Measuring 5 groove rifling

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  • Last Post 01 March 2022
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GBertolet posted this 20 February 2022

What is the best way to measure S&W 5 groove rifling? I have been trying to determine the groove diameter on a revolver. I slugged the bore, and had been measuring, by placing the bullet in a micrometer, and slowly turning the bullet, until it just rubs on the anvil, and taking the reading from there. I know there are some sophisticated ways of measuring, but what is the best method the average person can use, to get a reading?

I know there used to be a member here, that if you would send him the bullet, he had the proper equiptment to measure, and he would tell you the diameter.

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Shopdog posted this 21 February 2022

"Best"? Tough question.....

One way is a Starrett T485 micrometer. Another is V block(108*) with D.I. which is what I use.

Good luck with your project.

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kootne posted this 21 February 2022

Somebody should make a simple plate with a series of graduated holes, a 5 or 10 in 1 ring gage so to speak. Be a lot cheaper than buying precision measuring tools if you have no other need for them.

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GBertolet posted this 21 February 2022

Amen to that. I had considered drilling a series of holes in an aluminum block, having the sizes verified by pin gauges, and see if the bullet will just slip through.

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MP1886 posted this 21 February 2022

Some of you fellows should know this.  First the correct mic to use is a V mic for measuring round rods and tubing.  Since you don't have that you have to improvise.  Take a little strip of metal (rectrangle)  about 1.125 to 1.50 long by .50 wide and preferrably 3/16th to  1/4 inch thick. Bend it into a perfect 90 deg V.  Then file the bottom point of the V so it's flat and square to the main body of the V, but don't file it too thin as to weaken the V.  Okay you place the flat of the V on your regular mic's anvil and measure I know size jacketed bullets.  Let's say a 30 caliber that is .308.  You will get a larger reading because you're including the V block in the measurement.  Subtract the .308 diamter from the larger number and that is number to use in your measurements in other diameter bullets.  Let's say you get a total measurement of .358. So the difference is .050.  This will be your base number to use to subtract on measuring other caliber bullets.  You slug that  you drove thought your bore will have 5 rifling grooves on it and will fit perfect between the V block you just made so that it will be touching from groove to groove, not groove to land as you would be with a regular flat anvil mic.  I've included a drawing of the V block to make. 

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GBertolet posted this 22 February 2022

Great info. Thanks.

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GBertolet posted this 24 February 2022

I tried that method, and got confusing readings. I used a .308 rifle bullet, to get a baseline, and my 32 caliber bullets kept measuring at .308, the same as my rifle bullet. I was expecting .312 or .313. If accurate, my bore is really tight. Maybe I goofed in my measurements, but I got .308 again and again.

Update: I read of another method of wrapping a thin piece of shimstock tightly around the bullet, measure and deduct the thickness of the shimstock X 2, and that's your diameter. I used 1.5 thou shimstock. Guess what, I still got .308.

A S&W 32 H&R revolver, with a .308 bore. That's strange, but it is what it is.

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ALYMAN#1 posted this 25 February 2022

The Rifleman had a short article on measuring 5 groove slugs at some time in past.  It involved machining 36 degree angles lengthwise in a 1/2" by 3/8" block 3/4" long and measuring with a common mike. Mr. Homer S. Powley gave formulas to determine the actual size of the slug after determining the block constant dimension.  I have a photocopy of the page but it has no indication when it was printed and is probably copyrighted.  I have used it over the years and has served quite well. The page shows a diagram of the measuring arrangement above a short article on slugging a barrel, beside the tailend of an article on maybe an British Enfield (shown below the above texts) or some other large bore 30 cal military rifle if some one has an index of rifleman articles.

Hope this helps.  Al

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pisco posted this 25 February 2022

This is interesting as I can’t justify the cost of angle block if I get time I’m going to give mp1886 method a go

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David Reiss posted this 26 February 2022

I have the angle blocks to measure all the odd grove barrels. Send me a slug and a self addressed envelope for the return info.

David

David Reiss - NRA Life Member & PSC Range Member Retired Police Firearms Instructor/Armorer
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harleyrock posted this 01 March 2022

I tried that method, and got confusing readings. I used a .308 rifle bullet, to get a baseline, and my 32 caliber bullets kept measuring at .308, the same as my rifle bullet. I was expecting .312 or .313. If accurate, my bore is really tight. Maybe I goofed in my measurements, but I got .308 again and again.

Update: I read of another method of wrapping a thin piece of shimstock tightly around the bullet, measure and deduct the thickness of the shimstock X 2, and that's your diameter. I used 1.5 thou shimstock. Guess what, I still got .308.

A S&W 32 H&R revolver, with a .308 bore. That's strange, but it is what it is.

Not everyone has shimstock on hand.  Good quality paper (letterhead, tracing paper, etc.) will do just fine.

Lifetime NRA since 1956, NRA Benefactor, USN Member, CBA Member

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