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Sir;

Having built several double rifles on shotgun actions, I will give you some advice in regard to your question and other issues that you may not have thought about ( as there were issues that I had not thought about when I first started building double rifles).

The selection of a cartridge for a double rifle built on a shotgun action is first of all related to the design and alloy steel content of the shotgun action's (is it strong enough) and secondly to the design of this action's firing pin mechanisms (does the action have removable firing pin discs that screw into the face of the action---these discs are known in the British and European gun trade as "striker discs"). The majority of boxlock (such as the American, Belgium and British boxlocks) shotgun actions do not have these striker discs, but there are some very strong European actions that do have striker discs such as the Brno model ZP-49. Many Spanish boxlock actions have striker discs. If you will look back through the old posts of this website you will see a poster from Canada who built many double rifle on the ZP49 action and I have as well--the ones built in the late 1950's and the 1960 are the ones to use.

Striker discs in a shotgun action allow the builder to make new striker discs with the firing pin (striker) hole diameter to the correct smaller size for the primer size of the cartridge used and to reduce the concern about ruptured primers and the gases emitted during a primer rupture. A large diameter shotgun firing pin might work with a cartridge like a .45-70 but not for a cartridge say like a .30-30. Just take a look at whatever rifle you have and you will see the firing pin diameter is much smaller than your 12 gauge shotgun firing pin diameter.

All of the .45 and.50 caliber cartridges that you mention above could be built on a strong shotgun action by a person who is prudent in his work and might be OK with a 12 gauge shotgun firing pin, but in my experience I would not built with a shotgun action that did not have striker discs because I have had ruptured primers on double rifles.

Of the cartridges you mention I would choose the .45-70 because of its ease of regulating and building for a first time double rifle builder.

The late Bob Hynden, who posted here before his death built maybe 8-10 double rifles on .45-70 and he found them to be his favorite.

Further in regard to converting a shotgun action to striker discs weakens the action and a prudent person would not do this.

Also threading the barrels and the barrel monobloc stub weakens (takes away metal) the barrels at the point of most pressure and TiG welding is the process to use.

A important side issues is the equipment you have in your home shop or at another shop you have access---a lathe and a milling machine, and finally your understanding of building a double rifle such as the all important method of building the barrel set with the correct amount of bore convergence before you ever begin the regulation of the barrels which require good understanding of soldering and brazing techniques. In this regard knowing which method of moving the barrels at the muzzle during regulation needs to be decided before you begin i.e. are you going to use the method of move a wedge length wise to the barrels or are you going to use the Holland & Holland method of moving a wedge between the barrel muzzle up and down.

Having said all of this please feel free to PM me for any other details that you might want to discuss.

Stephen Howell