Many of you know me from the AR Big Bore Forum as the Inventor of the 12GaFH. If you have been following the thread (55K long now) you know that we have had a great deal of success with a low pressure version of the cartridge that operates at about 24,000 psi and still delivers 5K Ft-lbs of energy with bullets made from Brass and Aluminum. The cartridge is so impressive it simply screams to be put into a double. We know that a simple H&R/NEF single shot with a simple but sturdy under barrel locking system can handle these loads including Proofs to approx. 30Kpsi with no issues so I dont see why a strong double wont survive as long as things are kept to the "10Ton Rule". Well I've had a great deal of interest in a Double Rifle version. I have aquired a Zabala Mercury 10Ga SXS that may be suitable for this process. Because this cartridge generates considerable muzzel pressures due to the slow powders we use in this version, I need relatively thick muzzel wall thickness, thus the 10Ga approach. The questions I have are for those of you who have actually built Doubles before. BTW I have very good machine shop equipment available including manual and CNC lathes and Mills( can pgrm in CadCAM also)so very little is beyond my machining capability. just no direct experience building doubles. I own a few very nice English Double Guns and have always admired them. Now that I'm retired, I have the time to do this project right.

I was planning on a simple monoblock approach and was planning on threading the monoblock in the Lathe with a 4 jaw to hold it. What approach do you use to dial it in? Do you make a short tennon for a internal #3 jacobs taper ( or equivalent to orient the monoblock),clamp down the 4 jaw then dial in on the interior chamber walls? I could also see doing this on a faceplate with a appropriate fixture to hold the monoblock. What has worked for you? Can you post some pictures also?
Alternatively, I could use a CNC mill and use a coaxial indicator to center the chamber, then threadmill it. How far down do you thread and what thread pitch do you reccomend?
Finally, How much larger should the barrel be at the joint so that once soldered in place, a good peening can be done to hide the joint? I need to order barrels soon, so I need this info.
Thanks very much for your input ahead of time and yes, I'll be buying the book!-Rob