Yes I'm a novice and I'm trying to acquire as much knowledge as I can! I was curious to see if these numbers could be broken down for me, ie what they mean and how it applies to use.

12ga Damascus steel barrel

Right Barrel
.740
.695 wall for
.045

Left barrel
.736 choke
.695
.041

My guess is that (rt barrel) the bore is .740, the choke is .695 and the barrel thickness is .045 and would be safe to shoot just based on minimum wall thickness (.035 general consensus) given all other variables being addressed. Or am I looking at the numbers wrong?

Thanks guys!

You should know the bore diameter, ie: .740, choke diameter or amount of total construction, and minimum wall thickness in each barrel. The important spec is the minimum wall in each barrel, top, bottom side of each barrel. You cannot take the outside diameter and subtract the inside diameter and divide by two to get the wall thickness, you need to measure with a wall gauge.
From the specs you list, I am assuming the left barrel .736 is bore, .695 is choke dia and .054 is wall.
If your numbers are correct, that should be a rather heavy gun.

yep I had the numbers wrongs. The barrels were inspected, measured and given the all safe, I was just curious if I was reading the numbers right. There's a steep learning curve, this I know and I thank you guys for your patience.

one point to add

for minimum wall thickness - the where is important

the walls at the area of the breech/forcing cones are important to know

.054 just before the chokes is a stout gun
but 6 to 9 inches from the breech is too thin

It is likely that the right bore (and possibly both if the gun is Continental) has been honed, making it even more critical that someone with the interest, equipment and expertise to do so (like Bro. Dennis) has evaluated the barrels; NOT the seller.
If the report does not include MWT at the end of the chambers and the forcing cones (near out eyes) and every inch out to 16" - 18" (near our forward hand), it is inadequate.
I believe Osthaus meant .045" 9" from the breech.
More here
https://docs.google.com/document/d/1ZIo0y746UsSRZIgRuuxwAbZjSBHitO_EanvwLYc-kGA/edit

yes

kant type

wouldn't it be .740 O.D. - .695 I.D.= .045 divided by 2 = .0225 wall thickness? Also agree, gauge is a much better way to get accurate measurements. Remember when working with dia. measurements as above you have 2 wall thicknesses in the equation.

m-4

m-4,
That is true, but the two thicknesses might not be the same, because of being "struck" by hand, and sometimes overly so near the ribs. Unfortunately, it may be hard to get a good measurement here with a gauge also.
Mike

Mike, I understand all that. Just trying to show them the flaw in their equation for wall thickness. Could also get accurate measurements with an ultrasound gauge to determine wall thicknesses at any given point accessible.

m-4

Unfortunately, the barrel outside diameter is too small for virtually all commercially availiable ultrasonic transducers to get an accurate reading. That is why they are not used in this application.

For many years I measured the ID and OD, subtracted and divided by two to get the wall thickness thinking this is close enough. Without going into detail, I purchased Galazan's wall thickness gauge and began measuring my barrels. One of my guns measured .014 thickness whereas before I thought there was about .025 minimal wall thickness. Needless to say, I now love my Galazan gauge.

Roalco is correct.
The $1900 TI-007 Precision ultrasonic wall thickness gauge has a measurable range from .006" to 1" with a resolution of .001". However, the manual states that the minimum radius for convex surfaces measured is .5" or a little larger than a 4 bore, or 2.5" for a concave surface.
https://www.abqindustrial.net/store/inde...2ab880033fe5556

It's not unusual for me to discover a variance of .010 at the same point in a barrel while measuring wall thickness due to indifference while the barrel was struck.

Joe, I have seen .010 difference from top rib to bottom more than once.
At the Southern three years ago a member of our organization had me check his barrels wall thickness. Mine is a take-off of the Galazan gage and to do it right takes me 20 minutes or so going from top rib to bottom rib. Anyway I started 5 inches from breech and go every 3 inches till halfway and then go to 4 inches. Both of his barrels were almost concentric within .002.
I can go 15 inches and then turn it upside down with the breech up providing the choke area is at least.690 any tighter and it won't go over the rod.

Thanks guys, I spoke with the smith yesterday, I had it wrong! The MWT in each barrel was averaged out to .045 and .054, has factory 2 3/4 chambers and light target loads can be safely used. I thought I had written the numbers wrong, but I didn't they were correct the first time!

I know what you mean by the .45 and .54, but others won't. What you have written is 450 thousands, shy of 1/2" when it should be 45 thousands.
With those readings you should be fine to shoot loads under 8,000 psi. Like stated, readings from breech to for-end should be the ones to worry about.

Thanks David!I didnt notice the error before posting this morning!

Barrels vary in thickness at different points. They are often draw files after brazing and installation of the top and bottom rib, so they can end up thinner in some spots.

According to Brit methods, the barrel wall thickness is measured 9" ahead of the end of the chamber. I like to see 25 thousandths to 30 thousandths. Some 2" 12 gauge guns are below 18 thousandths...

Chukarguy: do you not mean 9" from the muzzle for .025" - .030"??

Courtesy of Hugh Lomas for English 12 bore Game guns.
Pre 1925 Proofs:
Chamber immediately prior to forcing cone - .105”
9” from breech - .045”
4-8” back from muzzle - .025”
Prior to 1924, the Belgian and British Service (using) maximum 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.

Post 1925 proofs:
Chamber immediately prior to forcing cone - .100”
9” from breech - .040”
4-8” back from muzzle - .025-.020” minimum
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 forumula) 9,682 psi.


Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives (C.I.P) http://www.cip-bobp.org/
Standard CIP proof pressure of 850 kg per sq. cm. (BAR) = 13,920 psi proof pressure = 10,730 psi service pressure (SAAMI)
Shotgun recommended minimum wall thickness (p.4) for Standard Steel with tensile strength 700 - 849 N/sq. mm = 101,526-123,137 psi
http://www.cip-bobp.org/sites/default/files/new_file/A-4-1_EN.pdf
10 & 12g
End of chamber - .079”
4” from breech - .075”
8” -.043”
12” -.030”

Most barrels that I have measured for myself and a few for others seemed to be thinner near the ribs and I believe that is from setting barrels for POI (point of impact)

I have a W.W. Greener circa 1892 that at the muzzle, inside left and right had to be filed and there is no land.

Have to find another site for uploading pictures, Photobucket now wants me to pay for 3rd party posting.
Photob

Pete Mazur uses an ultrasonic instrument to measure barrel wall thickness and it works very well.

The next time you speak with Pete, please inquire as to the specific ultrasonic wall thickness instrument he uses, and his minimal wall thickness criteria at the end of the chamber, 9" from breech, and overall MWT. Thank you.

Question: If you've been shooting your 12 gauge double for years and discover 0.014" wall thickness out near the muzzle, or anywhere else, do you quit shooting it?

In my case I had only shot it once at sporting clays. Ignorance is not bliss and although I had no problem with it once I knew things change. It is in the back of your mind and it affects your performance if not your safety and those around you. If it is way down the barrel where the pressures are lower there is probably little risk to life and limb. When looking at a feeler gauge at .014 and you see the thickness it gives you pause for thought. There are many guns out there that are fun. Better to replace it with one that reduces the risk. It is easy to dent and what do you think your barrel smith would say about fixing it? I sold it for parts and took a loss.

I wouldn't shoot a gun knowingly that had MWT of .014" near the chamber. I was told by a barrel man that at the muzzle, a garden hose could be safely used to contain the pressure. Gil

The critical measurement for wall thickness is traditionally 9" ahead of the breech. If this where most people measure them?

I'll probably talk to Pete in the next few days and I'll ask him.

The critical WT measurement(s) are every inch from breech to 16-18" (the location of the forward hand), esp. the end of the chamber and forcing cone if the chamber or cones have been lengthened.
End of chamber, 9" from breech and muzzle, and overall MWT are the numbers commonly reported. A dealer's listing of only MWT means little.
https://docs.google.com/document/d/1ZIo0y746UsSRZIgRuuxwAbZjSBHitO_EanvwLYc-kGA/edit

C guy - any follow up from Pete regarding his US wall thickness gauge and his acceptable numbers; end of chamber, 9" from breech and muzzle, and overall MWT?

Rule 18 of the "Rules, Regulations and Scales" published by the Guardians of the Birmingham Proof House stipulates the bore be measure 23 cm(9.055")from the breech face. Wall thickness must be considered the whole distance of the tube by especially in the area it could be dangerous as Drew has pointed out. The proof house does not have a minimum wall thickness required. Tube set must simply pass proof.

http://www.gunproof.com/Proof_Memoranda/RULESOFP.PDF

Haven't talked to Pete for a while. I am leaving shortly for fishing in Idaho, will talk to Pete when I get back and post up here to let you know.

Mike