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Old 01-26-2004, 02:38 PM
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Question Measuring the Bore

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What is the proper way to measure the bore of a rifle, or pistol for that matter? Do you measure on the land or the groove?

Thanks in advance.
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Old 01-26-2004, 02:57 PM
91Carcano's Avatar
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Join Date: Nov 2003
Location: Fallon Nevada
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Both, depending an what you want to do with it.

Usually groove. Drive a soft lead fishing weight into the barrel. First make sure it's a little bigger than the groove. Lubricate it; sizing lube is OK. Drive it into the breach with a hardwood dowel. When it comes out, measure across the widest part by slowly rotating the slug to find the greatest diameter. A cast bullet should be .0005" to .001" bigger to fit the barrel properly.

It's also good to know your bore diameter, too. However, you'll need a blade micrometer instead of the more common style and expect to spend $$$! Some people (I don't) like bore-rider cast bullets. A bore rider doesn't necessarily ride the bore (the tops of the lands) unless you carefully fit it by enlarging the bore-riding part of the mould until it just fits. If you're making paper-patched bullets, the bullet must be cast or swaged to .0005" to .001" bigger than the bore so it will properly engage the rifling.

Almost always, though, the groove is the critical diameter because you want your bullet to fill it up and not allow hot gasses to leak past it. Having said that, a surprising number of old guns, particularly old military arms, have grooves ~.002" or so larger than their specified diameters and, from a standpoint of keeping it from blowing up from overpressure, this is good.

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Old 01-26-2004, 03:11 PM
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Thanks 91.
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Old 01-30-2004, 09:09 AM
Darkker's Avatar
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The answer is both, but it depends on where you live, not what you want to do. Americans and europeans measure differently. I always get the two methods screwed-up. But I beleive this is how it goes. American's measure lands and Euros measure the rifleing. That is why the NATO boys call a .30 cal a 7.62mm. 7.62mm==.300"
Ask John Lazzaroni .308"==7.82mm(such as the Warbird).
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Old 01-30-2004, 10:18 AM
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Please don't think there is a system `cause that will get you in trouble. While bore and groove diameters are pretty when standardized now, the naming convention isn't. It's better to say there isn't one. So in our nominal .30 calibre we've got .300, .307 and .308 in the U.S. 8mm German bores are .315" and grooves are .318" OR .323". Remington split the difference with the .280 and .260. The Brits do it both ways too. The .375 H&H is groove diameter and the .300 H&H (Super 30) is bore diameter.

As I tell my Firearm Safety students, 'If you understand it, you haven't studied it well enough'.

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