How to measure "groove" dimension

[clayshooter]

I have a 44 Special that I wanted to measure the cast slug dimension from a fired round. The problem is the grooves do not line up directly across from each other. Micrometer measurement at that point is not accurate. Anyone have a trick way to measuring the "across" dimension. Incidentally, the revolver is a S&W 21 dash 4 classic (new gun).

 

[flat top]

Clayshooter; You need to "roll" the bullet in the micrometer to obtain the maximum dimension. Also, if you are intending this measurement to get a "true" reading of bore diameter you need to slug the bore....a spent bullet is not a good indicator of true bore dimension for many different reasons.

 

[5-Shot]

You're not going to believe this! Based on an article by Homer Powley, written many years ago, you need a v-block with the sides of the "v" 36 degrees from horizontal. Lay the slug in the v-block. Measure the distance from the top of the slug to the bottom of the groove. The slug diameter = 0.8944 x that distance. Let me know how it works out.

Ed

 

[dg101win]

I do not recall the math as 5-Shot does. However I recall many,many moons ago an article in American Rifleman about this same thing. All I can recall though is the V-Block and measuring from the top of the bullet to somewhere on the block.

 

[gregintenn]

You do have a 36 degree v block don't you?

 

[Alk8944]

Only it is a 108 degree included angle, not 36. This applies to only a 5 groove barrel. There are blocks available for three and seven groove barrels.

The bullet is placed in the V-groove with two lands against the legs of the V. It is then measured from the top land (of the bullet) to the bottom of the V block. This dimension is multiplied by the constant noted above and then this has a second constant, peculiar to that specific block, subtracted from the result for the final solution, the groove-to-groove diameter of the barrel.

And, yes, I do have one, and use it frequently. No, I don't recall where I bought it or I would be glad to tell you.

 

[gregintenn]

Wow. That's a specialized tool. Why couldn't you rotate thslug inside your caliper jaws and get an accurate reading?

 

[clayshooter]

how to measure groove dimension

I've tried rotating the slug within the jaws. The way the grooves are orientated I am only able to pick up a small edge of each of the grooves. I'm sure I'm getting a false reading. For a recent 44 special my slug measurement is .424". Should be somewhere around .429"

 

[NKJ nut]

Wow. That's a specialized tool. Why couldn't you rotate thslug inside your caliper jaws and get an accurate reading?

That works with bores that have an even number of grooves, but when you have a land opposite a groove the measurement is not so easy.

 

[gregintenn]

You just ain't spinning it fast enough.

 

[oldRoger]

Consider that you have two concentric circles the bore and the groove dia.
I have a set of plug/pin gauges which I use to measure the bore, but it can be done at the muzzle with calipers.. I can usually measure from a land across the bore to a groove with my dial calipers; I make this measurement for each land/groove. Average these measurements. You now have bore plus one land. Since you know the bore dimension, subtract it, multiply by two, add the bore dia back and you have the larger concentric circle. You can use a very similar approach with the slug from slugging the barrel.
Is this accurate, well it beats not having any idea.

 

[NKJ nut]

You just ain't spinning it fast enough.

Dang, now you tell me. I had the slug up to 1,500 rpm and the little needle on the caliber at 300 rpm and still couldn't get a good measurement.

 

[whelenshooter]

Dang, now you tell me. I had the slug up to 1,500 rpm and the little needle on the caliber at 300 rpm and still couldn't get a good measurement.

You almost had 'er! 1,501 rpm would have done it!

 

[sonny]

I just had this problem a little bit ago.
Take one of your very thin feeler gauges, maybe 0.001" or so. Wrap it around the lead slug and measure it, then subtract TWICE the thickness of feeler gauge. I've done it using several thicknesses of feeler gauges and got the same groove dimension. Yeah, it works well. Just don't try to use a real thick feeler gauge.
Sonny

 

[Nygma]

A V-anvil micrometer is the easiest way. They are expensive when bought new, but can be had for a reasonable price used. I bought a lighty used one at a machinery auction about ten years ago for $40.

They are for sale on eBay. Search Mitutoyo 114-202