Once or twice in your shooting life, you will have a shotgun cartridge that’s the right bore/gauge, but it won’t fit in the gun you are using. The cartridge is too long. More often in your shooting life, you will have a shotgun cartridge that will fit in the gun – but it’s still the wrong size – and that can damage the gun.
As well as the calibre (gauge/bore) of the shotgun, you have to consider the length of the cartridge, especially if you are using an old gun.
Matt Simpson from Simpson Brothers Gunshop explains why it’s vital to use the right shotgun cartridge for the right gun. “Commonly we get people come in the shop asking for certain shotgun cartridge sizes for their guns but it’ll be the wrong length.”
The usual problem is with 12-bore shotgun cartridge sizes: a 2½in (65mm) chamber will fit a 3in (76mm) cartridge, but it won’t do it any good.
Matt shows off a Cogswell & Harrison gun with a 2½in chamber. “That gun is proofed as being safe with a 65mm or 2½in-length cartridge,” he says.
He has another gun that’s proofed as being safe with a 3in cartridge.
“What commonly happens is people have put a 70mm into a 65mm chambered gun,” he says. “This is very unsafe. This gun is not proofed as being safe with that length cartridge.”
“If I take this CIP (EU firearms safety standards authority) measurement tool and insert it into the Cogswell & Harrison, it stops at just shy of 65mm, whereas if I were to put that into the Beretta, which is proofed as being safe with 3in cartridges, I get 76.2mm, which is 3in. Yet I can get this 3in cartridge into this 2½in chamber and it would go in and fire.”
The measurement of the cartridge is in the “fired form”, which is why a 70mm cartridge will fit into a 65mm chamber.
“There are a lot of pressures that shouldn’t be where they are – and it’s not proved as being safe with that cartridge,” he says, “so you can get massive ejector problems. They’re not ejecting. There’s just not the space in the forcing cone or the chamber because it hasn’t been allowed for that length cartridge. That is a very common problem.”
More information about Simpson Brothers Gunshop here.
