Fred (Doublegunfan) and all,
This is such a great thread that it deserves a bit of new life I think. A lot of thoughts are swimming around now, so bear with me if this post gets long. Obviously, all those who read the whole three pages of the thread can see the considerable controversy around proofing. My friends in England absolutely swear by the necessity of their Proof Houses (not the mention the English proof law) whereas I think some of the English customs that prevail with today's proofing are arguably not good.

In the above thread is eyewitness coverage submitted by Bramble of some proofings at the London Proof House in early 2007.He wrote:
"I should like to add to this thread my experiences from the London proof house last week.

There are apparently three methods.

If the cartridge is current and common, they may have an assembled proof load in hand.

For my .450#2 they did not and I was asked to bring primed cases and heads into which they were going to assemble a proof load in blackpowder. However they only do this with lead bullets.

When they discovered that I had Woodleigh solids they decided to use my service load which was near maximum and they took the cases and oiled them liberally before firing the rifle.

The rifle was fired with the foreend detached.

Both barrels were loaded alough they were fired one by one.

The examination before and after was visual and physical. Checking that the rifle was on face, examining the bore and chamber.

Immediatly after firing the technicion removed the rifle from the chamber and before opening it held the breach up to the light to check for any gap opening between the barrels and the action face. He then opened the rifle and removed and examined the cartridges remarking on the ammount of primer extrusion indicating the pressure levels. Both primers were volcanic in extrusion and one had a burr forced up that I could remove with a thumb nail. He examined the chambers and the exterior of the barrels. No measuring equiptment was used.

He did remark to me that a similar process had knocked the .600 from a major manufacturer off the face just a couple of weeks earlier.

My rifle was stamped up as proofed to CIP limit of 3500 BAR.

My 6mm BR Norma Ruger #1 was proofed to the same method at the same time using my Lapua factory rounds again because of the relative rarity of the round."

Now it is my hope that somehow the Proof House has done CUP or Piesoelectric pressure testing using a myriad of oiled cartridges and somehow knows that this constitutes a reliable Proof for various cartridges suitable to stamp the gun as proofed to 3500 BAR. One BAR is about 14.5 pounds per square inch. So by oiling the cartridges, including B.P. ones apparently, we are to believe those guns are safe up to almost 51,000 psi!! I submit that very likely the pressure testing with oiled cartridges has not been done to any degree and so this proof stamping becomes almost ludicrous. If I am wrong I will gladly say so and apologize if offered good evidence.

That brings us back to the U.S. and many other place where fear of lawsuits produces very good and safe guns almost all the time. Many a gun and cartridge now in production started off as a likely wildcat - with many telling the experimenter they were crazy, about to blow themselves up, etc. Modern steels are WAY better than they were when rifles that we shoot with aplomb were made.

We also know that a good many shotgun actions have been converted to fire rifle cartridges and since I have heard of few accidents, its seems the daunting efforts to do this weed out a lot of dunces along the way. So let's just look at some "bottom lines" in this regard.

First off, tho not a DR builder, I am rather familiar with that breed, and would highly recommend that anyone contemplating building one, or having done so, get W. Ellis Brown's book "Building Double Rifles on Shotgun Actions" - 2nd Edition. Having spent a few hours with Mr. Brown in his shop, and read his book thoroughly, I can say that he is unusual in being quite humble about his experiences in this cloudy forest - and not at all afraid to tell of mishaps and serious mistakes. His descriptions of gun building are quite good and the photo back up is superb and profuse. See the last page for what cartridges he will, and WILL NOT, build a double rifle on. Notably, the .444 Marlin is in the low pressure category that he WILL build with.

If you are building with liners you can call the manufacturer and find out what steel is used and whether it is heat treated or whatever. Brown's book has a formula to help figure out needed breech thickness, so that may help. If the liner is intended to hold the pressure of something like the .444 on its own, the mfgr. should be able to tell you. The engineers at the steel-maker can tell you more about it too.

If you are using new barrels on the monobloc breech section of the shotgun, as Brown often does, again the barrel maker can tell you what the steel is and what it is designed to hold. You don't have to reinvent the wheel if it is already designed to hold a good bit more pressure than any standard load in the .444/.

That leaves the action strength, and again Brown tells you what to look for in a suitable SG action. Other SG actions are already being used here in the U.S. and elsewhere. Notably, it is the action strength and resistance to stretch that the London Proof House seems to be wanting to test with their oiled cartridge method.

I couldn't agree more with those who say you are playing with fire by picking out this or that smokeless powder and assuming that 7% above the max load will give you a 30% increase in pressure and therefore yield a suitable proof. This is generalizing dangerously - to say the least. Hodgdon is very progressive in the powder field now and a good talk with one of their senior ballistic techs MIGHT reveal a powder to use for a given cartridge which MIGHT yield linear pressure as you add a small percentage more than the max they list in their tables. This would be "off the record" of course, and very possibly you won't get the info. Again, fall back on what Brown does in his discussion on proofing, - if you feel you MUST proof despite researching all of the above.

If all else fails, and assuming you have built a decently strong gun, it is very likely safe to just fill your .444 cases with 1.5F Swiss B.P., topped with a typical bullet for same, and have a happy life shooting your D.R.

Aloha, Ka'imiloa