I want to add a bit of clarity here regarding a second cylinder, as in being able to shoot different ammo. Having a second cylinder allow you to shoot the secondary ammo without the need for any moon clips. You just load the cylinder straight away with the secondary ammo. For instance, you might want to be able to shoot both .45LC ammo as well as .45AQCP and .45 Super, all with the same gun.
Or you may want to be able to shoot .357 Mag ammo, .38 Spec ammo, and 9MM ammo as well, all with the same gun. Both of these scenarios are possible, but only with single action revolvers, which allow you to swap out the cylinder very easily. This procedure takes only 5-10 seconds to do with single action revolvers.
But this is all very different with double action revolvers. You cannot take the cylinder out at all, unless you tear the gun apart, which is a very long process, and not one anyone would do when just shooting. To be able to shoot multiple 'calibers' with the same double action revolver, you will need either moon clips, so that the shorter non-rimmed ammo stays put, and does not fall into the cylinder.
In both cases I mention above, it is where you would want to be able to shoot both rimmed and non rimed ammo from the same gun. The non-rimmed ammo is what is used in semi auto guns, where the ammo needs to slide freely from its magazine into the chamber, which is also always the barrel.
Where the ammo ends up in the gun before being fired is the difference between revolvers and pistols. A pistol is a handgun were the round is loaded into the barrel before firing it. Think back to the 16th-18th century of the pirate days and such, where pistols were always loaded from the end of the barrel, just as muskets were. With the advent of the semi-auto weapons and its ammo, the cartridge is shoved from its magazine into the barrel, and then fired from there, instead of having to muzzle load the bullet manually.
Revolvers do not load anything into the barrel, and the ammo stays in the 'revolving' cylinder until it is actually fired. This very basic difference ends up with different ammo requirements between a semi-auto gun and a revolver, where the semi-auto ammo is non rimmed so it can proceed easily and quickly along its path, all prior to actually shooting it. And its path destination is the barrel! The round is actually in the barrel when it gets fired. But the ammo for a revolver is rimmed so it can stay at the firing end of the cylinder, and without the rim, the cartridge would simply fall into the cylinder and would never get fired.
So, given this very basic ammo difference, with single action revolvers you can take out the cylinder and replace it with one designed to hold the non-rimmed ammo so that the round does not slip into the cylinder. But with double action revolvers, you cannot take the cylinder out, so the use of moon clips is required when also shooting semi-auto ammo, which then holds this rimless semi-auto ammo at the firing end of the cylinder, making sure that the round does not fall into the cylinder, and hence, that it can be fired.
In Ruger terms, all the 'Blackhawks', including Super Blackhawks, are single action revolvers, and you can swap the cylinders easily. But all the 'Redhawks' and 'GPs' are double action revolvers. So, with these latter gun types, you cannot swap cylinders, and moon clips are required to shoot the same bore ammo which are shorter than the ammo the gun was designed for. So, this whole idea of shooting multiple ammo types from the same gun only works if the gun was initially designed for the longer ammo. If the gun is designed originally for the shorter ammo, then the larger ammo would not fit into the cylinder at all.
One last note regarding dual ammo shooting - all .357 Mag revolvers can shoot both .357 Mag ammo and .38 Special ammo, and without any cylinder swap or moon clips because both are rimmed ammo, and the cylinder holds both types at the firing end so they both can be fired. So, rimmed ammo is really 'revolver ammo', and non rimmed ammo is semi-auto ammo for pistols, which is 'pistol ammo'. So, when you are online, or just talking to someone, and they use the term 'pistol' ammo, as a result of their ignorance, they often use this term interchangeably, which of course could be very misleading.
Just about all the powerful handgun ammo is made for revolvers, and thus is rimmed ammo. This includes the following calibers: .41 Mag., .41 spec, .44 mag, .44 Spec, .45LC. .454 Casull, .45-70, .460 Roland, .460 S&W, .475 Linebaugh, .500 Linebaugh, and the super powerful .500 S&W.
