doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Barrel wall thickness, what do the numbers mean?

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 � 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