bullet diameter & calibers

Tom Finco

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Jan 28, 2002, 11:43:10 PM1/28/02

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Please help me out.... I confused about bullet diameters.

per reloading books 7mm has a bullet diameter of .284" 6mm is .243 .30cal
= .308" = 7.62mm correct?

I was taught that a 1 inch = 2.54cm or 25.4mm

Doing the math

7mm/25.4mm = .276"
6.5mm = .256"
6mm = .236"
..308" = 7.82mm

What am I missing?

Eric Ching

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Jan 29, 2002, 12:44:14 PM1/29/02

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> From: Tom Finco <fi...@worldnet.att.net>

---

Tom, you're not missing anything. The caliber names are very often
approximations of the real bullet diameter, chosen for a variety of reasons.
A lot of times, the reason is that it sounds or looks good, or a wildcatter
wants to distinguish his cartridge from others using bullets of the same
diameter. Best to just learn the true diameters for reloading purposes, and
the myriad of caliber/cartridge names for identification purposes.

Eric

Thom

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Jan 30, 2002, 1:59:58 PM1/30/02

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On Mon, 28 Jan 2002 22:43:10 -0600, Tom Finco <fi...@worldnet.att.net>
wrote:

Besides the commercialization of caliber names there is also the
difference in whether you measure the "gun" by the bullit diameter or
bore diameter.

Two examples:
1. The standard nato round the 7.62.51mm is a .308 bullet
2. The two standard Russian rounds, the 7.62x54R and 7.62x38mm are
actially .31 caliber guns (.310 bullets) because the Russians have a
7.62mm BORE!

There are dozens of rounds in .308 that hace commercial names like the
.300 Waetherby. 300 Norma Magnum, 300 Winchester magnum etc etc.

The only true modern .30 cal was the 7.35x52mm Carcano that actually
had a .300 bullet.

THOM

John Chase

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Jan 30, 2002, 2:00:47 PM1/30/02

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>
>
>Date: Mon, 28 Jan 2002 22:43:10 -0600
>From: Tom Finco <fi...@worldnet.att.net> <mailto:fi...@worldnet.att.net>

>
>Please help me out.... I confused about bullet diameters.
>
>per reloading books 7mm has a bullet diameter of .284" 6mm is .243 .30cal
>= .308" = 7.62mm correct?
>
>I was taught that a 1 inch = 2.54cm or 25.4mm
>
>Doing the math
>
>7mm/25.4mm = .276"
>6.5mm = .256"
>6mm = .236"
>..308" = 7.82mm
>
>What am I missing?
>

The caliber measurements are "bore" diameter; i.e., the diameter
measured across the grooves (also, bullet outside diameter). The metric
measurements are the diameter across the lands. Groove depth is
nominally .003 - .004 inch, or approx. 0.1mm. Thus, the 7mm or .284 cal
barrel starts out with a .276 (.277) hole drilled, and after the rifling
is cut (or "buttoned"; different process), the inside diameter measured
across the grooves is (nominally) .284 in, or approx. 7.2 mm.
Similarly, the .308 caliber barrels start out with a .300 in. (7.62 mm)
bore, which increases to approx. .308 (7.82 mm) when the rifling is cut
or buttoned.

I don't know why "they" do it that way; you'd probably have to ask the
government. ;-)

-jc-

Remember to prune the .excess to email me.

Ephiny(Co30)

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Jan 30, 2002, 2:01:24 PM1/30/02

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The euro/metric size is for bore rather then bullet, thus
7mm bore =.276 plus rifling of roughly .004 per grove
adds .008(groves being opposit each other)for .284.
Personally I think they did it just to try to confuse the
Brits!! ; )

Mike
They murdered his mother,they burnt his forrest,he's BACK!
And he's PISSED!!! Bambo!!!!

Tom Finco <fi...@worldnet.att.net> wrote in message
news:M2o48.2622$S67.1...@bgtnsc05-news.ops.worldnet.att.net...

> Please help me out.... I confused about bullet diameters.
>
> per reloading books 7mm has a bullet diameter of .284" 6mm is .243

..30cal