Measuring chamber throats: apparently, I have critics!

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This is an expansion on an email I replied to recently. A loyal reader noted that my name had been brought up on one of the forums (sadly, he couldn’t remember which one) regarding my blog article on measuring chamber throats.

Apparently, the gist of the discussion was that the forum’s “expert” (every forum has one) opined that I was full of it for suggesting that throats couldn’t be measured accurately with a caliper. What’s more, someone expressed the thought that a caliper would show an out-of-round condition, whereas a pin gage wouldn’t, and therefore anyone who didn’t use a caliper didn’t know what he/she was doing.

Sheesh! Let’s start from the top.

A caliper – whether vernier, dial, or digital – is most assuredly not a precision measurement tool. Feel free to ask any tool & die maker the question: “how accurate is a caliper?” I have yet to meet one who would trust a caliper for anything less than 2/1,000ths of an inch (.002″) For reference, this is the difference between measuring, say, .357″ and .359″. On a good day (meaning a very experienced operator) with good equipment (meaning not a Harbor Freight special) one might be able to do a bit better, but most people aren’t all that experienced, and most do not possess the top-quality equipment necessary.

This is actually extremely easy to test: take a caliper to a local tool & die shop, and ask the owner if he’ll let you measure his certified, calibrated toolroom gage blocks. If he lets you (he probably won’t), you’ll probably find that getting to within .002″ with any consistency is not possible. I have a set of said blocks, and I can’t do much better – even though I’m experienced, and have top-end Swiss Etalon calipers with which to work!

There’s a reason watchmakers measure parts that must be fitted to incredibly close tolerances with micrometers, and not calipers. The same goes for precision machinists. Do I need to keep flogging this deceased equine?

(I haven’t even touched on the need to hold the calipers perfectly perpendicular to the axis of the bore, and to get the jaws as close to centered on the inside surface as possible. It’s darned difficult to do under the absolute best toolroom conditions, let alone at a kitchen table! Errors multiply under less-than-ideal conditions.)

Let’s tackle the second criticism: that one can’t measure an out-of-round condition with a pin gage, therefore the best way to do it is with a caliper. By now, the answer should be obvious: if a hole is, say, .002″ out of round, and the measuring system can’t get within that range to begin with, it follows that one can’t measure the condition because it’s within the amount of “slop” already present!

In other words, if a caliper indicates that the hole isn’t round, we can’t trust it because we don’t know if what we’re seeing is real or simply the result of the errors inherent in the device. Conversely, the absence of a round error doesn’t mean that the throat is round – because it may be within the normal error of the caliper being used! (This is why one does not use imprecise instruments when one expects a precise result.)

The exception is if the condition is sufficiently severe that it exceeds the error of the tool – but if it’s that far out, it can be easily spotted with the pin gage anyhow. While we can’t measure an out-of-round condition with a pin gage, we can certainly identify that an out-of-round condition exists, and elect to measure it with more accurate means.

Whew!

Now I’d like to expand on the recommendation in my earlier article. The reason I suggested using calibrated pin gages for measurement is because they’re cheap (a set to cover, say, the range of a .357 cylinder costs less than $20), readily available, and last forever. There are other tools that can be used, but all are much more expensive and require occasional testing & recalibration, as well as a certain amount of technique.

The best choice is a “tri-mic”, made by various companies, which measures holes at 3 points spaced 120 degrees apart. This is extremely accurate – the most accurate way to measure a hole – but that accuracy comes with a price tag of several hundred dollars for the least expensive example. That’s why I didn’t recommend them, though in hindsight I should have at least acknowledged that they exist.

Bottom line: there is no substitute for knowledge, experience, and the proper quality tools when one is doing precision work.

I hope this puts the matter to rest – though I somehow doubt it!

-=[ Grant ]=-

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About the Author:

Grant Cunningham is a renowned author and teacher in the fields of self defense, defensive shooting education and personal safety. He’s written several popular books on handguns and defensive shooting, including "The Book of the Revolver", "Shooter’s Guide To Handguns", "Defensive Revolver Fundamentals", "Defensive Pistol Fundamentals", and "Practice Strategies for Defensive Shooting" (Fall 2015.) Grant has also written articles on shooting, self defense, training and teaching for many magazines and shooting websites, including Concealed Carry Magazine, Gun Digest Magazine, the Association of Defensive Shooting Instructors ADSI) and the popular Personal Defense Network training website. He’s produced a DVD in the National Rifle Association’s Personal Firearm Defense series titled "Defensive Revolver Fundamentals" and teaches defensive shooting and personal safety courses all over the United States.
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