calibers - confusion

[natural marksman]

I'm sure I initiated this thread earlier, but I think something went wrong and it never got out, so I'm just gonna make a new one

Anyone who knows a thing or two about any rifled firearm knows about the lands and groove in the rifling. We all know that calibers are confusing. Is .30 cal really exactly 7.62mm? In the US I understand that the caliber of the barrel is measured from the grooves. Does this mean that rifled barrels have two calibers? The caliber measured from the lands, and the caliber measured from the grooves?

The .308 cal bullet is the caliber measured from the barrels grooves, but the lands stick up further, even if for 0.2mm or so. So knowing that the caliber, measured from the lands is .30 cal (7.62mm) and the caliber measured from the grooves is .308 cal (7.82mm if I've done the maths right) - does the barrel stretch slightly when the bullets goes through it?

 

[Walkalong]

Bore and groove pic as posted here before by someone else.

Groove diameter is .308, the lands dig into the bullets since they stick out from the bottom of the groove. The bore diameter will be different for different barrels depending on the lands and how they are cut and how deep. (What bore diameter they started with before cutting the grooves)

I seriously doubt the barrel stretches around the bullet considering how soft it is compared to the barrel.

 

[Steve Marshall]

7.62MM is .300" exact. The groove diameter is about 7.82MM which comes to .308". Not that it matters for this post, but yes the barrel swells as the bullet travels. Not much mind you.

 

[mete]

Typical difference between bore and groove dia is .008".
Yes the barrel does stretch as the bullet goes through it and this is significant since this is what sets up vibrations in the barrel. The vibrations are very important because they involve accuracy. When shooting from a rest for example NEVER rest the barrel on the bench because it can interfere with the vibrations -accuracy and point of impact.

 

[NCsmitty]

Good information so far from the other posters, and I'll add that some confusion with metric designations come from the fact that European applications given to some European designed calibers are based on bore dimensions and others on groove dimensions. It's often confusing to new shooters and reloaders.



NCsmitty

 

[Jim Watson]

Yup.
It's not that there is no system, it is that there are several systems and you just have to learn which applies. Rote memory, a lot of them. Not to mention advertising; consider that .218 Bee, .219 Zipper, .220 Swift, .221 Fireball, .222 Remington, .223 Remington, .224 Weatherby, and .225 Winchester, although different in chamber dimensions, ALL shoot .224" bullets. But .22 Remington Jet and .22 Savage High Power don't.

There are a lot of Internet Experts who have trouble with the difference between bore and groove diameters although they will surely set you straight on the difference between a magazine and a clip.

The barrel does bulge as the bullet passes down it. The double rifle boards are full of warnings against shooting solid copper or bronze bullets in a double express rifle, because the strain can pop the soldered ribs loose.

 

[natural marksman]

Fascinating stuff, thank you guys

also, mete, I'd like to learn more about these vibrations and how they influence accuracy, if you know a lot of detail about all that stuff. Thanks

 

[zhyla]

You're better off considering the "caliber" as just a model name (e.g. 5.56 NATO, 7.62x39mm) and looking at the actually spec for the ammunition in question. There's no law that the name has to accurately describe the bullet diameter. Example: .38 special and .357 magnum are both .357-.358" bullets.

 

[Float Pilot]

There's no law that the name has to accurately describe the bullet diameter.

Just like the old 38-40, actually uses a 40 caliber (10mm) bullet and most were loaded with less than 40 grains of black powder. I guess 40-40 or 40-38 would have not been a good marketing name.

Then of course the famous 44-40 really used a .429 or .430 diameter bullet...

Plus those pesky CAP and Ball 44 caliber Remington Revolvers from the War of Northern Aggression (Civil War to public school folks) , actually used .451 caliber balls...

 

[DEFOXER]

As zhyla states; its just a name. Some rounds reflect the actual bore size as in the .303 Brit and all 300's. Others reflect the groove diameter e.g. 308Win others are close but neither the bore, groove nor proj size such as 307Win or 460 Weatherby. 404 Rimless NE (Jeffery) is actually .412 bore and .422 Proj.....I think the words that go along with the numbers are quite interesting and /or a mouthful; 'SUPER-EXPRESS-VOM-HOFE' or maybe - Today I shall bring along my 375 BELTED RIMLESS NITRO EXPRESS

defoxer