While there are exceptions to the rule the first number is the calibre and the second is the weight in grains of the black powder charge(7000 grains in a pound).

I was wondering why some of the older cartridges are named the way they are such as 45-70 45-120 30-30 38-55 etc, what method did they use to name old cartridges

Good question.
Mostly it is a caliber/powder charge designation, sometimes with a third number for bullet weight.
The original BP load for the 45-70 was 70g of black powder, eg.
The bore nominations are a little screwey, but we still do that today, with the 300 WSM for a .308" bullet. Mostly marketing now, likely the same back then.

There's always some exceptions to the rule, but the basic answer for the cartridges you listed is .caliber - black powder charge



Example: .45-70 = .45 caliber and 70 grains of black powder.



Also, the older designation took bullet weight into account.



Example: .45-70-405 = .45 caliber, 70 grains of black powder, 405 gr bullet.



From your list, the .30-30 is the exception. I think (I could be mistaken) that it meant 30 grains of smokeless powder. But Winchester also called this cartridge the .30 WCF if I'm not mistaken. About the time the .30-30 came out, cartridge nomenclature was in transition. The .30-30 and .30-40 Krag are good examples. The second number supposedly denoted the smokeless powder charge.



Some of the more knowledgable posters here will probably have more accurate, detailed responses to your question than I have provided.



-Bob F.

The bore designations are not that "screwey". Your 300 started with that size "bore" (as the British say).
.308 is the grove diameter after rifling. The 38's with their .357 bullets result from going from outside
lubed bullets (same size as the case OD to inside lubed bullets. There is a reason for most things.
Take care!

Another exception would be in the case of the 30-06 Springfield!

30-06
30 = caliber and 06 = the year it was introduced, 1906!

Before that, it was designated 30-03. I don't know what the exact difference is, but in 1906 it was redesignated the 30-06 Springfield!

30-Govt-03 was adopted in 1903 to replace the 30-Govt (30-40Krag), same 220gr RN Ball bullet as the Krag and a slightly longer neck than the -06 case.
In response to the new rounds comming from europe (read Mauser designs), the 30-Govt-06 was adopted with a 150gr Spitzer bullet and the -03 neck length shortened to what we are familier with today.

There`s not a lot of reasoning behind cartridge names here in the states, 30-40 = 30 cal/40 gr powder, 30-06 = 30 cal approved in 1906, 25-06 = 25 cal/ 06 parent case.
The europeans were a bit better with cal x case lenght in milimeters, 7x57 or 6.5x55.
I think it all boils down to "it`s mine and I`ll call it what I want" from the designers.................

Cummins:
FWIW, originally, most of the older cartridge designations included the case length. The .45-70 was the .45-2.1, the 45-90 being the .45-2.4 & the .45-120 is a .45-3 1/4. If you order a current production BP cartridge rifle such as a Sharps or Ballard, the above chamberings would be so marked on the barrel.
The reason being that BP cartridges were loaded heavily with powder. Unlike smokeless, you do not allow an air space with BP. The use of the powder weight designation (ie. 70 grs. for a .45-70) could be a bit misleading. In my .45-120 for example, I load 110 grs. of Fg with a 530 gr. paper patch bullet. There is no way would it could hold 120 grs. of Fg. It might hold 120 grs. of FFg but I assure you it would be very unpleasant to shoot.
There, have I muddied the waters for you even more?
Bear in Fairbanks

Designating caliber and stating cartridge names are two different processes.

Caliber is the bore (not groove) diameter expressed in hundredths of an inch. So, we have 45 Caliber (not .45 Caliber, as is so often written) for a bore that measures 0.45 inch. Rifling came later, so the groove diameter is nominally 7 or 8 thousandths larger, but the caliber remains an expression of the bore diameter, even so.

Cartridge nomenclature diverges into several paths and there is no real standard for anything. Metric uses bore diameter X case length (e.g., 7mm x 57 mm), but even there, we have slight variation, such as the baloney for the 8 mm Mauser, which can be 3 or so designations for the same cartridge. British use is largely proprietary, and so are many American (.600 Nitro Express, .45 Colt), but so many exceptions exist as to make any system useless. For example, .375 H&H is American for some other real name that I have forgotten by now, and .30 Army is now .30-40 Krag.

Sometimes, it pays to use a system, such as that already stated on the black powder cartridges; .45-70-500 is a good example. However, we have the .30-30 (.30 W.C.F.) and the .30-40 Krag that were named with the black powder system for their caliber bullet and velocity, but never loaded with black powder commerically.

Read a lot. Think on this a lot. Then just resolve to memorize the examples. It'll work and you'll have fun.

Geo.

Chuck, it is my understanding that the 30-03 was a case of 2.54" or 64 m/m, just like the .270 Winchester. Way back then it was loaded with the same bullets that the Krag used, a 220 grain round nose. In '06 the ordinance dept. went to a lighter spitzer bullet of 150 grains. With the shorter bullet it was decided that the case could be shortened .007", that's how we got the 2.494" case. All of this was done in response to what Germany had done with the 7.9X57

Geo; Just to muddy the water some more: The Navy uses "caliber" to describe the length of a gun
barrel in bore diameters. eg. 5x36. One thing about reading up on firearms, the more you learn the less
you know. Some things make at least a little sense, buy why is a 38/40 a .401 ?.
Good Luck!

As everyone above has stated, the first number is the bullet size, e.g., ".45", the second is the weight (or volume measure) of black powder, and to complicate matters even further, these old designation also often gave the bullet weight in grains. For example, .45/70/500 (the original government infantry .45/70 load) or .45/55/405, the original government .45/70 cavalry carbine load using the EXACT SAME CASE, but with less powder and a lighter bullet than the infantry load!

Irv--that's true, but then that is not small arms usage either. Bore diameters are still referred to as, in the case of the Yamoto Maru, 18.1 inch. 5 x 36 tells us more than the bore diamter, it tells the barrel length of those big rifles. The "rifles" that the navy uses are a class unto themselves. Geo.

Eldeg.--Irv hit the nail on the head nicely. There are examples of using case diameters, groove diameters, and so on. Look at the Lazzeroni cartridges system--he apparently uses groove diameter. Winchester and Sharps used essentially the same cases, but loaded different weight and diameter bullets because some were paper patched and some were lube-grooved cast bullets (.45-90 vs .45-2&4/10 in.). One is the sharps designation and one is the Winchester-type designation. It always has been a lot of fun, I hope that it is so for you.