Barrel diameter

[M-61]

What is the common way to give the barrels diameter? Is it land to land, land to grove, or grove to grove?
I recall a maker something like ‘Lazzeroni’ was using some different way to come up with some odd sizes at least on paper if not the same dimension.
I don’t have one but a conversation came up about a Ruger Mini 30 having two different bore diameters..308 and 312. After all these years I realized I have no idea of what is standard. Shooting a .312 bullet in a .308 bore seems catastrophic, but I don’t really know what the bore diameter is.

 

[Herzo]

In general, the US calls out the bore in terms if the dimension across the grooves, whereas the Europeans use the minor diameter (across the lands). In the case of the 7.62 X 39, it is a European (Russian) cartridge using the European convention.

 

[M-61]

Thank you.
But….a .308 bullet is .308 yet a 7.62 x39 measures .312 (I’m told, I have none to measure.)
Are the barrel diameters the same?
I am quite confused.

 

[Eric Leonard]

I think some mini 30s have .308 bores with a tapered throat to shoot the .311 factory ammo and still be able to shoot .308 bullets with some degree of accuracy.
In these rifles I think it’s supposed to be safe to shoot either bullet size

 

[rockhound78]

My x39 ammo is .310". Goes along with the .310" groove dia. of my #1 Ruger

 

[M-61]

My x39 ammo is .310". Goes along with the .310" groove dia. of my #1 Ruger

Thanks.
How did you measure the grove diameter? Did you measure grove to grove or what?

I think some mini 30s have .308 bores with a tapered throat to shoot the .311 factory ammo and still be able to shoot .308 bullets with some degree of accuracy.
In these rifles I think it’s supposed to be safe to shoot either bullet size

So you’re saying the bullet is being squeezed down as it transits the barrel?

 

[M-61]

At what point does the bullet become TOO large and disaster strikes?
Not just the Mini 30 but say a 38 S&W bullet ( .361) vs a S&W38 special bullet at .357

 

[Eric Leonard]

Thanks.
How did you measure the grove diameter? Did you measure grove to grove or what?
So you’re saying the bullet is being squeezed down as it transits the barrel?

Yes that’s how I understand it.

 

[jdh47]

Jack O'Connor wrote a chapter in "The Rifle Book" years ago titled "How cartridges are named" I think. Some are named for bore diameter ( 30-06) and others for groove diameter like the .257 Roberts. The .270 has a land diameter of .270 and a groove diameter of .277. The bullets loaded for it are .277. Grooves can be cut to whatever depth the manufacturer desires but it's either SAAMI or European standard. Bores remain constant. Yes you can measure groove diameter if you have the equipment to do so or you can just slug the barrel and measure across the grooves. Bullets are "squeezed down, yes. The grooves are rifled at different twist rates giving the bullet spin. It stabilizes the bullet and improves accuracy.

 

[Bill K]

Barrel diameter? Width from one side to the other. Bore Diameter varies, depending on who's measurements you use. US or European.

What is the common way to give the barrels diameter? Is it land to land, land to grove, or grove to grove?
I recall a maker something like ‘Lazzeroni’ was using some different way to come up with some odd sizes at least on paper if not the same dimension.
I don’t have one but a conversation came up about a Ruger Mini 30 having two different bore diameters..308 and 312. After all these years I realized I have no idea of what is standard. Shooting a .312 bullet in a .308 bore seems catastrophic, but I don’t really know what the bore diameter is.

 

[M-61]

Jack O'Connor wrote a chapter in "The Rifle Book" years ago titled "How cartridges are named" I think. Some are named for bore diameter ( 30-06) and others for groove diameter like the .257 Roberts. The .270 has a land diameter of .270 and a groove diameter of .277. The bullets loaded for it are .277. Grooves can be cut to whatever depth the manufacturer desires but it's either SAAMI or European standard. Bores remain constant. Yes you can measure groove diameter if you have the equipment to do so or you can just slug the barrel and measure across the grooves. Bullets are "squeezed down, yes. The grooves are rifled at different twist rates giving the bullet spin. It stabilizes the bullet and improves accuracy.

Thank you for taking the time to answer

Yes I understand that. The difference between US and European yes,now.
At what point does MORE become too much? .2
.312 in a .308? A .361 in a .357?
I have never heard of the tapered bore as stated above so that’s new to me.
The name manufacturers use are merely ‘catchy’ names.(numbers)