doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com 850 kg/cm sq Spanish proof (for the simple man)

i don't wish to take this thread farther off topic, but it seems a reasonable place to pose a question that i have wondered about....

is there any concrete evidence re: the effect tighter (or looser) than normal bore sizes have on pressure levels? we see guns with 13, 13/1, 12, 12/1 markings...as small as .710 and as large as .740. that's 30 thou difference between a "tight" 12, and a "loose" 12.

i have recently gotten the MF Ideal with the shallowly rifled right side tube. it was proofed at .716, and measures that in the lands, and about .726-7 in the grooves (not easily measured). i'm far away from attempting to shoot the barrel with a slug, but it has given me an interesting question to ponder. every commercial slug i've seen specs on has been somewhere in the area of .736. that's 7 thou to swage down on the small driving bands typical on the modern slugs...if you are .729 in the bore. but if you start out 8-10-12 thou smaller...the swaging effect is how much greater? then, i started to think about pressure differences of normal one piece wads working through tighter/looser bores.

i seem to recall burrard's book mentioning something about his preferred felt wads sealing forcing cones and bores better than hard card types. but that far predates the modern plastic wads. sherman bell's interesting experiments in DGJ mention pressure differences between tight and loose chambers, but i don't see anything related to bore size variations.

the only other reference point i see is the obvious strong correlation between oversize bullets and higher pressures in rifles. has there been any work that considers this factor in shotguns?

Is your Ideal's rifling designed for slugs, or is it designed to produce an even more open pattern to a shot charge than cylinder bore? There are quite a few French doubles with the right barrel designed as a "canon raye dispersant". Designed in particular for woodcock shooting.

I am not a Randy Wakeman fan on this topic.
If you read Drew Hause post above, you will see that CIP has a standard for cartridges to be used in 850 Bar proofed guns.
The problem is that the SAAMI standard is a higher pressure than acceptable in these guns. Also, we rarely know the actual pressure of the major US cartridge manufacturers, except the SAAMI upper limit.
Some smaller manufacturers of low pressure loads do publish their cartridges pressure.
It is up to you to choose wisely, or reload cartridges to confirm to CIP standards.

this barrel is not the short rifled section near the muzzle that is noted as "raye". this is the shallowly rifled full length rifling noted as "supra". follow this thread from

https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=492115&page=7

and i will add another observation; the english paradox system, and evidently the "raye" rifling is added inside the dimensions of the normal bore. it is effectively a whirled choke. this "supra" rifled bore is proofed at .716, and measures .716 on the lands; but has grooves 10-11 thou deeper than the bore. that fact seems significant to me.

British and CIP load pressures for comparison. See Bro. Vic's comments at the bottom.

The 1954 British Rules of Proof "Highest Mean Service Pressure" equivalent transducer values as converted from LUP by Burrard's formula for 3 1/4 tons = 9,800 psi = 676 BAR.
"Standard Service Pressure" for most loads however was about 8000 psi per John Brindle in The Double Gun Journal, "Black Powder & Smokeless, Damascus & Steel"; Volume 5, Issue 3, 1994, "Some Modern Fallacies Part 5", p. 11.

The equivalent transducer values provided by the Birmingham Proof House is simply Tons x 2240 x 1.33 - NOT simultaneous readings for psi by LUP and psi by piezo transducers:
3 tons/ sq. inch = 8,938 psi
3 1/4 tons = 9,682 psi
3 1/2 tons = 10,427 psi
4 tons = 11,917 psi

Great Britain adopted the 1969 Commission Internationale Permanente pour l'Epreuve des Armes d' Feu Portatives standards March 1, 1980 but continued using Lead Crushers to measure pressure until 1989.
The CIP transducer "Maximal Statistical Individual Pressure" is 850 BAR = 12,328 PSI for a "Maximal Average (Service) Pressure" of 740 BAR = 10,733 PSI, and "Mean Proof Pressure" of 960 BAR = 13,924 PSI.
900 BAR is for a "Maximal Average (Service) Pressure" of 780 BAR = 11,313 PSI and Proof pressure of 1020 BAR = 14,794 PSI.
High Performance (Magnum) MSIP is 1200 BAR = 17,405 PSI for a Service Pressure of 1050 BAR = 15,229 PSI, and Mean Proof Pressure of 1320 BAR = 19,145 PSI.

When a British shell box is labeled:
These cartridges are suitable for use in:
70mm case length: Guns with a chamber length of 2 3/4" or longer, nitro proofed to a service pressure of 3 1/4 tons per square inch (900 kg per square cm)
67 mm case length: Guns with a chamber length of 2 1/2" or longer, nitro proofed to a service pressure of 3 tons per square inch (850 kg per square cm)

Vic Venters, Shooting Sportsman, March-April 2012, "CIP Proof"
"Although the Maximum Mean Pressures for service loads for standard proof (850 BAR) guns are 740 BAR, CIP regulated cartridge manufacturers typically work to lower pressures...between 450 (6,527 psi) and 650 bars (9,427 psi) as measured by CIP piezo transducers."

Dr. Drew, Vic Venters has again urged me to ask you to put your research on Damascus and other topics into printed form. The internet is ephemeral. We are here for a limited time to do the best we can.

I appreciate Vic's encouragement, but am afraid I still don't have everything quite right Gene. I'm continually revising and correcting many of the docs on the DamascusKnowledge site.
And having been through 2 Google Docs format changes which required re-editing everything, and loss of the Picture Trail images I do have some concern about losing all the research...when no one is around to pay the subscription fees
But my Baptist preacher grandfather lived to 107 so DamascusKnowledge should be good for another 37 years or so

It appears that Spain did not adopt the 1969 CIP standards until at least after 1974

Proof certificate from 1972; still with 900 kg/cm2.
"This shotgun has been proofed with two equal firings through each barrel with a pressure of 900 kg/cm2 at 17mm (about 3/4 inch) from the face of the breech."



1100 kg/cm2 (15,646 psi) proof receives 4 shots through each barrel.

CIP certificates state "Esta escopeta ha side probada con dos disparos iguales por cada canon, desarrollando presion de 960 bar a 25mm. del plano de culata." (1 inch)

1974 date code Sarasqueta with 900 kg