Driftwood Johnson
Member
"
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."
What he fails to mention is exactly what you thought. Measuring the inside diameter of a fairly small hole with calipers has built in inaccuracy because your are placing the flat part of the inside measuring jaws of the calipers against a curved surface. The tiny flat surface of the jaws will 'bridge' the curved surface, giving a slightly smaller measurement. An error of around .002 or .003 sounds about right. For our application, I don't believe an error of .002 - .003 is acceptable. Not if you are slamming a lead projectile through a hole at high speed. A hole that is smaller than the projectile.
In the past I would bring a cylinder in to work and measure the throats with precision pin gauges. The set I would use has pin gauges from .250 to .500 in .001 increments. This was great for measuring any 38, 44, or 45 caliber chamber throat. Now that I am retired I don't have that option any more. I have considered buying my own set of gauges, but they run over $200 and I really can't justify spending that much on a set of tools I will hardly ever use. There are micrometers, not calipers, made specifically for measuring inside diameters. Sometimes they are called Bore Micrometers. Unfortunately they are very expensive and are usually not made small enough for our purposes.
There is another tool called a Hole Gauge. These are a type of Transfer Gage. You place the tool into a hole, and turn the barrel to expand the split ball until it contacts two sides of the hole, then you remove it and measure it with your caliper or micrometer. There are two types, the full ball type and the half ball type. Google Small Hole Gages and look at the illustrations and you will see photos of lots of these. The half ball type will be better for a shallow blind hole.
Here is a set I bought from from Starrett:
http://www.starrett.com/metrology/m...isplayMode=grid&itemsPerPage=24&sortBy=wp/asc
I bought the four piece set, No. 829EZ that comes in the red plastic folding case. I bought it on Amazon last year for $141.76. Yeah, for that money maybe I should have gone ahead and bought the set of pin gages. But a pin gage set takes up a lot of space, and this set of hole gages is compact and fits in my tool box. I use the .400 - .500 tool for 44 and 45 caliber chamber throats, the .300 - .400 tool is good for 38s.
You can also buy other brands for less money from Fowler and Mitutoyo. You can buy some real cheapies for around $20 too, but you know the old saying of you get what you pay for.
Using these gages takes a little bit of finesse. You turn the barrel to expand the split ball. You have to develop a feel for how hard to turn the barrel because if you expand it too much it is difficult to extract the tool from the hole. And if you turn it too much the split ball will probably spring out a couple of thousandths giving you an inaccurate measurement. I turn the barrel until I just feel the split ball contact the sides of the hole. I take a few measurements and average them. I also make sure to take a few more measurements with the tool rotated 45 degrees or so to get measurements across the diameter a couple of different ways. You might be surprised that some holes can be a bit less than perfectly round. So the convenience of the pin gage is you just select the one that slips easily through the hole and that tells you your diameter. With the hole gage you have to play around a bit and take a few measurements to get the same result. Not really all that difficult, in fact I think it is fun. And the hole gauge will give me an idea of how out of round the hole might be. Don't forget, we are talking about revolvers that are over 100 years old, and the chamber throats may have worn unevenly over he years.
Don't forget to take your measurements directly across the split ball, not at an angle or you will not be measuring the diameter of the hole. This will be obvious.
Regarding calipers: I find calipers are completely adequate for everything I do with guns. I have good vernier micrometers, but I seldom use them. I have a good Mitutoyo digital caliper and an old Starrett dial caliper. With the dial caliper I can interpolate between the marks on the dial for ten thousandths. The digital caliper goes four decimal places, but it rounds to 5 ten thousandths for the fourth digit, so that really does not mean much. However, I find that three decimal places is really all that is necessary for most messing about with guns.
I do not have any cheap plastic calipers. Only good quality metal ones.
Hope this is of some help.
P.S. For a modern revolver made with modern steel, chamber throat diameter is not all that critical, within a couple of thousandths of so. There are of course issues of accuracy, but we are talking about very old revolvers here made from very old steel. For this application, I did not want to be firing a .428 bullet through a .424 chamber throat. Even with soft lead. When the 45 Colt cartridge was first designed, it used a hollow based bullet that would expand slightly to fill chamber throats of various diameters. We are not talking about hollow based bullets here, we are trying to match bullet diameters pretty closely to chamber throats. Bill reamed the chamber throats of my Pocket Army Merwin Hulbert to .429 before I fired a shot out of it. Yes, I found smokeless fouling down inside it when I first opened it up, so Lord only knows what type of cartridges with what diameter bullets had been fired through it over the years. But I wanted to err on the side of caution, which is why I had Bill ream the chamber throats. He did the same with a 44-40 Colt New Service that also had tight chamber throats. Again, a 100+ year old gun that had probably had thousands of rounds fired through it, but I wanted to err on the side of caution.
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."
What he fails to mention is exactly what you thought. Measuring the inside diameter of a fairly small hole with calipers has built in inaccuracy because your are placing the flat part of the inside measuring jaws of the calipers against a curved surface. The tiny flat surface of the jaws will 'bridge' the curved surface, giving a slightly smaller measurement. An error of around .002 or .003 sounds about right. For our application, I don't believe an error of .002 - .003 is acceptable. Not if you are slamming a lead projectile through a hole at high speed. A hole that is smaller than the projectile.
In the past I would bring a cylinder in to work and measure the throats with precision pin gauges. The set I would use has pin gauges from .250 to .500 in .001 increments. This was great for measuring any 38, 44, or 45 caliber chamber throat. Now that I am retired I don't have that option any more. I have considered buying my own set of gauges, but they run over $200 and I really can't justify spending that much on a set of tools I will hardly ever use. There are micrometers, not calipers, made specifically for measuring inside diameters. Sometimes they are called Bore Micrometers. Unfortunately they are very expensive and are usually not made small enough for our purposes.
There is another tool called a Hole Gauge. These are a type of Transfer Gage. You place the tool into a hole, and turn the barrel to expand the split ball until it contacts two sides of the hole, then you remove it and measure it with your caliper or micrometer. There are two types, the full ball type and the half ball type. Google Small Hole Gages and look at the illustrations and you will see photos of lots of these. The half ball type will be better for a shallow blind hole.
Here is a set I bought from from Starrett:
http://www.starrett.com/metrology/m...isplayMode=grid&itemsPerPage=24&sortBy=wp/asc
I bought the four piece set, No. 829EZ that comes in the red plastic folding case. I bought it on Amazon last year for $141.76. Yeah, for that money maybe I should have gone ahead and bought the set of pin gages. But a pin gage set takes up a lot of space, and this set of hole gages is compact and fits in my tool box. I use the .400 - .500 tool for 44 and 45 caliber chamber throats, the .300 - .400 tool is good for 38s.
You can also buy other brands for less money from Fowler and Mitutoyo. You can buy some real cheapies for around $20 too, but you know the old saying of you get what you pay for.
Using these gages takes a little bit of finesse. You turn the barrel to expand the split ball. You have to develop a feel for how hard to turn the barrel because if you expand it too much it is difficult to extract the tool from the hole. And if you turn it too much the split ball will probably spring out a couple of thousandths giving you an inaccurate measurement. I turn the barrel until I just feel the split ball contact the sides of the hole. I take a few measurements and average them. I also make sure to take a few more measurements with the tool rotated 45 degrees or so to get measurements across the diameter a couple of different ways. You might be surprised that some holes can be a bit less than perfectly round. So the convenience of the pin gage is you just select the one that slips easily through the hole and that tells you your diameter. With the hole gage you have to play around a bit and take a few measurements to get the same result. Not really all that difficult, in fact I think it is fun. And the hole gauge will give me an idea of how out of round the hole might be. Don't forget, we are talking about revolvers that are over 100 years old, and the chamber throats may have worn unevenly over he years.
Don't forget to take your measurements directly across the split ball, not at an angle or you will not be measuring the diameter of the hole. This will be obvious.
Regarding calipers: I find calipers are completely adequate for everything I do with guns. I have good vernier micrometers, but I seldom use them. I have a good Mitutoyo digital caliper and an old Starrett dial caliper. With the dial caliper I can interpolate between the marks on the dial for ten thousandths. The digital caliper goes four decimal places, but it rounds to 5 ten thousandths for the fourth digit, so that really does not mean much. However, I find that three decimal places is really all that is necessary for most messing about with guns.
I do not have any cheap plastic calipers. Only good quality metal ones.
Hope this is of some help.
P.S. For a modern revolver made with modern steel, chamber throat diameter is not all that critical, within a couple of thousandths of so. There are of course issues of accuracy, but we are talking about very old revolvers here made from very old steel. For this application, I did not want to be firing a .428 bullet through a .424 chamber throat. Even with soft lead. When the 45 Colt cartridge was first designed, it used a hollow based bullet that would expand slightly to fill chamber throats of various diameters. We are not talking about hollow based bullets here, we are trying to match bullet diameters pretty closely to chamber throats. Bill reamed the chamber throats of my Pocket Army Merwin Hulbert to .429 before I fired a shot out of it. Yes, I found smokeless fouling down inside it when I first opened it up, so Lord only knows what type of cartridges with what diameter bullets had been fired through it over the years. But I wanted to err on the side of caution, which is why I had Bill ream the chamber throats. He did the same with a 44-40 Colt New Service that also had tight chamber throats. Again, a 100+ year old gun that had probably had thousands of rounds fired through it, but I wanted to err on the side of caution.
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