doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com what should be the minimum wall thickness of 12

So you are saying it should be at least .045 9 inches from the from the breech and .025 at the thinest.I am assuming when someone post the minimum it could be anywhere.
I talked to Kurt Marrington today about the gun and gave him the same info as I gave you and he said it would be fine but maybe I left something out.I'll have to get my figures correct where it is .025 but it only would have started out as .729 bore and now is .733 and .737.

What makes you say that it started out at .729"? Was it prooved at 12?
It may have been proved at 13 which would have made it .710" originally.

I'll have it in my hands next week to find out.I'm confused on the .025 9 inches from the breech?

It's a Holland and Holland if that make any difference.I'll get the guy to snap a picture of the flats.

Yes, .045" 9" from the breech. Also, take heed to what blue grouse is saying above.

The specs at present do not suggest it was born at .728" bores. It could have been only .721" and still been proofed .729".

More likely it was a 13/1 proof around .710" originally.

Put bluntly, It is missing .020"-.030" of metal at 9" from the breech that should still be there. Now you figure out where It went. Honed away inside? Polished and filed off outside? Both? I definitely wouldn't shoot it.

Stay away from it Montey.

Some serious misinformation here. First, 13/1 did not exist in 1880. Fractional bore sizes first appeared under the 1887 rules of proof. Second, if it's marked 12, then it would have been proved as a 12, not as a 13. It would only have been proved as a 13 if it was marked 13. With the bores currently being 733 and 737, it would be marked 12 even if it has been reproofed.

Photos of the barrel flats would clear up any questions about proof/reproof.

Re pressure and barrel wall thickness, the graphs I have--from Dupont in 1933 and from Sherman Bell's "Finding Out for Myself" DGJ article in 2002, show pressures significantly reduced at 9" from the breech. The Dupont graphs show them between 3500-4,000, while Bell's graphs have them even lower: 2-3,000. I expect part of that difference is that Dupont was measuring LUP with the old lead crusher method, while Bell was measuring psi with electronic transducers.

Anyone have a reference stating that .045 should be the minimum at 9" from the breech? Thanks.

Forget about that amateur Kirk Merrington.

HomeyjOe is the ultimate authority!

(on something...)

A few things of note; assuming proofed after 1887 then 13 = .710"-719", 13/1 = .719"-.729+. 12 = .729"-.740" & 12/1 = .740"-.751". At this point no exact bore size was given, only that it would accept a plug of the size marked to a depth of 9" from the breech & would not accept the next larger size plug to that depth. Thus a .711" & .718" bore would have both been marked with a 13 with no distinction between the two.If proofed prior to 1887 then the whole gauge numbers remain the same, but there were no /1 sizes in between. so a gun proofed as a 13 could have been from .710"-.729" wath a 12 being .729"-.751"
The next thing of consideration is; at a point 9" from the breech the bbls are not really overly concerned about what the "MAXIMUM CHAMBER" pressure of the load is. IF you push a certain amount of shot out the muzzle to a given velocity then the same amount of work has been done, consequently, you will have a very close "Average" pressure over the length of the bbl. If one load has a significantly lower max chamber pressure then it will have higher pressure at some other points in the bbl to compensate.
Bottom line is assuming same overall balistics the load which would stress the bbl the least @ the 9" popint is almost certain not to be the lowest pressure load, but rather the one which had the highest max chamber pressure, as its pressure curve would fall quicker than would the one using a slower burn powder.
I am in agreement with HoJoe on this one, .025" at the 9" point doesn't have much of a built in safety factor. It would not concern me at all if it were 15" down the bbl.

Going to jump in here for some advice. I just got a 1875 W.C. Scott 12b hammer gun that look in very good cosmetic condition. Ahead of the flats are the three proofmarks and also each barrel is stamped 14. I assume it was a 12 bore proofed at 14 bore.

Using OD-ID/2 I get both barrels with 0.70 wall thickness 9" from the breech and 9" from the muzzle I get .050 (rt) & .040 (left).

The bores are consistent along the barrel, .710 (rt) and .718 (left). The chokes are actually negative, the right barrel opens up .009 and the left opens up .002.

So this tells me that if it was proofed at 14 bore, .693, then the barrels have been honed out .017 (rt) and .025 (left).

But with those wall thicknesses (.070 9 inches from the breech) and everything else being sound would you consider it to be safe with low pressure or blackpowder loads? The barrels are quite heavy, the breech walls are between 0.235 and 0.248 depending on where on the circumference they are measured.

Amazing thing is that for a 138 year old gun the stock has shootable dimensions.


thanks,

Rob