.257 equals 6.5mm

[engineer40]

I have a friend looking to get into longer range shooting. Currently he only has his AR 15. While I was broadening his horizons, I was explaining how the rifle calibers match up from inches to mm's.

Within our shooting and reloading, obviously most of us realize:
.224 = 5.56mm
.243 = 6mm
.264 = 6.5mm
.277 = 6.8mm
.284 = 7mm
.308 = 7.62mm


I was surprised though if you actually do the math conversion:
.257 actually equals 6.5mm
and
.264 actually equals 6.7mm

Weird right? I never realized this.

 

[7mmShooter]

And .277 actually equals 7mm...

 

[MagnumManiac]

Your problem with your quotes is that you have qoted bullet diameter, not true bore diameter, which is the calibre.
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gun)

 

[engineer40]

Your problem with your quotes is that you have qoted bullet diameter, not true bore diameter, which is the calibre.
lightbulb

gun)

Hey MagnumManiac, I don't understand what you're trying to say.

Are you saying that either the American or the Metric measurement is the bore diameter (caliber) and not the bullet diameter?

I just measured one of my ".264/6.5" bullets. It is 0.264 inch or 6.70mm. Not actually 6.5mm. But it is actually .264 of an inch. Measured a 142gr SMK bullet.

If there is something super basic here that I just never realized, I definitely would like to know.

 

[7mmShooter]

Hey MagnumManiac, I don't understand what you're trying to say.

Are you saying that either the American or the Metric measurement is the bore diameter (caliber) and not the bullet diameter?

I just measured one of my ".264/6.5" bullets. It is 0.264 inch or 6.70mm. Not actually 6.5mm. But it is actually .264 of an inch. Measured a 142gr SMK bullet.

If there is something super basic here that I just never realized, I definitely would like to know.

I think what MagnumManiac is trying to say is that the caliber designation is tied to the bore diameter of the barrel. Regardless of the measurement system... a 6.5 caliber barrel will have a .256 bore (which equals 6.5mm roughly). The groove diameter of the rifling is what determines the bullet diameter. .264 groove diameter of the rifling, therefore .264 bullets.

 

[MagnumManiac]

I think what MagnumManiac is trying to say is that the caliber designation is tied to the bore diameter of the barrel. Regardless of the measurement system... a 6.5 caliber barrel will have a .256 bore (which equals 6.5mm roughly). The groove diameter of the rifling is what determines the bullet diameter. .264 groove diameter of the rifling, therefore .264 bullets.

What he said.

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[Michael Eichele]

Same thing with the 7mm. It's actually 7.22mm (.284") groove to groove and 7mm (.277) rifling to rifling. And so on...

 

[engineer40]

I think what MagnumManiac is trying to say is that the caliber designation is tied to the bore diameter of the barrel. Regardless of the measurement system... a 6.5 caliber barrel will have a .256 bore (which equals 6.5mm roughly). The groove diameter of the rifling is what determines the bullet diameter. .264 groove diameter of the rifling, therefore .264 bullets.

What he said.

lightbulb

I've always understood the difference between bore and groove diameters.

But now that I've read what you guys have responded with, what I don't think I realized was that the American measurement nomenclature is based on groove diameter (.264 in our example). And the Metric measurement nomenclature is based on bore diameter (6.5mm in our example).

I had always assumed both naming structures were based on the same measurement location of the internal barrel dimensions.

Thanks.

 

[Mikecr]

The there are Lazzeroni cartridges(metric grove).
My 6.5 Scramjet was 25cal.

 

[MagnumManiac]

I've always understood the difference between bore and groove diameters.

But now that I've read what you guys have responded with, what I don't think I realized was that the American measurement nomenclature is based on groove diameter (.264 in our example). And the Metric measurement nomenclature is based on bore diameter (6.5mm in our example).

I had always assumed both naming structures were based on the same measurement location of the internal barrel dimensions.

Thanks.

No, not exactly.
Cartridges may be called by bullet diameter, or true bore diameter. Military cartridges are always referred to as bore size, or calibre. For instance, the 30-06 has a .300" bore, as do all 30 cal rifles made in the US. European 30 cal rifles are .303" cal.

Metric cartridges are also quoted as bore size, not bullet diameter.
This is where you are confused.

Cheers.
gun)

 

[Edd]

There is no American standard for the labels. It could be the bore diameter, the groove diameter or something else. Some examples..... 308 Winchester, 300 Winchester, 30-30 Winchester, 30 Govt 06, 284 Winchester, 280 Remington, 223 Remington, 222 Remington, and the list goes on.