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Measuring chamber throats – calipers vs pin gages.

Measuring chamber throats – calipers vs pin gages.

There is a huge amount of misinformation regarding revolver accuracy. Folks, assuming that you have a gun in proper repair – timing, lockup, chamber-to-bore alignment – the most important factor in accuracy is the chamber throat dimension.

What is the chamber throat? It is the slightly constricted opening in the chamber, just in front of the cartridge mouth, that the bullet passes through on its way into the forcing cone. The throat gives the bullet its first stabilizing guidance, and many people better than I have demonstrated that it is critical to good accuracy – perhaps more than the bore itself!

The best accuracy is obtained when the bullet diameter and the throat diameter are exactly the same; in the case of lead bullets, it can be up to .001″ smaller than the bullet diameter with good results. If the throat is larger than the bullet, then the bullet sort of wallows through the throat and never does get that initial guidance. Accuracy will suffer.

It is therefore important to serious shooters to know what their throat diameters actually measure. Now, I took heat from some internet experts recently when I stated that one cannot get proper measurements of throat diameters using calipers – dial, vernier, or digital. One fellow wrote me that he’d been doing it for years with nothing more than a cheap dial caliper, and the readings were always “nuts on!” While I don’t wish to argue with anyone, let me relate a little test I did.

I took a cylinder that happened to be on my workbench – a S&W Model 60 “J” frame cylinder – and measured its throats with calipers, then with a set of certified pin gages. There were three different calipers – a vernier, a dial, and a digital electronic – all of Swiss origin. The Swiss make the finest calipers on the face of the earth, and substantially better than the Chinese tools most stores sell. In addition, I’ve been measuring very precise watch and clock parts since I was a teenager, and have more experience using quality measuring devices than the vast majority of people you are likely to meet. In other words, I know what I’m doing and I’ve got the best tools to use!

I started by checking the throats from several angles, to eliminate the possibility that they were oval instead of cylindrical. Since this is a brand-new cylinder, the readings were identical, showing that the throats were indeed machined correctly.

What did I find? The vernier caliper indicated the throat diameter was .355+”, the dial caliper showed .3560″, and the digital read .3555″. Now for the moment of truth: the certified pin gages, which are the most accurate method of determining a bore size, proved that the bore was in fact .3585″ ! That is between .0025″ and .003″ discrepancy!

Precision machinists will quickly tell you that a caliper – even the best, like I have – are only good to a “couple of thousandths” (.002″), and not reliable at all for inside measurements under a couple of inches. (Frankly, I was surprised that I got as close as I did!) The verdict? One simply cannot measure throats precisely with a caliper, even using the best that money can buy – they aren’t sufficiently accurate.

(It should not come as a surprise that I’m not a big fan of calipers; I don’t use them for anything remotely critical. I consider them to be “ballpark” instruments at best, and rely on best-quality Swiss micrometers for about 90% of my work. What does your gunsmith use??)

-=[ Grant ]=-

  • Posted by Grant Cunningham
  • On August 4, 2006

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