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Discussion Starter · #1 ·
Can somebody who KNOWS what these marks mean, share their knowledge (and eyesight)?

a) indirect reading bore-gage of 12ga barrel
b) measure 8ga hull head

P/TY

Bob








 

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Bob,

The first micrometer is reading .750 (seven hundred fifty thousands) or 3/4 inch.

The second micrometer is reading 1.026 (one inch and .026 thousands).

Bucko43
 

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Discussion Starter · #4 ·
Thanks, Buck. That's what I thought, and I was measuring the bore-gage a little skewed against the anvils. The bore is really more .729-.730 ... where it should be. So, each of the marks on the barrel? is .025?

Bob
 

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Discussion Starter · #5 ·
TM251: "And your metric vernier is reading 22.92 MM which is .902/1000 .03937x 22.92=.902 a MM = .03937 Looks like 10 gauge brass shell."

Inside ring is metric. Outside is decimal ... I think. It says it is dual-reading anyhow.

That is an 8ga hull: CNC from Rocky Mountain. I was measuring it to get a chamber reamer made.


Bob
 

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I can't read a vernier any more, got a metric and an inch.

went to a dial, got a digital now, inch or metric.

The thread on a micrometer barrel is 40 TPI, yes one turn=.025. If your mic reads tenths, that.s the little lines on the barrel bgoing longitudinally, and it's a vernier scale.

HM
 

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In the first picture it is obviously more than .750", since I can't see the vernier completely, I'm going to say it's more like .7503" : )<br>
In the second picture, the calipers read .920" and the micrometer says 1.0261" again, assuming the extra tenth...)

Then completely guessing, is that a Starrett micrometer? Josh
 

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Discussion Starter · #11 ·
Starrett, mdl 230 (whatever that may mean.)

And .7503/4 was what I made it, too - which is wrong, but my fault for clumsiness. And 1.0261/2, also.

The dial was measuring just above the cove on the rim.

BTW, Cartridges of the World has (at least some of) the measurements for an 8bore wrong.


Bob
 

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Looks like .7503", you have about 3 tenths but I can't quite see the barrel markings. Second pic is .0001" or .0002" heavy.

Nice pictures. I have the same mics, set of 4. Still going strong after 44 years.

HM
 

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Jolly is correct. Don't be confused by the alternate metric markings on the plastic dial caliper (0.92" = a little over 23mm, just as it shows).

The micrometer below the dial caliper is technically off it's scale (beyond it's max.) but Jolly's 1.026" looks right (only other possibility would be 1.051", but I don't think the barrel is far enough past the scale for that).

Friendly suggestion... always take readings from non-metallic calipers with a grain of salt. Even fiberglas reinforced plastics are not stable or wear-resistant enough to stand up to long term use for precise measurements such as those necessary in gunsmithing. They're fine for woodwork or quick estimates but a name-brand of stainless calipers is essential if you need reliable precision. A good micrometer with a tenths vernier is better yet and is generally one order of magnitude more accurate than calipers (X.XXXX vs X.XXX).

-Gary
 

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Discussion Starter · #16 ·
Point taken.

It seems I have some need almost daily to use calipers for 'close-enough', and those fiberglass jobbies live on my desk. I have a Mitutoyo dial and some misc. Midway SS digital, but they are never to hand when I have a phone in my ear.

This was my first experience with indirect-reading T-gages, and it was educational. I've got a gun that needs some choke work done, and I was trying to dimension the chokes. The .750 was the (mic) reading I got in the BORE. Impossible. But in this instance, the plastic calipers were more accurate than my ham-handed lining-up of two round bearing surfaces between anvils without x, y precision.

Bob
 
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