Calipers are an instrument for taking an inside or an outside measurement, having either two bowed legs or a sliding jaw.
The plain calipers seen here are often used in woodworking to determine the diameter of a part by first tightening the jaws until they touch the work, and then removing the caliper from the work and measuring the gap between the end points.
A gauge is what’s used to give a magnitude, such as in inches or centimeters, to that gap in the calipers.
“The diameter of the wood leg is 3.7 inches.”
The “3.7 inches” is read from the gauge.
In other words, calipers measure but do not quantify, whereas a gauge quantifies but does not measure.
When you combine the two, you have gauged calipers and that’s what we use in reloading, gauged calipers.
There are three types of gauged calipers to choose from: digital, dial and vernier.
- Digital (gauged) calipers display nice large numbers on an LCD.
- Dial calipers have a mechanical rotary dial that is added to the reading from a linear ruler-type measure on the same instrument.
- Vernier calipers, like the dial caliper, have two scales to read and add together for the measurement.
Digital calipers — the first kind — can be very inexpensive and are the easiest to use. You turn them on, close the jaws, press the “Zero” button, open the jaws and place them on the space to be measured. The answer is then displayed in the LCD.
I had two calipers of that style and would periodically use one to check the other. If both gave the same reading, I assumed they were both good. (Nothing wrong with that.) If they differed (which they never did), then I would know that one of them was wrong, but not which one.
But one day when I was ready to do some reloading, I discovered that one of my digital calipers was stuck to a certain reading, and the other would not turn on. I tried everything including two fresh batteries in each but, no matter what I did, they were broken.
Fortunately, I had purchased a dial caliper to use in case I needed a tie breaker. I had practiced with it a few times and could reliably get the same measurements with the same accuracy as with the digital.
So I switched to the dial caliper.
The dial caliper is 100% mechanical. There are no batteries to replace and no electronics to get befuddled or fried.
Simple is better.
But the dial caliper contains a gear that spins the dial and, if the slide is pushed too hard too fast, that gear could jump a cog and give an incorrect measurement. Worse, it might even strip off a couple of teeth and get worse and worse and worse.
I put a note on the dial caliper to “Push GENTLY” for that reason.
And like the digital caliper, the dial caliper must also be “zeroed.” You loosen a mechanical set screw, rotate the dial to zero and then re-tighten the set screw. It’s important to check the zero each time you use it. Failing to do so, you’ll get the wrong readings. But that sets you up to forget to re-lock the set screw and, again, you’ll start getting the wrong readings.
That zero adjustment, someone might say, is the Achilles’s Heel of the dial caliper.
To me, the dial-type caliper has two big strikes against it: the strength or weakness of the gearing, and the requirement to be mechanically zeroed.
Again, simple is better.
Are there gauged calipers that are even simpler?
That brings us to the third, and oldest type of caliper, the one with a vernier scale.
In the vernier caliper, there are no other moving parts other than the slide. There are no gears, no digital somethings-or-others to be counted up or down, and there’s no “zero” to be set, nor any lock screw to be forgotten or to work itself loose.
It’s a very simple instrument.
It has a body with a scale, and it has a slide with a scale. Both scales are permanently attached. There is nothing to zero.
To use it, the jaws are opened and then snugged up to the dimension to be measured, and then the scales are consulted to read the numbers. Other than the slide, there are no other moving parts with vernier calipers.
This simplicity makes the vernier caliper very robust, but it also requires a bit more from the user. You’ll need fairly good eyesight and you’ll need to be able to add.
Fortunately for us Bullseye shooters, we probably have those qualifications, either naturally or via some aids such as eyeglasses and calculators.
There are two parts to the reading on vernier calipers: the linear scale on the slide and the vernier scale on the body.
Here’s the linear part of the scale where it lines up with the lower (vernier) scale’s “0” point.
I’ve magnified this quite a bit. The “0” at the extreme far left of the upper scale is the “zero inches” mark. The “1”, “2”, “3” and so forth after that are tenths of an inch. The lower scale’s “0” says this measurement is between 0.4 and 0.5 inches.
Notice that each tenth is divided up into four parts, each part being a quarter of a tenth or 0.025″. Thus, between 0.4″ and 0.5″, the marks are at 0.425″, 0.450″ and 0.475″.
In this picture, the jaws of the calipers have been placed on the bullet’s crimp.
Look now at the bottom scale which is shifted over to the right in this picture. Its “0” mark is between the 0.450″ and 0.475″ marks in the top scale. That tells me the measurement is going to be slightly more than 0.450″ but less than 0.475″.
Notice that we’re taking this reading from the upper scale, 0.450″. When using the vernier caliper, you’ll take one reading from the top scale across from the lower scale’s “0”, and then you’ll take the second reading from the lower “vernier” scale — but this is where the reading gets tricky.
First, you’ll need a good light and your best reading glasses.
To read the vernier part of the scale, find where one of its marks lines up perfectly with one of the marks in the upper (linear) scale. Note that only one set of marks, upper to lower, will line up perfectly. You need to find that one set of perfectly lined up marks.
This is where the good light and best reading glasses comes in.
In this photograph, all of the lines except one in the lower scale don’t line up. They’re either to the left or the right from the marks in the upper scale.
To find the perfectly aligned marks in this picture, examine the marks between “15” and “20” in the lower scale. Notice that the marks for 15-18 are slightly to the right of the marks in the upper scale but the marks starting at “20” and up are slightly to the left. What we want is right where that change occurs.
Where the upper and lower marks are perfectly lined up that’s our second reading.
At that point, read the number on the lower scale.
In the above picture, the marks line up perfectly at “19” on the lower scale.
Here is that same photograph with a red circle added at the “19” mark on the lower scale so you can see the upper and lower marks perfectly lined up. (If you’re having trouble seeing this on a real vernier caliper, don’t despair. Farther down I’ll give you a tip that’ll make finding that mark a bit easier.)
The marks in the lower scale are in thousandths of an inch so “19” is actually 19/1000s or 0.019″. We’re now ready to add the upper scale’s measurement to that from the lower scale.
Add 0.450″ we got from the upper scale to the 0.019″ from the lower scale.
0.450″ + 0.019″ = 0.469″
That’s the measurement, 0.469″.
The first time or two you use a vernier caliper, this may feel laborious. Getting the jaws in the right place, especially for the crimp, is difficult. Then, you have to read one scale, then the other and finally add the two together.
But after a few minutes practice with common objects around the house, this will go faster and faster. Take a regular ruler along as you practice to make sure you’re vernier caliper readings are in the ballpark and pretty soon, you’ll be zipping right along.
It’s actually very simple once you get the hang of it.
I’ve found a couple of short-cuts that might make things easier.
First, what I find myself doing is looking at the left-most mark to see how far off it is and then quickly scanning across to see where it goes from “off to the right” to “off to the left.” Right in the middle of that change is where the marks line up.
Notice I don’t look for the aligned marks. I look for the un-aligned marks at each end and, somewhere in the middle, I know that’s where the aligned marks will be.
Another practical tip is that, in practice, your crimp only needs to be within a certain tolerance. You don’t need the exact measurement. Instead, you need to know it’s not outside of the tolerance.
I asked Bullseye shooters what tolerance they allowed in crimp measurements and the most common answer was “plus or minus a couple of thousandths”.
My crimp for these bullets, therefore, is 0.469″ ± 0.002″, so anything between 0.467″ and 0.471″ is acceptable. On the vernier scale, that anywhere between 17 and 21 (0.450″ + 0.017″ = 0.467″, and 0.450″ + 0.021″ = 0.471″).
Doing the measurement on the vernier caliper, if the alignment of lower to upper occurs in the area of 0.017 to 0.021, then the crimp is good. If the “off side” at 17 is different from the “off side” at 21, the crimp is within tolerance.
My eyes with my good glasses (and in good light) can see that.
So, while the vernier caliper may be a bit more challenging to use, the fact that it is mechanical means there are no batteries to replace. And because there are no gears or levers, there is almost nothing to break by racking through a big change quickly. And because the scales are permanently attached, one to the slide and one to the body, there is nothing to zero.
Vernier calipers are the simplest and, therefore, the most reliable.
But there’s nothing wrong with dial and digital calipers as long as you understand their strengths and weaknesses.
So, if someone says, “Shooters to the line … but no closer than two thousandths of an inch please”, you can volunteer to make sure everyone is in compliance!
(Yeah, I know that’s silly. I get that way sometimes.)