Q: Are the terms “magnum” and “3in” synonymous? In other words, if a gun is marked “magnum” does that mean it will have a 3in chamber — and is a 3in shotgun shell the same thing as a 3in magnum shotgun cartridge case?
A: With regard to the proofing of shotguns, “magnum” is a legacy term that is no longer current. Under the 1954 Rules of Proof, 3in 12-bore shotguns were proofed at either 3½ton or 4ton per square inch, the abbreviation MAG being applied to the latter. However, the word magnum is now only used in relation to proofing rifles of particular designations such as 7mm Remington Magnum or .300 Winchester Short Magnum.
Different types of proof
Modern shotgun proof operates to International Proof Commission (CIP) standards, in which the level of proof is matched to chamber length. Shotguns are proofed either to Standard Proof, to take a 70mm cartridge with a maximum mean service pressure of 740 bar, or to Superior Proof, to take a cartridge of 76mm or more with a maximum mean service pressure of 1,050 bar.
When you buy a box of CIP-approved cartridges, you simply match the cartridge length, shown on the box in millimetres, to that of the chambers of your gun, ensuring that the cartridge you select is no greater in length than the chamber for which it is intended.
What if you have an older shotgun?
If you have an older shotgun marked 123MAG, it may be used with CIP-approved cartridges marked 12/76. In addition, there is a further standard of proof for High Performance steel shot cartridges. High Performance steel may only be used in guns that are marked with the CIP-approved eur-de-lis symbol. If you are in any doubt as to the suitability of your gun for use with a particular cartridge, consult a competent gunsmith.