Proof & marks

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A very brief history of Gun Proof

Before one start with the technical details, one must understand somewhat of the history of the way in which shotguns have been proofed in modern times. Over time powder has evolved and measurement methods and units has changed. On a national level a lot of countries have used different proof methods, but at the bottom line - the simple fact is that there have been no evolution in cartridge performance since we invented smokeless (nitro) powder. So the pellet of our grand grand fathers guns did not travel at higher speed or were propelled by higher pressure. So the proof cartridges might have been different in different countries as the proof laws, but when speed and pressure is measured with the same technique the difference is in many respects not of any importance at all in practical means.

Today, all International Proof Commission (CIP) Proof House use the Radial Transducer system to measure pressure, with bar as the unit of measurement. The old systems are not simply convertible by mathematical means. However, one can make approximate conversions.

The CIP requires that all cartridges that are sold must be tested to see that they comply with internationally agreed standards of pressure, dimensions and performance. The information given on boxes will show the case length in millimetres, the chamber length in both inches and millimetres and the shot load in both ounces and grams.

Under the CIP system, the minimum level of proof is based on chamber length. All that you need to do is to match CIP-approved cartridges, marked on the side of the box, to the chamber length of your CIP-proved gun.


Gauge and chamber length

Highest mean service pres. Tons per square inch lead crusher system

Converted to CIP Copper Crusher (bar)

Old proof Pressure converted to CIP  Copper Crusher (bar)

CIP proof pressure (min-max)

12 2 1/2”





12 2 3/4”

3 1/4




12 3”

3 1/2




12 3” MAG





Mathematical conversion factors

To convert tons per square inch to bar Multiply by 154.4441

Kilogram per square centimetre Multiply by 157.48775

To convert kilogram per square centimetre to bar Multiply by 0.980665

Tons per square inch Multiply by 0.0063497

To convert bar to kilogram per square centimetre Multiply by 1.019716

Tons per square inch Multiply by 0.0064748

Bore measurement

The measurement of bore shall be done 15-22 cm (5,9 to 8.7 inches) from the standing breech of the gun.


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Last updated: 2012-10-15