Regarding bore size, bullet diameter, and cartridge names, there is a relationship, but once you get past the first number, its entirely a matter of what the maker wants to call it.
English or metric system, rounding up or down, measuring bore (land) or groove diameter, or even a name based on the historical parentage of the round and not related to actual bullet diameter (after the first digit).
Also the cartridge name can even be a combination of these things, or just something chosen for its market appeal and different from other names.
And, straight math conversions (primarily metric to english) don't always work out "right" (to match bullet diameter to 3 decimal places) either.
5.56mm converts to .218"
7mm is .275"
7.62mm is .300"
etc.
Nation of origin and its manufacturer's customs also play a part in the cartridge name.
examples abound. 7x57mm, 7mm Mauser, and .275 Rigby are all the same round, depending if you are in Europe, the USA, or England.
9mm Luger is 9mm Parabellum (original maker's name) 9x19mm (standard metric designation) or 9mm Luger (US name).
8mm Mauser, 8x57mm are commonly used, but the German military designation is 7.92mm...
.22 Hornet, .218 Bee, .219 Zipper, .221 Fireball, .222Rem, .222 Rem Mag, .223 Rem/5.56x45mm, .224 Weatherby, .225 Winchester, and .22-250 all shoot the same .224" diameter bullets.
US .30 caliber rifles use .308 bullets, but Russian ".30s" 7.62mm usually use .311-.312" bullets, as does the .303 British, and the 7.7x58mm Arisaka rounds.
It gets even more "into the weeds" when you look at the names of pistol rounds vs. their bullet diameters, today.
Like why our ".38s" shoot .36 caliber bullets (.358") and why our ".44s" shoot ".43" (.429") slugs. The reason is due to the history of metallic cartridge and bullet development, AND, as always, marketing.
Regarding IMR powder number designations in their 4 digit series, I don't know the actual reason for each number, but I've always suspected it had something to do with each one's characteristics with each number of the code system meaning something specific to the maker.
Like the military ID systems. For example, the Army designator for a WWII "deuce and a half" truck was CCKW the letters being code for a 6 wheel drive truck. The amphibious version was the DUKW, becoming nicknamed the "duck" for obvious reasons (it swam).
When it comes to names for firearms related things, there are general conventions that are usually followed, but there are no hard and fast rules, not even SAAMI.
English or metric system, rounding up or down, measuring bore (land) or groove diameter, or even a name based on the historical parentage of the round and not related to actual bullet diameter (after the first digit).
Also the cartridge name can even be a combination of these things, or just something chosen for its market appeal and different from other names.
And, straight math conversions (primarily metric to english) don't always work out "right" (to match bullet diameter to 3 decimal places) either.
5.56mm converts to .218"
7mm is .275"
7.62mm is .300"
etc.
Nation of origin and its manufacturer's customs also play a part in the cartridge name.
examples abound. 7x57mm, 7mm Mauser, and .275 Rigby are all the same round, depending if you are in Europe, the USA, or England.
9mm Luger is 9mm Parabellum (original maker's name) 9x19mm (standard metric designation) or 9mm Luger (US name).
8mm Mauser, 8x57mm are commonly used, but the German military designation is 7.92mm...
.22 Hornet, .218 Bee, .219 Zipper, .221 Fireball, .222Rem, .222 Rem Mag, .223 Rem/5.56x45mm, .224 Weatherby, .225 Winchester, and .22-250 all shoot the same .224" diameter bullets.
US .30 caliber rifles use .308 bullets, but Russian ".30s" 7.62mm usually use .311-.312" bullets, as does the .303 British, and the 7.7x58mm Arisaka rounds.
It gets even more "into the weeds" when you look at the names of pistol rounds vs. their bullet diameters, today.
Like why our ".38s" shoot .36 caliber bullets (.358") and why our ".44s" shoot ".43" (.429") slugs. The reason is due to the history of metallic cartridge and bullet development, AND, as always, marketing.
Regarding IMR powder number designations in their 4 digit series, I don't know the actual reason for each number, but I've always suspected it had something to do with each one's characteristics with each number of the code system meaning something specific to the maker.
Like the military ID systems. For example, the Army designator for a WWII "deuce and a half" truck was CCKW the letters being code for a 6 wheel drive truck. The amphibious version was the DUKW, becoming nicknamed the "duck" for obvious reasons (it swam).
When it comes to names for firearms related things, there are general conventions that are usually followed, but there are no hard and fast rules, not even SAAMI.