What's the difference between a caliber and a cartridge?

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I like to consider the way things are used.
When you go to your LGS to buy ammo, are you buying calibers or cartridges?
Q.“Can I help you, fella?” A. “Well, yeah, I need a few boxes of calibers.”
When you load magazines for your pistol……what is it that you are loading into the mags?
 
...Its almost like the industry wants things to be a bit confusing.
It's just good old fashioned capitalism, going back to a glorious era before there was any attempt at standardization. A time when gunmakers fought tooth and nail for their share of the American market and laid down the foundation for the traditions we, in turn, enjoy and complain about. It's given us a history, with it's twists and turns, ego and humility, that's completely fascinating. One can either stare at the Gordian knot of the various calibers in confusion, or grab a thread in both hands and, in the name of research, dive headlong into the tangled mess to see where it takes us.

That's why I love it so.
 
Caliber can also be the length of the barrel divided by the bore diameter.

ie. 5" 50 caliber... A Navel gun with a 5" diameter bore and a 250" (20 ft, 10 in) long barrel. https://en.m.wikipedia.org/wiki/5-inch/50-caliber_gun

It becomes useful when there are guns of the same bore diameter, but use different sized cartridges. It is easier to see/measure the barrel length than the chamber dimension when selecting ammo for it.
Actually, that use of caliber is the same as the SAAMI definition given above, ie the bore diameter. And you have the equation backwards - the length of the barrel is the caliber times 50.

Let me add to the confusion.
In artillery, "caliber" is the ratio between bore diameter and bore length, expressed as Caliber (Navy) or Length (Army).
USS Arizona had 5" bore guns in 5"/51 Caliber (255" barrel) and 5"/25 Caliber (125" barrel) for anti-ship and anti-aircraft secondary armament.
In US Army terms those would be 127mm L/51 and 127mm L/25.
Again you have it backwards, the barrel length (less chamber) is defined in bore diameters (calibers)

The reason why is because when you start to mess with interior ballistics and barrel design, one ratio that will keep popping up the the length to bore diameter, so you always just define the barrel length in bore diameters, or calibers.
 
Actually, that use of caliber is the same as the SAAMI definition given above, ie the bore diameter. And you have the equation backwards - the length of the barrel is the caliber times 50.


Again you have it backwards, the barrel length (less chamber) is defined in bore diameters (calibers)

The reason why is because when you start to mess with interior ballistics and barrel design, one ratio that will keep popping up the the length to bore diameter, so you always just define the barrel length in bore diameters, or calibers.

I have always wondered why barrel length denoted in calibers has not crept into small arms, specifically rifles and long range shooting. It's not going to change things too dramatically but it is interesting from an internal ballistics point of view. Maybe I have just spent too much time with Quickloads and internal ballistics text books...

Are you sure the barrel length is measure in less chamber in the big guns? I have tried to find that explicitly mentioned in text for both historic and modern larger weapon systems and have not found definitively if the chamber was included. With small arms we included the chamber for most measurements (except revolvers). Any sources on this you can share would be appreciated.
 
Actually, it appears to be with chamber, but for those I can find the chamber is not more than an extension of the bore, ie not a bottle neck cartridge shape.

By the way, an M16 with a 20 inch barrel would be .219/L91, an M4 with a 14.5 inch barrel would be a .219/L66.

Ballistically, very long compared to the 24 caliber length of a 105mm howitzer, or even the 62 caliber gun in the Mk 45, Mod 4 gun mount . . .
 
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