bore vs caliber

[nanuk-O-dah-Nort]

does the bore diameter = caliber?

ie/ 30 cal = 0.300 bore?   27 cal = .270 bore?

[PowPow]

pretty much.
30 cal is 0.300" dia to the top of the lands and 0.308" dia to the bottom of the grooves.
270 cal is .270" to the lands, .277" grooves.


Then some they just make up a name thats close to sound unique, like .307, which is a 30 cal.
Then some are just wacky, like 38 cal which are .357" dia.

[mcwoodduck]

Bore is based on how many round balls can be made from a pound of lead
Caliber is based on either inches or metric.
So a 12 bore is a bore diameter that will allow 12 perfectly round balls to be made from a pound of lead or .729 of an inch.  He h a 10 bore rifle or an 8 bore rifle means he has a rifle that shoots 10 ga or 8 ga. 
Bore can also be hole in any firearm.  As in "What is the bore diameter of that Mauser?"
now when you are talking abut cartridges and the names people give them
Well there is no rhyme or reason for most.  The europeans kind of have the best system.  Bullet diameter X case length.
so 6.5X55 is a 6.5mm bullet wth a case that is 55mm long.  the problem with using it is te 300 WSM and the 308 Win have the same bullet diameter and the same length.  So both would be 7.62X51.  and 30-30 is 7.62 X51R the R stands for Rimed but 307 Win is the same dimentions and it too has a rim. 
a 303 Brit round uses a bullet .311.  But the brits measure the distance bewteen the lands not the grooves.
300 H&H or 300 Winchester use the same .308 bullets, have the same base but the only difference is the length and the shoulder of the case.  the difference between 300 H&H and 300 WBY is the shoulder.
And the 10mm an the .40 S&W use the same size bullets, case is length is different.
 

[Richard P]

To add to the confusion using the reference of 38 cal having a bullet dia of 357; remember the early 38s used a heeled bullet. The bullet was the diameter of the casing. When we learned to put the bullet completely inside the casing the bullet diameter had to change. It's just a part of the evolution in firearms and of course all related parts and pieces.  rp   

[sharps4590]

nanuk...the heck of it is, everyone one of the guys are right!!!!  mcwoodduck is spot on regarding caliber and  bore....so is Richard and so is powpow.  There is not nor has there ever been a method to anyone's madness when it comes to naming cartridges.  250-3000, 25 cal that pushes an 87 grain bullet to 3000 fps.  Clever marketing as it was the first standard cartridge to reach 3000 fps.  30-06....another dandy.  30 caliber adopted by the US Army in 1906.  22-250...the 250-3000 necked to 22 caliber.  257 Roberts?  Another 25 caliber named for Ned Roberts....and based on the 7 X 57 Mauser case!!!
 No wonder newcomers pull their hair out.  I'm not saying you are....that was just an observation in general...it's nuts.