Bramble- Thanks thats exactly what I was looking for. I thought a chamber Tenon would be the best way to go. This allows for proper alignment of the monoblock irrespective of any angular effects of their convergence. Check out the 12GaFH Thread on Accurate reloading. I built the first 12GaFH on a scaled up Borchardt action for full power loads. Ed Hubel went with a Savage 210 action much modified. We even used a TC Encore at one point! I also have a scaled up winchester High wall in the works. However, both actions will wind up as pretty heavy unfortunately if you want a full power gun!. The Borchardt gun weighs 26lbs and has driven a 1000gr bullet over 3100fps and some 400gr Sabots over 4500fps or a 2000gr Bore rider over 1500fps. No 700NE can ever match it! It simply has more case capacity( 355grs of Water) and can be operated at far higher pressures. .50 BMG cases are readily available, cheap and good to 55KPSI or more. I have their modification down to a science now! Using a .50BMG primer alows for reliable ignition of large slow powder charges which as you know has proven to be very problematic in the .700NE. The 12GaFH was built specifically to get around the California and US Federal Destructive devices laws and regulations, as basically anything in 12 ga is exempt from those laws. Pretty much true Globally in fact! The gun fires a 12 ga 2 3/4 inch birdshot cartridge easily! We have found that a H&R NEF single shot at 10lbs and a cost of about $300 US. can handle 1000gr bullets at 26KPSI at 1500fps and survive. It actually a great deal of fun! I think a similar load in a double should be no problem and provides for that all important second shot. In addition, its a Challenge as I've never built a double rifle before. I'm not at all concerned that a single load for a double will be necessary. We just need to determine what it is. This cartridge is extremely flexible and I've fired it from 2000gr bore riders at 1500fps to 240gr Aluminum bullets at 3000fps.
I see from your pictures, that you only thread the monoblock partially. Out of curiosity, why not all the way? Finally, do you Tin the inside threads and then recut them on the lathe as a last step or do you rely on the tinning of the barrel threads?-Rob