doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Minimum wall thickness

Yes, I used the anglocised acronym to avoid confusion, but doing so I created it. Oops.

The CIP sells a series of technical publications-decisions and among them is one with the minimum barrel thicknesses for the three ISO types of steel the Commission says are most often used for shotgun barrels. Note that this is a recommendation and not a binding rule. In other words a gun can still be submited for proof and will be stamped as proved if it passes even if the wall thickness is less than any specified in the recommendation.

Thank you.

The Permanent International Commission for the Proof of Small Arms is better known as C.I.P., Commission Internationale Permanente pour l'Epreuve des Armes � Feu Portatives
http://www.cip-bobp.org

Here on p.4. I'm working on harvesting from the PDF
http://www.cip-bobp.org/sites/default/files/new_file/A-4-1_EN.pdf

Corrected

Well, Drew, your data points just eliminated 99.5% of the guns built before WW1.

I think the measurements are in millimeters and 0,30 of a millimetera is about 15 thousandths of an inch so most current guns do not fail.

But notice that this is the minimum recommended thickness for modern high strength steels. some extra is needed to provide a safety factor over and above the elastic limits of steel and for some metal removal in case of honing or external polishing.

Thought of that, but 1mm = about .040" and no way the MWT in Categorie 1 steel would be .0787" just past the forcing cones and the end of the chamber in 12g, or .040" in Categorie 4.
8" 1.1mm = .043", 12".75mm = .030", 16" .60mm = .024"???

Someone a lot smarter needs to explain the steel categories



Assuming Categorie 1 is Standard Steel, for 12g the MWT numbers are:
End of the chamber - .079"
Just past the forcing cone/ 4� from breech - .075�
8� - .043�
12� - .030�
16� - .024�
20� - .022�

Post 1925 British proof recommended courtesy of Hugh Lomas:
Chamber Immediately prior to Forcing Cone - .100"
9" from Breech - .040"
4-8" back from muzzle - .025-.020"minimum




I'm still amazed at the end of the chamber and 4" numbers.

The SXSs that I have measured made in the last 50 years are very close to the chart. Pre WWII some are pretty robust.

I have a Spanish Aramberri 12 ga made in 71 2 3/4" proofed at 1200 kgs. The walls at chamber's end is .082; 4" .096
I have a 3" Ugartechea Falcon 12 ga. made in 72 proofed at 900 kgs. Chamber's end wall is .075 I don't shoot 3" in any field gun though.

I believe that the barrels are thinnest up 8-10 inches behind the muzzles is for two reasons. 1) it is safe to have the barrels thinner up there and 2) if you have ever filed on a set of barrels the middle of the barrels is where it is easiest to grind steel off! After trying to even up a set of Ruger barrels I have profound respect for craftsman who make barrels perfectly smooth without taking too much metal off.
Regards,
Jeff

The CIP figures are plain engineering figures pitting pressure against steel strength and the resulting figures are somewhat alarming. There is no way in the world I would trust a 12 gauge with 1.05 mm thickness at the chamber ends!!!

However, practically it is virtually impossible to work with those dimensions, firstly because the barrels would be impossibly light and whippy, secondly because the CIP figures do not include safety factors, gunmakers usually work to a factor of 2, so they would choose at least twice the thickness, and also because you need some spare metal for repairs and rebluing.

Even a dense designer must realise his creation will span several generations of users and will face repairs.

Having miked some of my doubles, it is reassuring to see that the thickness at the chamber/forcing cone junction is at least twice the recommended CIP thickness and in the 100 to 120 thou range in all except a Westley Richards droplock which is due for rebarreling anyway.

Other doubles I miked turned out to be in the 80 thou range for most Spanish and over 100 for most Italian, British and French.

Striking up barrels is a tough one. It gets easier with hollowed out backing blocks for the emery cloth, but it is never an easy job. Here you appreciate the monobloc method of barrel making that lets you do the maximum finishing on the tubes, using a lathe before they are put into the monobloc. In some modern lathes there are sensors that continuously monitor tube thickness and automatically adjust for the desired contour.