doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com What do proof houses measure?

I was wondering what is actually measured when a shotgun is proofed.

I have some basic knowledge in metallurgy (Mechanical Engineer) and a simple idea of what is fired for the proof load (150% load). But I really don't know what a proof house would inspect.

My assumption is that they are working on the basic theory that steel (non-damascus) does not deform elastically (stretch out of shape) until it reaches stress levels above 2/3 of its ultimate tensil strength (i.e. failure, think blown or cracked barrel).

So if we fire a 150% load without deforming the barrel, we should be able to fire a load 2/3 of that (normal or 100% load) on a consistant basis without stretching the barrel (assuming no interior wear from shot).

I would think the main work at the proof house would be determining if the barrel had deformed and a visual inspection for cracks or bulges. In theory measurement of the deformation would seem a simple task:
1. Determine the point of maximum pressure to the barrel (generally about 9 inches from what I've read)
2. Determine the inner diameter of the barrel at that point before firing
3. Determine the outer diameter of the barrel at that point before firing
4. Fire a proof load (determined by your choice of what your normal load would be multiplied by 150%)
5. Measure inner diameter of the barrel at the same point as in step 2
6. Measure the outer diameter at the same point as in step 3
7. Determine if there has been any deformation by subtracting step 2 from step 5 and step 3 from step 6
8. Examine for surface cracks

Basically it would be a determination of wall thickness at a given point and then a determination that the bore did not enlarge (i.e. wall thickness was not reduced). Followed by a surface inspection for fatigue cracks.

Does that sound about right for what a proof house does? Does anyone know for sure?

Here is some infro:
http://www.basc.org.uk/media/100_proof.pdf

There were multiple tests and measurements that used to occur. In the case of shotguns, the barrels were tested 1st. After they passed, then the barrels fitted to the action were tested. At one time, in Liege, there were 5 separate firings that could take place. Today, I am sure that has changed.

A general discussion
http://en.wikipedia.org/wiki/Proof_mark

Birmingham proof house.
http://www.gunproof.co.uk/Proofing/proofing.html

SAAMI standards
http://www.saami.org/Publications.cfm

Some interesting reading.
http://forums.nitroexpress.com/showflat....=&fpart=all

Any instrument used to measure has to be calibrated and certified. Also it is not just the barrels that are measured but also the action and the fit or lock-up of the two.

Pete