How Many Different Calibers Should I Own?

Simply put: as few as possible.

It is okay to have some variety, but having a dozen (or even half a dozen) different caliber rounds with associated weaponry is totally unnecessary in terms of survival practicality. Especially if you're mobile and you can only carry so much with you.

If you're a gun collector then this probably won't apply to you.

Here are few guidelines to help you understand my reasoning...

(*For the purpose of this conversation let's put shotguns in a category of their own because everyone should own one for at least home defense)

• Stick mostly with "common" calibers
• Overall availability, purchasing prices and the ability to easily find and scavenge later on if needed (I doubt you'll find a huge cache of 6mm Remington anywhere)
• You don't need multiple calibers for multiple firearms
• It's usually good to have a couple firearms that use the same ammunition (i.e. Springfield M1A and a Remington Model 700 both chambered in .308)
• Between rifles and pistols 3-5 different types of ammunition is good
• It's okay to have at least a couple less-than-common calibers and associated weapons as backups or even for bartering in the future (i.e. .300 Savage or 9x18mm), but don't rely heavily on them

An example of what an arsenal could be: 

• S&W M&P9 (9x19mm) [self defense/short range]
• GLOCK 17 (9x19mm)  [self defense/short range]
• GLOCK 30 (.45 ACP) [self defense/short range]
• Olympic Arms AR-15 (.223) [short-medium range]
• Mossberg MVP (.223) [medium-long range]
• AMD-65 (7.62x39mm) [short-medium range]

There are a couple of basic principals for both the firearms and the ammunition they use in the list above.

1. Useability
2. Practicality
3. Availability
4. Compatibility

In the example list I have: three pistols, two tactical rifles and one hunting rifle.  There are six guns overall and only 4 different calibers.

My theory on shotguns is that one should be loaded and near your bed (if you live in a state that actually trusts you with a loaded gun) so it shouldn't need to be listed.