Other than during the process of actually firing the shotgun, I cannot imagine another use for C0. This is certainly not a recommend condition for carry, handling, or storage.
Condition 1 - A shell is in the chamber, the hammer cocked, the magazine is loaded, and the safety is on.
C1 is usually go-mode in my opinion. The possibility of having to discharge the shotgun is imminent. Depending upon individual circumstance, some will maintain their shotgun ready in this condition. Personally, I do not, nor do I recommend it as a day-to-day practice. Some will say that shotguns are not drop safe. Drop safe or not, to the best of my knowledge most modern, higher quality, well maintained, shotguns will not discharge if simply knocked over, or at least I’ve never heard of it happening. However, why take that risk? The decision is yours, based upon your reality and requirements.
Condition 2 - The chamber is empty, the hammer is uncocked (thus unlocking the action), the magazine is loaded, and the safety is on*.
AKA “Cruiser Ready”. Unlike C2 for an automatic pistol (hammer down on a loaded chamber), C2 for a shotgun has the hammer down on an empty chamber. This is because, to the best of my knowledge, so-called “hammerless” shotguns cannot be loaded (or unloaded) without the hammer being cocked. To ready the C2 shotgun for firing, one must operate the shotgun’s pump action (or bolt charging handle for an automatic), which is unlocked. This instead of thumb cocking a C2 pistol’s hammer. Depending upon the condition of the gun and the circumstances of storage/handling, i.e. bouncing around in a vehicle for one, I have heard that there is a possibility of the action opening on it’s own. This is something of which to be aware.
Condition 3 - The chamber is empty, the hammer is cocked (thus locking the action), the magazine is loaded, and the safety is on.
AKA “Cruiser Safe”. It’s the same as C2 except one is now required to manually unlock the action in order to chamber a shell. It’s an additional step, albeit a minor one. As far as I am aware, this is the SOP for many police and security organizations. It assures the action will remain closed and, as a bonus, it’s an additional precaution against unauthorized and untrained people using the weapon.
Condition 4 - The chamber is empty, hammer may or may not be cocked (your option), the magazine is empty, and the safety is on.
In other words, the gun is completely unloaded. It is essentially a safe handling, transportation, and storage condition. This is also the prelude to disassembly and cleaning. Be that as it may, the absolute first thing that one must do when the shotgun, pump or automatic, is handled, is to clear or proof it. The action must be opened and the chamber, carrier or elevator, and magazine checked. Furthermore, when handing the shotgun (or any firearm) to anyone, the action should be cleared and open.
* With any empty chamber method, the weapon can be kept with the safety off, it’s your option. Nonetheless, as a matter of practice, I default to safety on for consistency's sake.
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If you own a pump or automatic shotgun at some point you will undoubtedly use all of the above. Assuming you want to keep a shotgun handy, just in case, your choice is really down to conditions 1, 2, or 3. My advice is to get some training shells and try them. Then pick a set that works for you and stick with those. Make them your SOP. Always let safety be the deciding factor. You own firearms to protect yourself, your family, and possibly others, not to endanger them.
Addendum: Re C2 and C3; please don’t think that the sound of the action being operated is a benefit. In my estimation it is not. If anything it is a detriment as it lets any malefactors know your location and that you’ve got a really big gun. Don’t bet your life on that mystical sound putting fear into the hearts of villains and causing them to flee for their lives, possibly even giving up a life of crime altogether. If it does, ya got lucky, no more, no less. This is not to say that I am recommending C1 instead, I am not. Only you can decide what works out best for you.
As well, I was going to either add comments, pertaining specifically to the automatic shotgun, or make a separate set of conditions. Finally, due to operational differences and brand specific variables, I decided upon neither. C0, C1, and C4 all apply directly to either. C2 and C3 simply become empty chamber carry for the semi. I suppose one could have the bolt locked back with a loaded magazine, however I cannot imagine any reason to carry or store the shotgun in that condition.
Truth be known and as I’m sure you know as well, for automatic pistols and most repeating long guns, there’s really only two basic ready conditions.
Condition 1: Chamber is loaded, magazine full, and safety on.
Condition 2: Chamber is empty, magazine full, and safety on or off.
All the rest is just gun rag tripe, but people like that stuff.