We will have to stick to some standards here, and ASSUME that we are talking MODERN rounds, made in the last 10 years or so.
Rimfire and Shotgun ammunition will degrade faster than modern rifle rounds.
Simply because they aren't sealed and draw moisture.
You won't find a rimfire round that epoxies the bullet head onto the case, although some have wax, (lubricant) on the bullet, this is NOT a MOISTURE SEAL...
In all actuality, it's detrimental to the round since the wax will eventually break down and mix with powder/primer material.
(Remember, we are talking MODERN ROUNDS here...)
Most shot gun shells are not sealed at the front crimp.
The plastic cases ARE water tight sealed at the 'Brass' base, but primers are rarely sealed with shellac or epoxied in place to keep the moisture out.
Eventually, Moisture will work it's way into the round from the opening in the crimp in the nose, and will effect the powder charge also,
Most shot gun rounds now use synthetic for wadding, spacers, ect.
In the days of cardboard and corn/walnut media for packing, that stuff would swell and make the round hard to chamber/eject even if it didn't go off!
Paper husks on shot shells is ALWAYS a bad idea!
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Civilian RIFLE Ammo.
Most modern Center Fire ammunition uses a heavy, but even crimp on the bullet, and that helps keep a lot of moisture out of the cartridge case.
A lot of civilian 'Hunting' rounds use a shellac or epoxy to seal the primer from moisture, and that will contribute to keeping the rounds alive longer.
Modern powders use epoxy based binders in the powders they make, and that also helps keep the moisture/chemical reactions inside the cartridge to a minimum.
MILIARY/POLICE PRODUCTION.
The military REQUIRES that all primers be 'Swagged' into place once they are installed,
An arcane practice from the days when proper primer pocket holes couldn't be formed, and primers used to back out of the cartridges.
This 'Press Fitting' of the primer virtually ensures an air/water tight seal around the primer,
BUT,
The military still INSISTS that primers be shellac or epoxy sealed anyway...
(that's what leaves the colored 'goo' on your bolt faces!)
Military production rounds are HARD CRIMPED to make sure the bullet doesn't push back into the case as the bolts of Auto-Loading Firearms cycle and the bullet is banged on the breech on the way in.
That hard crimp around the bullet (all military rounds have a 'Cannlure' around the bullet, that's a groove with a textured finish like knurling).
Again, that cannlure is a throw back to early days when LEAD bullets used to shave off at the crimp, and get forced backwards into the case.
Besides the cannlure and hard crimp, military rounds use a sealer at the case neck to keep moisture out.
Military uses a 'Ball' powder that is slightly more stable for long term storage.
(Civilians use 'Flake, Stick, ect. since it has more consistent burning properties)
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WHAT YOU MUST REMEMBER!
The nickle plated, copper cup and steel pieces of the primer, along with the unstable primer material...
WILL REACT WITH,
The brass in the case, the powder, any moisture or trace of lubricants in the case from production, ect.
If you are storing open base bullets, you are adding lead and copper to the mix of chemicals all reacting with each other!
It's MUCH better to ROTATE STOCK so the ammo doesn't have a long time to 'Cook' Chemically!
What started out as 'Non-Corrosive' and 'Smokless' rounds can become duds, explosive, corrosive, squibs (under powered that stick the bullet in the barrel!)....
Something else bench shooters have known for years...
Old ammo IS NOT accurate ammo.
The longer it's stored, the less accurate it becomes.
Differences in muzzle velocity, corrosion on one side of the bullet from the caustic chemicals, and a host of other things will effect accuracy...
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DRY, COOL, STABLE ENVIRONMENT.
1. DRY!
There is NO SUCH THING as 'TOO DRY' for ammo!
The more moisture you can take out of the air around it, the less moisture that can migrate inside of cartridges!
Sealed packages inside of cans with desiccant would be ideal!
2. COOL.
NO HIGH TEMPS! Heating increases the chemical changes going on inside the cartridges.
3. STEADY TEMPERATURES.
NO BIG temp swings, NO FAST temp swings.
4. NO FREEZING!
Freezing will condense ANY vapors to liquids in most cases, and that includes off gassing from the powder/primer (usually acids),
Any off gassing from the trace elements, lubricants, case cleaners, ect. used during manufacture.
Any moisture or odd gasses that got in when putting the round together from the open air in the plant.
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IF YOU DON'T HAVE MILITARY LONG TERM 'BATTLE PACKS'...
Find an old refrigerator with magnetic door seal, and use some 'Great Stuff' expanding foam to seal up where the compressor and hardware come out from the bottom.
These things are FREE or dirt cheap at any repair shop or 'Coolant Evacuation' center in your towns...
If the seal is messed up, SPRING FOR THE $15 for the new door seals!
Install a Humidity control rod inside.
I use 'Golden Rod' brand because it's rebuildable, rechargeable, and CHEAP to operate.
This thing will remove the moisture from your power supplies 24/7/365 for 3 to 5 watts of power!
Refrigerator gives you a LIGHTED cabinet,
Sealed moisture free environment,
That can be easily secured with lock and key.
I keep powder & primer, welding rods/wire, ect. and it's worked great for the last 15 years.
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Another way to evacuate the oxygen/moisture from your storage cans...
1. Don't forget to grease the seals with SILICONE grease.
Petroleum grease will eat up the seals in military cans!
2. Use a MIG or TIG welder on 'PURGE' setting so all you get is gas out at the tip.
3. Get everything set up in the can, close the lid on the Welder nozzle, and use the nozzle to 'Purge' the air from the can...
Pull the nozzle out, then seal up the can right quick.
I find this works VERY WELL for everything from paint in a can you don't want the oxygen to skim over, to keeping the oxygen/moisture out of my powder cans,
To keeping my welding rods from absorbing moisture in there opened containers,
To ammo cans...
Keeping the 'AIR' out REALLY HELPS with corrosion of sensitive parts,
Stops Degradation, and keeps the effectiveness of the powders I'm using,
And has saved me a fortune in the Urethane coatings everyone wants for 'Cammo' now...
