It’s a problem most gun owners never encounter, but it’s one just about every gunsmith has seen. Someone brings in a gun with no caliber designation. The question becomes what caliber is it and how can that be determined.
Over the years I’ve encountered this situation a number of times and have seen it dealt with in many different ways—some good, some bad. Perhaps the worst and potentially most dangerous were the cases where folks just kept trying to chamber different cartridges until the bolt closed on one. As far as they were concerned, that was the correct caliber!
Needless to say, that procedure is dangerous. In fact, I heard recently of a young guy who did it and ended up touching off a 7.62x39mm round in a rifle that was actually chambered for .243 Winchester. There was considerable damage to the rifle, but fortunately the shooter’s injuries were minimal. I also recall a guy who came into my shop years ago with an old, beat-up Japanese Arisaka rifle in 7.7mm. I happened to ask him what he was shooting in it since ammo in that caliber was kind of hard to find at the time. He proudly pulled out a box of Remington 8mm Mauser cartridges! I remember being absolutely speechless at the time. I couldn’t believe it. It was truly a testament to the strength of the old Arisaka that neither he nor his rifle had been injured.