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Old October 6, 2011, 10:59 PM   #1
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Odd no. of Grooves on a Slug

I just slugged my 357 mag (by taking 3 grains of 2400 and shooting into a pile of towels, worked like a charm) and i looked at the slug only to find that the slug has an odd number of groves (there is no groove completely opposing another to make measuring anything easily done). Have any of you wiser gentlemen found a way to find the diameter in this situation? Thanks in advance!
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Old October 7, 2011, 08:44 AM   #2
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Years ago, I think in the American Rifleman, was an article with a drawing of a miniature "v-block" that machinists could make to measure the odd number of grooves on a S&W fired bullet. There was a specific number that was subtracted from the measurement to give the correct diameter of the slug. I had made a drawing to enable me to make one, but never got around to it, and that was in the early 70's so I do not have it now. But, there may be some math wiz who reads this that can make a sketch and figure the angles to post it here.

I have added the device here, if some math person wants to add the correct angles, formula to construct it.

Last edited by dahermit; October 7, 2011 at 02:29 PM.
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Old October 9, 2011, 10:36 AM   #3
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360 (the no. of degrees in a circle) divided by 5 = 72

Then divide 72 by 2 =36

The angle needs to be 36 degrees on each side .

But it`s easier to just look at the bullet to see if it`s scored or rubbed by the bore not the rifling (or lands).

Now that ya have this info the big ?? is does the throats match the bore ????

To answer ya question I have taken a ribbon of pop can & wrapped the bullet to make it round then subtract the thickness of the can .

Don`t forget to subtract twice the thickness though .

Where ya getting leading at ????
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Old October 9, 2011, 11:28 PM   #4
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A couple of ways. Get a "V" anvil micrometer, CAUTION, they're very expensive. Or, get a precision set of hole gauges.

Another way is to take the slug, try to pass it through the cylinder throats. IF you know what the throats measure at, you'll know if it's loose, just fits, or you can't get it to go through.

I'll bet it's a Ruger like my GP-100, five groove barrel. I slugged both the Cyl. throats with.380 pure lead balls, and the barrel. The Cyl. throats were .357, the barrel slug was a tight slip fit in the Cly. throats, so my revolver has matched throats and barrel the same diameter.
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