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Original Post (Thread Starter)
by J.B.Patton
J.B.Patton
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
Liked Replies
by Drew Hause
Drew Hause
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.
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