Having read DGJ’s articles on Damascus pressure testing-
I was considering the capabilities of the standard 1 1/8 oz. / 2 1/2” proof English guns.
Sherman’s tests concluded negligible pressure increase from shooting 2 3/4” loads ( not high velocity/high pressure) in 2 1/2” chambered guns.
Would it be unnecessarily risky in shooting 2 3/4” loads in these guns. The guns I’m considering are relatively heavy ( 7 1/4- 7 1/2 #), ( FLUID STEEL/ London ~ 1930 era) with original proof measurements, and 30/ 1000s min wall thicknesses / 45/1000 @ 9” from breech.
Also, there be any reason why an English gun would have original 12 gauge 2 1/2” proof at .740 rather than .729? The gun has no indication of anything other than .740 proof marks.
Thanks in advance,
Best Regards,
JBP

Under the 1896 Rules of Proof Pressures 12g 2 1/2” and 2 5/8” chambers (bore .710-.740) were proved for a maximum service load of 3 1/4 Dram Eq. with 1 1/4 oz. shot. (1220 fps)
The Proof House did not publish pressures but other period published pressures with Bulk Smokeless powder were 7,000 - 8,500 psi

William Bruette, Guncraft: Guns, Ammunition, Wing & Trap Shooting, 1912
https://books.google.com/books?id=5g51K93as84C&pg=PA210&dq
Pressures converted from long tons/sq. inch to psi by Burrard’s formula
Curtis & Harvey’s (42 gr. = 3 Dram) Amberite
12g 2 1/2” case 47 gr. (3 1/3 Dram) 1 1/4 oz. shot = 11,480 psi
12g 2 1/2” case 42 gr. (3 Dram) 1 1/8 oz. = 8,960 psi
Curtis & Harvey’s Smokeless Diamond (33 gr. = 3 Dram)
12g 2 1/2” case 39 gr. (3 1/2 Dram) 1 3/16 oz. = 11,984 psi
12g 2 1/2” case 33 gr. (3 Dram) 1 1/8 oz. = 9,632 psi
12g 2 1/2” case 33 gr. (3 Dram) 1 oz. = 8,288 psi

In 1925, the 2 1/2” & 2 5/8” 12g maximum service load was reduced to 3 Dr. Eq. with 1 1/8 oz. shot with a mean pressure of 3 1/4 tons by LUP = 9,800 psi by Burrard’s conversion.

Bell's study (here about 1/3 down https://docs.google.com/document/d/1ZIo0y746UsSRZIgRuuxwAbZjSBHitO_EanvwLYc-kGA/edit ) showed an increase pressure using 2 3/4" shells in 2 1/2" 12g chambers from 228 to 1216 psi.
His summary statement was "“Shooting 2 3/4” shells in 2 1/2” chambers does make them produce more pressure-but in most cases it is less than a 1000 psi increase. I see no reason, related to safety, to modify an original 2 1/2” chambered gun to shoot 2 3/4” shells, if the 2 3/4” load you intend to use would develop pressure that is safe in that gun, when fired in a standard chamber!”

It should be noted that modern nominal 2 3/4” (after firing) shells may be shorter than 2 3/4”; I've measured some Fiocchi and Federals at 2 11/16".

If the case mouth of a 2 3/4" hull is “feathered”, it very likely extended into the forcing cone on firing with the potential to significantly increase pressure and recoil.

I’ve shot tens of thousands of 2 3/4” handloaded ammunition in my 2 1/2” chambered guns with no ill effects. And most of these have been in damascus barreled guns. But I do load with pressures the guns were originally intended for. Velocities are kept between 1100 to 1200 fps and pressures (as published) below 8,000 psi, usually in the 7,000 psi range. As long as the hull does not open into the bore but stays in the forcing cone all should be fine. As Drew mentioned above if the mouth of the fired hull is ragged it is probably opening up into the bore. Frankly, I have never had this happen.

Pressures drop of dramatically after the shot leaves the chamber. The most important wall thickness, in my opinion, is immediately in front of the forcing cone. I’ve never found a consensus of opinion of what the minimum should be in this area but I like a lot, say near .100”, sometimes a little less. Many factory guns were made with them even thinner and I shoot some…..but then, I’m a grown up boy and do it heads up.

I very much agree that the critical wall thickness numbers are the end of the chamber, the forcing cone, and every inch out to 9", which is almost never reported by sellers. "MWT of .030" is (almost) meaningless.
As Joe said, there is no published consensus as to MWT at the above locations, probably because vintage doubles have barrel steel of different composition and tensile strength.
Which makes the CIP standards of limited help in that the numbers start for Category 1 “Standard Steel” (Non-alloy AISI 1045) with tensile strength 101,526-123,137 psi

But we can make our best recommendations based on measuring lots of presumed unmolested vintage barrels, and there are lots of numbers here
http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=379803&page=3
http://parkerguns.org/forums/showthread.php?t=31791

This is an example of feathering; 67 mm hull on left and 65mm on right

I believe that it was Charles Fergus, in an article in Shooting Sportsman, who reported problems using even standard British 67MM shells in a British 2 1/2" gun from the 1890's. Some guns from that era had very short and sharply tapered forcing cones. Fergus reported increased recoil and the ends blown off the hulls. Only true 2 1/2" shells should be used in those guns. He fired the same shells in a gun from the 1930's with no issues.

I haven't fired as many reloaded 2 3/4" hulls in 2 1/2" guns as Joe has. But I shot a bunch of home rolled pheasant loads (1 1/8 oz) in a pair of Webley & Scott made Army & Navy doubles from the 30's. No ill effects . . . other than on the pheasants. Like Joe, I kept my pressures in the sub-8,000 psi range.

I agree with Joe Wood. years ago I developed a method of cutting off a small portion of a 2 3/4" hull, reloading using an overshot card so you ended up with a crimp but no center plastic. They worked fine but I really felt no difference between those shells and a reg 2 3/4" hull. Keep in mind all of these were loaded with 7/8 oz at below 1200 fps . My short chambered guns are from 1893, 1905 periods. Some had lengthened forcing cones some not. My resource for barrel questions wants to see over .105 right in front of the chamber, but another knowledgeable source quoted the .025 9" in front of breech. A recent discussion about reproofing an old gun with a UK gunmaker he said the barrels aren't usually the problem but the lockup and making sure the gun stayed tight was often an issue. Another friend whom I consider very knowledgeable about American and English best guns said unless you were planning to sell a gun in England he wouldn't send it back for reproof since it isn't needed here. Just use appropriate loads and enjoy it

I shoot several very thin walled doubles. It’s more where than how thin that I consider. I have a 20 bore that’s .018/.017 at 25+” where the pressure is much lower. It’s more a dent risk that a burst risk. Being reasonable is subjective but if my barrels are .025 out to 20+” I am fairly confident I can load and shoot ammo which will be safe and effective. Chambers need to be thick and pit free.

I once came across a Fox A grade 16 with ruined, pitted, chambers. Chamber sleeved it down to a 20 so it was a overbored 20 with 30” barrels. Made a great pass shooting gun. But with the original pitted chambers I would not shot snap caps. You decide your level of comfort and take reasonable measure.

Eley`s Grand Prix 12b loaded plastic 2 3/4" cases for years after they were first introduced ,they adjusted the powder charges to suit !! I still have a couple of hundred yet to be shot ....

First thing I would try would be the 2 3/4" B & P Comp One loads. They're low enough pressure that the small increase from the extra 1/4" would be inconsequential. The only problem I've found with them is that they seat deeper in the chamber than many other loads and can misfire in some guns.

That photograph Drewe posted of the two cartridge cases came from me, this was a start of a problem that started to become more common here with some cartridge makers.The cartridge case on the left was an Eley "Blue" 67 mm this cartridge rattled my teeth because it improved the recoil no end. The other cartridge case is my usual go to Hull 65 mm and as you can see they do not agree on size. I have a number of guns that are 21/2 inch cambers and they are exactly that size, the guns where built before 1880. I did contact Eley at the time their reply was it was within accepted specifications. If I lived on your side of the pond this would not be a problem just extend the chamber to accept 67 mm on each of the veteran guns BUT!! If that is done on this side of the pond the guns would then be out of proof and required to go through proof again, as each gun is in proof there is that chance that a couple of costly gun's could end up in pieces after test firing I will give that a miss. At that time it was a sort of Russian Roulette when purchasing 21/2 inch cartridges what case length you would end up with.

How would anyone know that a chamber had been altered from 65 to 67mm ? Surely the test is by plug guages on a go/no go basis and if 70mm doesn`t go.......

Plug gauges wouldn’t work on older guns. Prior to establishing SAMMI standards there was no standard size for chambers. Guns I have from 1870 and later have chamber dimensions all over the board.