Dear Twobob...;

There does not seem to be evidence that there is or was a formula for "beefing" up the actions of British gunmakers in order to accommodate the increased pressures from shotgun cartridges to rifle cartridges. However, we have an excellent description of what a present day gunmaker, David McKay Brown did to the size of his famous round action shotgun action to have them suitable for higher pressures of rifle cartridges, starting with the .375 H&H flanged magnum cartridges. All of this is recorded in the excellent book of the history of David McKay Brown's guns and rifles authored by Donald Dallas.

What is evident in David McKay's Brown's example is that he used his experience as a gun and rifle maker as well as good engineering practices and higher strength steel to accomplish his task at hand.

We see that on page 122 of Dallas's book that Brown uses for his steel in both his guns and rifles, EN-34 for actions and parts to be casehardened (EN-32 was formerly used, as common in the trade) and he uses EN-24 for working parts, EN-45 for springs and EN-19 for barrels.

Of course, we should remember that the Scottish round actions have always been considered more stronger than the typical side lock and boxlock actions of the gun trade in the UK because of for at least one reason that the design that has holes drilled (and few at that) in areas that minimizes risk of action strength reduction. (Note: I have no evidence that this is correct, nor have I found any technical data to support this assumption, but it appears practical and reasonable) On page 99 of the same book, Brown tells us that when he resized his shotgun action for the first .375 H&H flanged magnum cartridge that...."The bar of the action was lengthened by 1/4", the back of the action was increased(made thicker)by 3/16" and the depth of the action was increased by 1/16". The joint/hinge pin was fully supported across the entire action knuckle.....The centre of the radius of the rear barrel lump was lowered so that when the barrel are closed the radius(known as the circle) comes into contact with the matching radius(known in the trade as he "draw") within the action slot. This was designed to take some pressure off the joint pin." (Note: this is a very very important mechanical and engineering design issue that most amateur gun and rifle makers do not understand and therefore do not take into account when they are building a double rifle from a double shotgun action--and it is not just amateurs who do not know this but individuals who are "professional gunsmiths". Holland & Holland is pne of the most well known maker who has even redesigned the "circle and draw" to make them stronger on their double rifles. H&H double rifle now even have a replaceable draw.)

In the following paragraph Brown (via Dallas) says that: ...."There was no need to fit side bolsters as required on side lock double rifles." (This statement of course, adds credibility that the Scottish round action must be stronger than the side lock and boxlock actions). Brown says futher: "Disc set strikers are a NECESSARY inclusion in double rile design. The nose of the striker has to be of a smaller diameter than that of a shotgun." (another item missed by some amateurs and professionals) Continuing on he says: "The pressure is much greater on a rifle and so a smaller area at the nose reduces the backward pressure and should the smaller diameter striker be snapped in two when on safari, it can easily be replaced with the spare kept in the well of the grip cap. Another design feature of the strikers is a collar or flange on its main body to act like a valve. Should any gases enter the striker hole, the striker would be forced back with the flange of the striker coming against the shoulder and thereby seal the hole preventing the powerful gases from getting into the head of the stock." (It should be noted that the striker and its flange design spoken of by Brown is a feature that the best double rifle makers have built into their actions for many decades).

On the following page we read that the next cartridge size that Brown built double rifles was the the .470 Nitro Express, and Donald Dallas records this: "Due to the increased power of the .470 Nitro Express the action body had to be scaled up even further to take the cartridge. The diameter of the joint pin was also increased from 5/16 inch to 3/8 inch"......"Soon other even bigger double rifles followed, again with the action being beefed up to compensate for the bigger pressures". (Note: David McKay Brown's double rifles built include the .577 and .600 Nitro Express).

Bob; I suspect that a form of the above has been the process that all the double rifle makers have progressed through in the engineering and building of double rifles, and is very similar to what Ben posted in his response to you.

Maybe and hopefully Mark Mitchell, the renown double rifle builder to the trade in Scotland, who is a member of the forum, will add knowledge to this post. Then we will have information from a master not from an amateur like me.

Kindest Regards;
Stephen Howell