Caliper vs. Pin guage for measuring chamber throats

aterry33

aterry33

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I have heard that a pin guage is the most effective way to measure the diameter of chamber throats on a 25-2. Someone asked me if a caliper would be as effective. I don't have a lot of technical knowledge on this.

Can someone share their thoughts?
 
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IMHO, a good-quality dial or digital caliper will give just as accurate a reading, but is a mite more difficult to apply correctly. The plug gauge (its correct technical name) will only fit one way, given that it's the correct size, and will wiggle sidewise if it's not. The caliper can be cocked either radially or axially, yielding an undersize measurement while feeling right. In practical terms, the caliper can be carried easily in a pocket, and will measure over a wide range. You'd have to carry a whole set of plug gauges to have that range available. If you want to measure only the 25-2, you could get by with just a few gauges, ranging from .451 to .454. If the throat is above or below that, you'll need more gauges.
Is that helpful, or just more confusing?

Larry
 
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Personally, I think you can get a pretty accurate reading with a caliper, but you have to be careful to get a solid squared up reading on the diameter of the throat. You may not be quite as accurate as you can using plug guages, but you will be able to tell if the throats are .452 or .456. Mostly, the throats will be very close in diameter for each gun. I only found one gun that was .010 larger than the remainder of the chambers. Smith replaced the cylinder.
 
Caliper vs. Pin Gauge for measuring chamber throats

I have tried both methods, and find that the pin gauges are more accurate than
calipers in general. Calipers may provide a quick general check, if desired, but are
difficult to align properly. I prefer the (-) minus gauges for such measurements,
but neither caliper nor gauge will measure accurately if the chambers to be measured are not free of all fouling.
 
I've measured lots of throats that are slightly oval. I normally use an expanding ball gauge and a micrometer. A pin gauge will only measure the minor diameter.
 
When measuring an ID with calipers don't forget that most caliper ears have small flats which will prevent them from giving a 100% accurate reading against a inside curve. Also most calipers are only accurate to a resolution of .001. Some are graduated to .0005 but that is about the limit. The rule of ten says that if you want to measure to an accuracy of .001" you need a tool that resolves .0001".


The best, if you can find them are Bore Mics, they have a limited range but have three pads in a shaft, you insert the head into the hole, turn the spindle on the mic to expand the head and read the diameter on the spindle like any micrometer. I think they run a couple hundred each though, although if anyone has an affordable set it might be Fowler.

I am convinced that for the average user a 1 inch micrometer graduated to .0001 and a lead ball is the way to go for cylinder throats and barrels with an even number of lands and grooves.
 
I am a Journeyman Tool & Die Maker and can tell you that a Dial or Digtal Caliper is only accurate to + or - .001" and that accuracy is also subject to the skill of person doing the measuring. Pin gages are the next most accurate means of measuring and slugging the chamber mouth with a soft lead ball being the means of obtaining the most accurate measurement. For most intents and purposes the measurement obtained with plug gages will be accurate enough. Granted the slugging method will pick up out of round condition, but most times it is on the order of a few .0001" and of little or no consequence.
 
Peter M. Eick said:
I would add that my pin guage set works great, but as was pointed out above if you want to get it right, slug the barrel.
Click to expand...

Peter,

He is interested in determining the size of his throats for best accuracy from a 25-2 since so many are plagued with oversized throats and don't shoot that great.

Slugging the barrel will tell him nothing about that, and accurately measuring a slug taken from a 5 groove rifling system takes specialized tools to read it correctly.
 
Marksman is correct. After almost 40 years in Quality Assurance anf metrology. I would choose a Dial (or digital) caliper to use only if my tolerance is 0.005 or greater and I do not consider their accuracy to be any closer than 0.002. ( just because a tool has a readout with a discrimination of 0.0001 or 0.0005 does not mean it has an accuracy of 0.0001 or 0.0005.)
A pin or plug gage ( YES GAGE - a gauge has a dial, scale or other readout and shows variable data while a gage will only Go or not-go or what is called attribute data) is a simple, accurate cheap tool that, if you have a range of sizes will quickly tell you the largest pin that will go.
The most accurate ways to check(in order of increasing accuracy / precision would be a combination of Small hole gage and micrometer, a bore micrometer, or (assuming you had a setting master) an air gauge.
 
I just remembered a tool that might work, although it has the disadvantage of being a transfer tool, ie you take the measurement with one tool and measure that with another. Going to be somewhat finess sensitive, IE two users might get different results depending on touch. (Kind of like when I try to shoot like Jerry M., I go through the same process and get somewhat different results.)

Transfer Instruments - 6: Small Hole Gages

Excellent illustration and explanation above.

You would need a pair of digital or vernier mics to determine the final size, but 1" mics are not that expensive, even ones good to .0001.

Not recommending that brand, I know nothing about them, they popped up when I googled the tool. I'm a Starrett and Mitutoya user whenever possible.
 
RonS said:
I just remembered a tool that might work, although it has the disadvantage of being a transfer tool, ie you take the measurement with one tool and measure that with another. Going to be somewhat finess sensitive, IE two users might get different results depending on touch. (Kind of like when I try to shoot like Jerry M., I go through the same process and get somewhat different results.)

Transfer Instruments - 6: Small Hole Gages

Excellent illustration and explanation above.

You would need a pair of digital or vernier mics to determine the final size, but 1" mics are not that expensive, even ones good to .0001.

Not recommending that brand, I know nothing about them, they popped up when I googled the tool. I'm a Starrett and Mitutoya user whenever possible.
Click to expand...

Looks like a handy little tool to have in your pocket, when walking the floor at the gun show.

A method that I use is simply checking the throat w/ a variety of caliper measured bullets.

For example, a .452" measured hard cast bullet that is a light to firm push through, will normally indicate a .4525" throat - as w/ my 625-6 MG, in .45 Colt.

A .451" JHP will easily slide into the throats (w/ the slightest amount of wiggle room) of my recently acquired 625-8 (.45acp), but a .452" will stop dead - indicating to me, that the throats are .452" on the money.

My personal preference, is firm push though bullet sizing.
 
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