Never mix cartridges of different calibres
Q: I know that a 20-bore cartridge will fit inside a 12-bore gun and allow a 12-bore cartridge to be loaded on top of it, but does the same apply to other bore and cartridge sizes?
A: Yes. If a .410 cartridge is loaded into a 28-bore, it will drop right through the barrel tube, but will lodge in the forcing cone of a tight choke, allowing a 28-bore cartridge to be loaded and fired. You may not lose any fingers when the gun blows up, but you will almost certainly destroy the barrel.
A 28-bore cartridge will lodge in the forcing cone of a 20-bore, and as we all know, a 20-bore cartridge will lodge in the forcing cone of a 12-bore.
Despite the fact that most 20-bore cartridges are coloured yellow, almost every season one hears of an occurrence of this particular accident.
Incidentally, a 16-bore cartridge will also fit inside a 12-bore chamber, but not sufficiently deeply to allow a 12-bore cartridge to be loaded as well.
The same cartridge safety rules warning applies to the larger calibres. A 12-bore cartridge will lodge in the forcing cone of a 10-bore, while both 12- and 10-bore cartridges will lodge in the forcing cone of an 8-bore. As to the mighty 4-bore, well, an 8-bore cartridge will lodge in the forcing cone, while a 10-bore cartridge will lodge in the choke.
The moral: never mix cartridges of different calibres. To do so is exceptionally dangerous, and could result in serious injury or even death, not to say the needless destruction of your gun.
The first rule of good shooting is always safety
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Why does nobody reload shotgun cartridges in the UK?
Mike George advises: There are two main reasons. The first is that, for many years, cartridges in the UK have been much cheaper than in the USA, and most other European countries.
Price comparisons now show not that much difference between US and UK prices.
Nevertheless, British shooters never really acquired the reloading habit. More importantly, perhaps, you may not use home-loaded cartridges in CPSA competitions.
There’s the safety angle to be considered, plus the possibility of loading more than 28g of shot in a cartridge, and therefore cheating.
I don’t think many shooting grounds would allow you to use home loads in informal or club competitions, either.
You could load your own for field shooting, but you would have to load a very large number of cartridges to justify the cost of the equipment and components – unless you get together with a group of friends to share the start up costs.
As you suggest, the only type of shooting in which home loading makes any sense is wildfowling, and then only if you shoot a big-bore gun for which the ammunition may be either very expensive, or unobtainable.