doublegunshop.com - home Forums General Discussion DIY Gunsmithing @ doublegunshop.com Tool making: el-cheapo bore gage

It's always been a clumsy task for me to get a good bore diameter reading with the tools at hand. What I have works but something better is needed. Looking at the inside micrometers offered by many name brand companies I quickly realized a simple fact. I can't afford those things! Especially the deep bore models. Checking the chamber diameter, bore diameter at 10" up from the breech and bore diameter again at around 20" and finally at the muzzle gives a good indication IMO of how much life is left in a set of old barrels. Wall thickness also comes into play and we'll touch on that a bit later. So, what I'm looking for is a tool that can be used to accurately check bore diameter at each depth mentioned above. Lacking the extra mega bucks to get one ready made we're going to build one. A really cheap one.

We start with this:

This bore gauge set came from Harbor Freight, $15.00 out the door. The three smaller sizes A, B & C are of particular interest as they are suitable for every barrel size from .410 up to 10 bore. They're obviously too short to do what we need to do but this is where the fun begins. We're going to stretch 'em out. How you ask? First we cut some parts:

In the pic above there is a 32" length of 5/16" steel tubing, a 34" length of 0.090" diameter wire (it's a straightened coat hanger) and the barrels (handle part) from the A & B size gauges. I'm out of time for now but next we'll pull everything together and see how this thing works.

Hank

Hank,
To insure accurate measurements with the telescoping gauges, I recommend Adam Booth's method. He shows it on one of his You Tube Videos- at abom79.
Mike

I have a set of american starrett telescoping gauges that i bought or was give as an apprentice many years ago, they were not cheap, but as with guns, you buy cheap-you get cheap.

Thanks Mike. I'll look that up.

Bonny that's a good point but I wouldn't use a high end set of gauges to prototype something like this. Will this contraption give acceptable results? We'll find out when it's finished.

This type of gauge operates on a very simple principle: Spring tension holds the button (anvil) against the part being measured. Inside the gauge handle is a steel rod that fits inside the lock nut and locking portion of the anvil. Turning the lock nut at the top of the handle locks the anvil in position. Measuring the width of the anvil with a micrometer or caliper reveals the inside diameter of the work piece. We have the parts ready so let's put this thing together.

The handles from the A & B gauges is soft soldered into the tubing. Both fit perfectly inside the tube so it took very little solder to lock them in place.

Working from the anvil end the wire is inserted and trimmed to length.

With the anvil installed the wire is again checked for proper length at the lock nut end.

When the wire length is set the lock nut is installed. Here's the final product:

At 36" total length we can reach the muzzle of even the longest barrel from the breech which makes things much simpler when checking the inside diameter of a choke bored barrel.

We'll put it to use next and do some testing for accuracy.

Hank

bonny,
Of course, starrett tools are top of the line, no one can argue with that. With using telescoping gauges, what is more important is a reliable measuring instrument, such as Starrett.
Mike

I have a set of the same gauges. The concern is that the contact surfaces are flat, rather than conforming to the inside curve of the tube. It appears to me that the surfaces need to be modified for better accuracy. That's why wall thickness tools sometimes use pointed pin type pieces for the inside contact surface of the tool.

Regards
Ken

Both sets of my telescoping gauges have rounded contact surfaces. I don't think you could get them out, otherwise, if you are using the Abom79 method.
Mike

Spent most of today working with this new bore gage to figure out its reliability and accuracy. Those familiar with gage R&R studies know what this is all about. Examining three barrels sets with a few hundred data points between all three provided excellent results using both an old fashioned micrometer and a digital caliper set. More consistent results are had with the micrometer so we'll be using it in the future for all barrel measurements.

By making the gage over 30" long the inside diameter of the whole barrel can be checked and mapped beginning at chamber opening all the way to the tip of the muzzle to check the choke diameter. The process is still sorta clumsy but we're able to do what needs to be done with a great deal of accuracy. One last thing to do is add rule markings to the tube for depth reference. Here's a couple demonstration pics:

For now the tube is marked with a strip of tape to show how deep we want it to go and the outside of the barrel is marked accordingly. In this example it's 13" from the beginning of the chamber.

While in position the lock nut is tightened to lock the anvil.

Removing the gage and measuring the anvil, obviously, gives the bore diameter at 13".

By measuring the outside diameter of the barrel at the point marked and doing a simple calculation the barrel wall thickness is determined. The wall thickness of any tube can be calculated as (OD-ID)/2. So, for this example the ID is 18.1 mm and 21.3 mm is the OD at this point. Subtracting 18.1 from 21.3 gives 3.2 then dividing that by 2 gives a wall thickness of 1.6 mm or roughly 0.062". The same can be applied to any point on the barrel to determine wall thickness.

I honestly thought the whole thing would turn out to be a flop but this gage exceeds my expectations having a total variance of less than 10% meaning the results can be trusted to be quite accurate. Here's the cost break down:

Gage set: $15.00
Tubing: $0 (salvaged from scrap HVAC equipment)
Wire: $0 (it's a coat hanger)
Solder and gas for torch: $? (Do I really care?)
Total: $15.00

Just for fun I looked up a Dyer deep bore gage. You have to ask for pricing. WOW- It's too rich for me.

Hank

Nice job, Hank. However, wall thickness cannot be determined using your method. All you get is the average wall thickness when you divide by two. One could be .010" thinner or thicker than other, and there's no way you could know that with your method. There are almost always thinner and thicker areas on these old barrel sets, due to the hand striking that was done during their completion.

The proper way to map wall thickness is using a specially made wall thickness gauge. You can google that name and read all about them. Probably even watch them in use on "YouTube".

All my best, SRH

Thanks Stan but I can't fully agree with your comment. It's true that any tube (or pipe) whether it be a Damascus gun barrel or modern made product contains eccentricity. It's unavoidable even with the best process technology has to offer. Locating and quantifying eccentricity can be problematic regardless of the method used but, this method can detect eccentricity. Let's take a look at this drawing and crunch some numbers:

Measuring and comparing the ID, OD and calculated wall thickness of the three points will reveal eccentricity. For example:

It's easy to see there is considerable difference in wall thickness between points A & B and B & C but only 0.002" difference between points A & C. One can safely assume the barrel contains eccentricity. The only downfall is trying to determine which side of the barrel has the eccentricity. We know it lies at point B but is at the top or bottom side of the barrel or both? Does it really matter?

In the extreme, if the location of the eccentricity must be known then this gage can be used to better understand the nature of the eccentricity.

If one has the means and can justify the cost then this high tech ultrasonic gage from Olympus is the way to go for finding the true wall thickness of any tube or gun barrel. But, since I'm a real cheapskate, I'll keep working with this el cheapo bore gage to figure out its limitations and full capability. Hey, it works for me!

Hank