doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com "In" and "out" of proof?

I don't understand bore diameter and its impact on proof.

Why, for a 12 gauge, is a bore diameter of .740 or under "in" proof, but one of .743 is "out" of proof?

I can understand that a gun that is out of proof will be harder to sell and will sell for less than one in proof. Any issue with a gun increases buyer resistance and the price gets lowered as a result.

But, I do not understand the technical aspects. Does the greater bore diameter increase the risk of gas blowback? Is it possible for a gun with a .743 diameter to be reproved?

Can someone point me to an article that explains this?

Thank you.

The measurement taken at proof are the ones marked on the gun. There are allowed dimensional changes that can be made to the bore , honing out pits etc. , that will allow the gun to be in proof. If too much material is removed the over sized bores are then out of proof. Forcing cones can be lengthened, but the chamber length can not be changed. Each proof house has a set of rules that are published, and probably on line.

Vintage Guns: Collecting, Restoring and Shooting Classic Firearms by Diggory Hadoke is a good start, for British Rules of Proof
A short preview is available
http://books.google.com/books?id=lvBk8df8PjUC&pg=PA139&lpg

Bore dimensions and marks post-1887 Rules of Proof
"At proof if a plug gauge of 0.729" diameter (but not one of 0.740") will enter the bore to a depth of 9", that barrel would have been marked 12."



So if the gun is marked 12, and a plug gauge of .740 can pass at 9", the gun is out of proof.

Yes, a gun with bore of .743 can be returned to the Birmingham Proof House for re-proof (and must be in order to be re-sold in England) and will be so marked. There are labs in the U.S. that will prove guns, and provide a report documenting the results.

H.P. White Laboratory, Inc. in Maryland and Cortland, New York will proof Twist or Damascus barrels for $500 (in 2014) to a pressure of 20,000 psi. A minimum pressure test would be 1 and 1/2 times the pressure of your intended load.
www.hpwhite.com

Chesapeake Testing
http://chesapeaketesting.com/services/outdoor-blast-testing/

Dayton T. Brown
http://www.dtbtest.com/Default.aspx

Out of proof only means too much metal has been removed. A out of proof gun can be safe to shoot. It can not be sold out of proof in Great Britain. Out of proof guns are reprooofed and then can be sold there. Out of proof means nothing in the US and out of proof guns are sold all the time.

What you worry about is thin barrel walls. Too easy to dent. Thin In the wrong places can be a burst risk by you have to be extremely thin. Worst than thin is reaming 2 1/2� chamber out to 2 3/4�. That area has the highest pressure and no metal removal improves barrel integerty. Worse people go from7 or 8k psi shells to 10k thinking 2 3/4� chambers are automatically saf for all 2 3/4� ammo.

Note that under the "Old" system as Drew has stated the marks are not actual diameters but simply the largest plug gage in a series which would enter the bore to the required depth of 9".

To give an example of "Possibilities", consider two barrels have gone through proof. One had a bore diameter of .7295 & the other .7395.Both would accept the 12 gauge plug but neither the 12/1 plug so both would be marked 12. The smaller barrel could have the bore enlarged by .010" & remain in proof as it would still not accept the 12/1 plug. If on the other hand the larger one were enlarged by only .001" it would then accept the 12/1 plug & be "Out of Proof". This situation "Eventually", after many many years, led to the marking of actual bore diameter. From that point on the gun was considered In Proof until it was enlarged by .2 mm (.008"), a much better system. Barrel wall thickness is of course what determines whether a chamber can be lengthened or not. Many guns have had their chambers safely lengthened, including many British guns, which do have to be re-proofed in order to sell one of them. Even many guns which had original 2 3/4" chambers should not be used with modern SAAMI spec shells, even with the longer chambers they were not proofed for that pressure level. The load put in that shell is of far greater importance than the actual length of the hull.

"NO" shell should be used however which has a "Loaded"length long enough that the crimp enters the forcing cone as this can delay the opening of the crimp & may raise the max chamber pressure to a dangerous level.

Also guns with a "Stepped" or very abrupt cone should not be used with hulls longer than the chamber as the crimp will then open into the actual bore.

Fortunately, thanks to the wonders of internet digitization of vintage publications, we do not have to guess "For what load was my gun designed?"; which is of course very different that "What load should I use in this here gun?"

Prior to 1924, the British Maximum Service (using) load for 12g 2 1/2� and 2 5/8� shells was 1 1/4 oz. / 3 1/4 Dram Eq. (1220 fps). The pressure of that load would have been about 8500 psi with BULK smokeless powder; with DENSE smokeless 9,500 - 10,500 psi.
The standard 2 1/2� shells however were usually loaded with 1 1/8 oz. shot and 3 Dr. Eq. of BULK Smokeless with a pressure of 6500 - 7500 psi; DENSE Smokeless was 9000 - 10,000 psi.

During WWI the standard English 12g load was dropped by law (to conserve the supplies of lead and powder) to 1 oz. and 3 Dr. Eq. Bulk smokeless. After the War, 2 1/2� shells were generally loaded with 1 1/16 oz. shot and 3 Dr. Eq. Bulk or Dense smokeless powder.

In the 1925 British Proof House revisions, the 2 1/2� & 2 5/8� 12g service load was reduced to 3 Drams with 1 1/8 oz. shot with a mean service pressure of 3 1/4 tons = (converted using Burrard's formula) 9,682 psi.
The 2 3/4� 12g max. service load was 3 3/8 Dr. Eq. with 1 1/4 oz. shot with a mean pressure of 3 1/2 tons = 10,640 psi by Burrard�s conversion.
(Primarily for heavier �Waterfowl� guns)

Most makers were kind enough to simply tell us what to use



42 gr. "Schultze" = 3 Dr. Eq.

It's good to remember here that the "tons" proofmarks didn't appear until the proof rules were changed in 1954. But in spite of the different proofmarks, proof loads really didn't change. This is a quote from Roger Lees, Birmingham Proofmaster both before and after the change from the 1925 to 1954 rules: "The proof loads now set down in writing (in the 54 Rules) are almost in every instance the proof loading in use under the 1925 rules. In general it may be said that under the new Rules of Proof no arm will receive a more severe proof than hitherto." Roger Lees, as quoted by Vic Venters in his book "Gun Craft", p. 206.

I like to buy "out of proof" doubles with adequate wall thickness. Please bring me your tired, your poor, and destitute Damascus guns. I would hate for these "unsafe" guns to remain in the hands of Joe Public.

regarding lengthening of chambers to 2 3/4"... it is my understanding that so long as at least .090 thousands of barrel wall is left in front of the new chambers, then it is safe to lengthen the chambers...

experience over the years has indicated, that most american made guns with fluid steel barrels have heavy barrel walls and can be safely altered to shoot 2 3/4" field loads...not so much so with european made guns, particularly light weight field guns which often have thin walled barrels...

That's probably too much of a generality Ed, but I agree with you...Geo