I hope it is okay to ask the question here.
As I am prepping to get into reloading, I find myself confused, why is a 6.5 Creedmoor/ PRC/ Swede called "6.5" and not its actual diameter 6.71mm/ 0.264 in.? I could have bought the wrong bullets. The 25-06 is a 6.5mm/ 0.257 in. diameter bullet but it is called a "25"-06. Even the 8mm mauser is called 7.92x57 in some circles, but its actual bullet diameter is 8.22mm/ 0.324 in., why? A lot of cartridges are called by what they are in relation to their bullet diameter.
Can anyone help me understand? Thanks.
There are tons of cartridges that are named "x" but their actual dimension is "y". Some have to do with history, some have to do with nomenclature of the time being overtaken by modern measurement standards, black powder vs smokeless, metric vs English...there are a hundred different reasons each one specific to each unique modern cartridge.
Example: present day .38 special is a .357 groove diameter. Old days .38 special (just before 1900 I think) was originally a black powder cartridge with a 'heeled' bullet much like a present day smokeless .22lr. The bullet used to be the same diameter as the case just like a .22, thus the .38 designation. When they (Smith & Wesson) changed the design of the bullet and the revolver they didn't change the caliber number to .357. Probably for thrift and advertising reasons. Seems like the .357 magnum should be called the .38 magnum if they had stuck with it but they didn't so...? I have read that the old .36 Navy black powder cap and ball is the true parent of the .38 special but...
@AndyinEverson is a major black powder guy...care to weigh in Andy?
I think I got the above example right but if I missed something ANYBODY feel free to correct me.
Bottom line is it pays to do your homework Before you buy your projectiles. There's been attempts to standardize all this over the last century, but it's still not even close to standardized. Figuring out the history of why things are as they are now is kinda interesting if you're sick like me though

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