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AI Overview
Modern shotgun proof pressures are standardized by bodies like SAAMI (USA) and CIP (Europe) to ensure safety. For a 12-gauge shotgun, the maximum service pressure (MAP) is generally 11,500 psi (793 bar) under SAAMI, while CIP standards differ slightly. Proof loads, which are significantly higher, are used to test the strength of the barrel and ensure it can withstand these pressures and more, with tests often involving pressures around 19,800 psi (1365 bar) for a 12-gauge.

Proof Pressure vs. Service Pressure
Proof Pressure: This is the high-pressure load used to test the barrel's strength and integrity to ensure it's free of flaws and capable of withstanding the forces of normal operation.
Service Pressure (Maximum Average Pressure - MAP): This is the maximum pressure that modern ammunition is allowed to develop during normal firing for safe use in a shotgun.

Examples for 12-Gauge Shotguns
SAAMI (USA): The maximum average pressure (MAP) for 2¾-inch and 3-inch 12-gauge loads is 11,500 psi. The proof load used to test the barrels is much higher, around 19,800 psi.
CIP (Europe): Standards for 12-gauge shotguns are similar, often around 740 bar (10,730 psi) for standard loads, and 1320 bar (19,144 psi) for high-performance or magnum loads.

Proofing and Shotgun Strength
Barrel Strength: Shotgun barrels are built significantly stronger than the service pressure limits, creating a large safety margin.
Safety Factor: Proof loads are about 60% higher than the maximum average pressure (MAP) for standard loads.
Modern vs. Vintage: Proof marks on a firearm indicate that it has passed these tests, and for modern guns, this typically means they are safe for use with ammunition at or below the established service pressure limits.

AI responses may include mistakes.

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So...
there are some very high pressures quoted there, that would likely pipe-bomb a vintage game gun, with or without chokes.
You will see the proof marks on Clayton's new acquisition showing 3 tons per square inch. By direct calculation this is 2240 x 3 = 6720 psi.
It is likely the service pressure of a modern gun would equal or exceed the proof pressure of that early Boswell (and most of my SxS game guns as well!).

A lot of internet sites make a big deal about the "new" proof laws of 1954, and there is a general acceptance that British shotguns nitro-proved between 1896 and 1954 may only be safe with a 3 ton service load, perhaps 3 1/4 tons.

This brings us to the critical part of Simon Reinhold's interview where inspection of the gun, particularly barrel wall thickness (and general size of the locking components including breech dimensions IMHO), will usually tell you more than proof marks in an older gun.

Opening the chokes on a 100-year-old pigeon gun or fowler in good condition will probably give you safe use of normal commercial steel shot cartridges till the end of your life, whereas using them in a vintage game gun will probably give you the results you deserve!

Sad thing is, the gun wouldn't deserve it!

Something to clarify: are the "high performance" steel shotshells mentioned in the video all 3-inch, or can they be had in a 2 3/4 shell?
If so, an accident waiting to happen?
Must look that up.