Question: How to calculate bore diameter

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  • Last Post 21 January 2015
john.dejarnette posted this 21 January 2015

Friends,

Does anyone have instructions and formula for accurately calculating bore diameter of barrels with odd numbers of grooves, like S&W revolvers?

Thanks, John

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Glenn R. Latham posted this 21 January 2015

John,  you can do it with pin gages and a lead slug from the barrel.  Determine bore diameter with pin gage, then measure a lead slug across one land and opposite groove.  Subtract pin diameter from slug reading, this is your groove depth, per side.  Two times groove depth plus pin diameter is your groove diameter.

However, on a S&W revolver, the important dimension is the cylinder throats.

Glenn R. Latham

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john.dejarnette posted this 21 January 2015

Thanks Glenn

I don't yet have a set of pin gauges. I was hoping for some fancy math that could be done with a v block and micrometer. Looks like I just need to buy a set of gauges.

I underatand about throat fit in a wheel gun. My suspicion is that barrel is larger than throat. Trying to get accurate measurement to confirm

thanks -john

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Ken Campbell Iowa posted this 21 January 2015

hah ! bet you meant * groove* diameter of your barrel ... very common useage in the real world.

not sure what you have to measure with, but one way is to find the axis center and measure how far the od of the slug is from center. ie, think of it as ” spinning ” the slug.

the v block approach needs to match your # of grooves ... 5 grooves are 72 degrees apart.

ken

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OU812 posted this 21 January 2015

I have heard of people using a strip of aluminum soda can wrapped around the bullet. Pepsi can wall is about .002” thick, double that to .004 (measure thickness to be sure). You can cut strips using scissors. Wrap strip around pure lead bullet slug then measure.

You can use a measured jacketed bullet to check cylinder throat fit. My 1995 vintage 686 will allow a jacketed .3571 diameter bullet to slip thru all cylinder throats. Bullet will slip thru with very little finger pressure. This tells me throats are about .3571-.3575".

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john.dejarnette posted this 21 January 2015

Ken Campbell Iowa wrote: hah ! bet you meant * groove* diameter of your barrel ... very common useage in the real world.

not sure what you have to measure with, but one way is to find the axis center and measure how far the od of the slug is from center. ie, think of it as ” spinning ” the slug.

the v block approach needs to match your # of grooves ... 5 grooves are 72 degrees apart.

kenThanks Ken.  I have been known to make nomenclature mistakes now and then.

I just remembered that I have some small hole gauges tucked away somewhere that will help me get close enough.  Not as good as pin gauges, but it should work. 

-john

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john.dejarnette posted this 21 January 2015

OU812 wrote: I have heard of people using a strip of aluminum soda can wrapped around the bullet. Pepsi can wall is about .002” thick, double that to .004 (measure thickness to be sure). You can cut strips using scissors. Wrap strip around pure lead bullet slug then measure.

You can use a measured jacketed bullet to check cylinder throat fit. My 1995 vintage 686 will allow a jacketed .3571 diameter bullet to slip thru all cylinder throats. Bullet will slip thru with very little finger pressure. This tells me throats are about .3571-.3575".   Thanks.  I like the simplicity of that approach!

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