Q: Articles on guns and shooting often warn about the dangers of how a 20-bore cartridge can be loaded accidentally into a 12-bore shotgun, and a 12-bore cartridge loaded on top. But does this also apply to other calibres?
A: I once conducted an experiment in order to find out which calibres and cartridges this risk applies to. Of course, chamber and bore dimensions vary quite widely in large-bore guns, particularly older ones. However, an 8-bore cartridge will lodge in the forcing cone of a 4-bore, allowing a 4-bore cartridge to be loaded on top of it, while a 10-bore cartridge can actually lodge in the choke. A 12-bore cartridge will lodge in an 8-bore, allowing an 8-bore cartridge to be loaded, and a 16-bore cartridge may lodge in the choke cone if the gun is tightly choked. If you are using a 10-bore, a 16-bore cartridge will lodge in the forcing cone of the chamber.
Never mix cartridges of different calibres
Turning to the smaller calibres, a 28-bore cartridge will lodge in the chamber of a 20-bore, while a .410 cartridge will lodge in the choke cone of a tightly choked 28-bore. The message from all this is plain: never mix cartridges of different calibres, and always check that the barrel is free from any obstruction or blockage before loading your gun.