Don't mix cartridges
Every responsible shooter should be aware of the all-important cartridge safety rules. You must 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.
If you are going to have both a 12-bore and 20-bore, you need to be absolutely scrupulous about making sure the cartridges don’t get mixed up. (Read how to choose the right cartridge for your shotgun.)
For an absolutely fail-safe arrangement, ensure that you only load your 20-bore from a 20-bore belt. As a 12-bore cartridge will not fit in the loops, that prevents any likelihood of a mix-up of cartridges.
Tipping a few loose cartridges into a pocket or bag can cause potential danger.
You should always check the bore of your gun before putting a cartridge into the chamber. You might miss a bird in a hot spot by doing so, but at least you will keep all your fingers by not double-loading. (Read the shooting safety rules here.)
Shotgun cartridge safety rules
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 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.