesses. Since they were already bought and paid for, I chose to bore them out and I'm glad I did. However, when (or if) it comes time for me to add more 6PD's, I'll probably be looking very seriously at the Dereelight CL1H since it has the forward clicky, a HAIII finish that the 6P does not have, good build quality, compatible with the 18650, upgradable, good customer service via email or PM through CPF, etc.
