I look at ammo cans the same way I do other cases, as a form of insurance. Spending less than twenty bucks on an ammo can to protect a few hundred bucks in ammo is a simple decision for me. Check the seal, put the ammo in the can, toss in some silica gel and it will be there waiting for you in a year or a decade.
We use mostly 50 caliber ammo cans but have a few of the big 120mm mortar cans too. The 50 is a good size because they hold a lot but yet small enough to easily move and fit into nooks and crannies. The big 120mm mortar ones hold a ton of ammo but loaded weigh a lot. Probably a couple hundred pounds when fully loaded. Really a hassle but they have the advantage of a really low cost per round/case stored. Odds are I will reallocate things at some point and use them for something a bit lighter.
Now comes the question of how to load your cans. I would recommend having some sort of logical system before you get started. It is a lot easier to maintain a system (and inventory) then it is to impose one retroactively. We will discuss what I have going on and what is probably optimal.
I have one can loaded with the stuff I regularly use when home on leave. It has mags for the Glock 19 and AK as well as ammo to fill them and some more to keep handy. Also a holster for the Glock and a Cold Steel folder. The purpose of this can is to avoid digging around to find the things I need and keep things easy. I did this the trip home before last and it seems to be working pretty well.
The rest of my cans, I am ashameed to say are a complete mess. Some are all one caliber, others are a mix, there really is no rhym or reason. If I need something it is guess the ammo can time. Really the only good thing I can say about the system (or lack theirof) I am using is that a) my ammo is stored in ammo cans and b) they have sillica gel inside.
The optimal system, as far as I can see, is as follows:
-Special purpose cans. I have the home carry can. Another guy might have a hunting can or whatever else suits your specific purposes.
-Combat loaded cans. Combat loading in this context is a way of loading your supplies so things are spread around. The goal is to have a logical mix of whatever you need in multiple locations. So if hypothetically you can only move a few cans of ammo to a new location in a hurry you wouldn’t accidentally end up with just pistol and shotgun ammo but no rifle and rimfire or whatever. I don’t think you need to get too crazy about this. For example you probably don’t need a box of .220 Swift for the old varmiter Grampa left you or .32 S&W for the antique safe queen you picked up years ago in every can. I would focus mostly on the core guns you use and rely on like your defensive rifle and pistol, a hunting rifle, rimfire and shotgun.
Having a portion of your ammunition combat loaded makes sense. How much I can’t say exactly. I would say at least as much as you plan to have make the first cut if you have to leave town for whatever reason in a hurry. [Yes I would like to say that I would take all my ammo but that may not be realistic. Lets say we define (this is off the cuff) the first cut as in and on top of the vehicle, the second as a trailer and the third as a potential second trip, etc. What is loaded in our BOB's and chest rigs plus 2-3 cans would probably be all that could make the first cut when you consider extra food, camping gear, photo albums, etc. The rest of the cans would have to go in a trailer or whatever. Obviously bringing 30k rounds of ammo or whatever would be more of a concern during a Mad Max scenario than if we are leaving because of a more mundane event like hurricane or other natural disaster. ] So maybe 2-4 50 cal cans worth would probably make sense for most folks.
-The remainder of ammo would be loaded into cans by caliber and if applicable type of cartridge.
All cans would be labeled with at least caliber and quantity and if practical/ applicable type, it could be as simple as “7.62×39 2,200 rounds”. I would probably do it using permenant black marker on duct tape so it is easy to remove/ change. Depending on your scenario you may want to have it be discrete for opsec. However folks probably won’t think you are storing chocolate bars in ammo cans anyway so I would just think about where I store them and who I let go there.
Anyway those are my thoughts on that.