DIY strain gauge

[MagicPaddler]

The story

I have been doing something I thought would be interesting to some of you. Don’t laugh at my gun it’s a hammer and a hammer does not need a purl handle to hit a basketball. I have a Savage 220 for those that don’t know that is a 20 gauge bolt action slug gun. I wanted to load up some rounds and produce my own projectiles. Reading manuals on reloading shot shell always have a statement in there about never substitute any component. Other reloaders have said that what they do is they use the powder charge from a reloading manual but use a lighter projectile and look for deformed cases to insure they did not have excess pressure. I did not like that as a way of working up a load. I decided I needed a strain gauge to be able to compare the pressure of my loads to commercial loads. I designed and built 3 amplifiers which I collect data from with an oscilloscope. I save the data to a thumb drive load it into Excel. I use cheap ebay strain gauges and use them in full bridge configuration. The way the gun is built a strain gauge cannot be placed over the cartridge.

What I have

I have placed 3 strain gauge sets on the barrel. The first one is about 1.75” down the barrel from the bolt face. This means the bullet must move about 1.25” to 1.5” before the gauge can sense any pressure from the burning powder. The next set of gauges is about another 1.6 inches down the barrel. And the third is about 17.75” farther down the barrel. The third sensors are a little more sensitive than the others so there is a bigger signal for the same pressure. I shot and recorded several different commercial loads. Comparing 2 I saw something that was more pronounced than I had expected. Below readings are highest voltage (- the noise) while the bullet is still in the barrel.

Round ID___Gauge 1__Gauge 2 __Gauge 3

1600F/S slug__ .41V ___ .58 V _____.17 V

1900F/S slug___.39V ___.49V______.28V

The time it takes for the 1600F/S slug to get from 1 to 2 is shorter than for the 1900 F/S slug. Which agrees with what the voltage is telling me. The time it takes for the 1600 F/S slug to get from 2 to 3 is longer than for the 1900F/S slug again agreeing with what the voltage and the package labeling is telling me. Did not expect the 1600 F/S slug to have higher pressures on gauge 1 &2. With 1 &2 having higher pressure I did not expect gauge 3 to be lower.

 

[Rocketvapor]

Powder burn rate difference?

 

[RegionRat]

The velocity of the slug, is from a time integration of the pressure. The peaks are certainly important, but the area under the time curve also is very important if you are trying to back out velocity.

If you were to imagine having a pressure gage that rode the back of the slug, rather than a static point in the barrel, you would be able to integrate that pressure over time. Having one gage over the chamber may also help shed some light.

 

[MagicPaddler]

I think Rocketvapor said it simplest “Powder burn rate difference”. Considering the main reasons we reload, faster bullets and more accurate bullets does this information help. I was told by a reloader that if you keep adding powder many times before the pressure gets too high the round will lose accuracy. He said that if the pressure is high when the round leaves the barrel the muzzle blast will push the bullet off course. I don’t have enough experience to know if this is true. Going to the speed issue. If I were building up a load and got a gauge pattern like the 1900 F/S slug would it indicate that I should try a faster powder to bring the early pressures up but not so fast as to bring the late pressures down?

One observation the 1900F/S slug have a lot of muzzle blast and they are noisy. They also have about 1&1/2 times as much powder as any other of the rounds I have taken apart.

 

[MagicPaddler]

I am still working on getting a round worked up on my 20 gauge. I now have 3 strain gauges on the barrel and I have learned some things. I have tried to go to slower burning powders to get the early pressure down while keeping the later pressures up. Didn’t work. I am using a lighter slug than any of the commercial rounds. I have several of them so that is what I am using to learn what works. One round I tested had higher pressure early than any of the commercial loads and higher than I was comfortable with. I increased the area the burning gas could expand into without moving the slug, Using the same powder and slug caused a very low initial pressure and slightly lower later. So the burn rate is more sensitive to this volume than the powder type. That is why shotguns use fast powders for slower projectiles while rifles use slow powders and have faster projectiles. Shotguns have a wad that compresses allowing the gas to expand and slowing the burn. I have .4 Oz moving over 1900 F/S with significantly lower pressures than the commercial loads.

 

[gunsandgunsmithing]

Just IME, I tend to lean toward the faster powder when given the choice, especially if the area under the curve is suitable. I THINK most of my good results are related to lower muzzle pressures at equal velocities. There seems to be a correlation. Another correlation is the ability of very efficient cartridges to utilize fastter powders as compared to similar cartridges. Someone here, there is an overbore comparison chart. The most accurate cartridges ever developed also seem to be very near the least overbore...or very efficient, which lends well to faster powders. Frankly, lots I don't know or understand in this regard but there are some clear patterns that nearly must be more than coincidence..just too much evidence to be luck, or chance. Something to it. I'm just not smart enough to put all the pieces together well enough...So, I just do what works.

 

[JEFFPPC]

I had the same gun. Killed some big Michigan bucks with it. The most accurate rounds I fired where with one sabot bullet....Barnes. Try them.

 

[RegionRat]

Just IME, I tend to lean toward the faster powder when given the choice, especially if the area under the curve is suitable. I THINK most of my good results are related to lower muzzle pressures at equal velocities. There seems to be a correlation. Another correlation is the ability of very efficient cartridges to utilize fastter powders as compared to similar cartridges. Someone here, there is an overbore comparison chart. The most accurate cartridges ever developed also seem to be very near the least overbore...or very efficient, which lends well to faster powders. Frankly, lots I don't know or understand in this regard but there are some clear patterns that nearly must be more than coincidence..just too much evidence to be luck, or chance. Something to it. I'm just not smart enough to put all the pieces together well enough...So, I just do what works.

Agree. I had the privilege of spending your tax dollars doing lots of this from 16" on down.

What I would say... is it takes several parameters to understand pressure with respect to internal ballistics and what we would say is "good" or "bad".... Not just burn rate.

That said, you can have what looks like great pressure traces and still not have a great performance on the target. Good target performance still requires what we call load tuning.

In design, just keeping the peak pressure inside the safety margin for the gun, barrel, cartridge materials, projectile limits, etc., is cold comfort.

We don't expect "the boss" to accept blown up guns, so by default we don't bother with bad pressure values, but we also don't get credit for good ones unless the load actually performs.

By themselves, pressure traces don't show you enough to say you can tell the difference between a good outcome or a mediocre one. They can show you stayed inside material limits, otherwise known as margins of safety.

Good traces can even tend to give you an idea of barrel exit times versus calculated barrel harmonics. But in the end, you still can't know which exit time is the one that matters on the target till you are done testing on the target.

There are many exit times that correlate to harmonics, we just don't know ahead of time which ones matter. You would rather look at a set of traces that are consistent and overlap each other well, than scattered ones that are ugly and all over the place, but we just don't get any credit for those.

While knowing pressure is very important if you want to be safe, it still requires target testing and that tends to follow field testing rather than lead it, if what you want is target performance. That is not to say pressure testing isn't very important, it just isn't the missing link of load development when it comes to target performance.

Wildcatting with a shotgun can go bad fast, so play it safe and think about having a sturdy pressure test barrel made if you want to do a lot of this. Sooner or later, the odds can catch up with you.

Stay safe. YMMV

 

[Dave M.]

The story

I have been doing something I thought would be interesting to some of you. Don’t laugh at my gun it’s a hammer and a hammer does not need a purl handle to hit a basketball. I have a Savage 220 for those that don’t know that is a 20 gauge bolt action slug gun. I wanted to load up some rounds and produce my own projectiles. Reading manuals on reloading shot shell always have a statement in there about never substitute any component. Other reloaders have said that what they do is they use the powder charge from a reloading manual but use a lighter projectile and look for deformed cases to insure they did not have excess pressure. I did not like that as a way of working up a load. I decided I needed a strain gauge to be able to compare the pressure of my loads to commercial loads. I designed and built 3 amplifiers which I collect data from with an oscilloscope. I save the data to a thumb drive load it into Excel. I use cheap ebay strain gauges and use them in full bridge configuration. The way the gun is built a strain gauge cannot be placed over the cartridge.

What I have

I have placed 3 strain gauge sets on the barrel. The first one is about 1.75” down the barrel from the bolt face. This means the bullet must move about 1.25” to 1.5” before the gauge can sense any pressure from the burning powder. The next set of gauges is about another 1.6 inches down the barrel. And the third is about 17.75” farther down the barrel. The third sensors are a little more sensitive than the others so there is a bigger signal for the same pressure. I shot and recorded several different commercial loads. Comparing 2 I saw something that was more pronounced than I had expected. Below readings are highest voltage (- the noise) while the bullet is still in the barrel.

Round ID___Gauge 1__Gauge 2 __Gauge 3

1600F/S slug__ .41V ___ .58 V _____.17 V

1900F/S slug___.39V ___.49V______.28V

The time it takes for the 1600F/S slug to get from 1 to 2 is shorter than for the 1900 F/S slug. Which agrees with what the voltage is telling me. The time it takes for the 1600 F/S slug to get from 2 to 3 is longer than for the 1900F/S slug again agreeing with what the voltage and the package labeling is telling me. Did not expect the 1600 F/S slug to have higher pressures on gauge 1 &2. With 1 &2 having higher pressure I did not expect gauge 3 to be lower.View attachment 1307777View attachment 1307778

So what exactly did you learn? What pressure is the chamber seeing. Fun project but you could have started 10-15% under max stated load and worked your way up to the beginning signs of overpressure. Your could do that in about 1-2 hours and that includes your drive to the range. I’m sure the strain gage experiment was fun, but it sounds like not only did it not answer your question explicitly, it left you scratching your head with additional questions. Don’t overthink this. What you are trying to do has be done thousands of times by professionals, and their results are printed in black and white in dozens of reputable reloading manuals.
Dave.

 

[RegionRat]

What you are trying to do has be done thousands of times by professionals, and their results are printed in black and white in dozens of reputable reloading manuals.

I am sure he can speak for himself, but what he is trying to do isn't in the load manuals, which was the whole point. How does one proceed with shotgun when they are off the load manuals, while trying to avoid over pressure?

The strain gage DIY approach isn't wrong, it is just one part of the puzzle and many times it doesn't point the way home by itself. YMMV

 

[chop house]

I THINK most of my good results are related to lower muzzle pressures at equal velocities. There seems to be a correlation.

! well stated.

 

[MagicPaddler]

So what did I learn? With a 10% reduction in charge and a lighter load then the manual load you can over pressure your gun and the strain gauges showed me that. They also showed me how to avoid that.



RegionRat

You are way ahead of me. I got to get some loads that will safely come out of the barrel at a good speed first. Then it will be time to try to find a accurate one. I assume that will be by making small changes in the parameters I am playing with now (Powder/burn volume/seating).

I agree Barnes are great.

 

[RegionRat]

So what did I learn? With a 10% reduction in charge and a lighter load then the manual load you can over pressure your gun and the strain gauges showed me that. They also showed me how to avoid that.



RegionRat

You are way ahead of me. I got to get some loads that will safely come out of the barrel at a good speed first. Then it will be time to try to find a accurate one. I assume that will be by making small changes in the parameters I am playing with now (Powder/burn volume/seating).

I agree Barnes are great.

Don't get me wrong, I am just trying to make sure you place a high priority on safety whenever you try for speed/pressure with a shotgun. I give anyone who does a bridge circuit and amplifier from scratch some credit, and want you to keep your eyes and fingers intact.

In some ways, shotguns are just like rifles but in others the consequences of a burst are severe and more likely. So just play it safe and keep a blast shield between you and the test if you are not going to spend on a pressure test barrel.

As a kid, I used to attempt to make special shells like slugs out of hulls and wads that were not listed. That was jut due to being poor, not smart. Many years later when I had labs at my disposal and learned what was really happening, I realized how close I was to disaster on several occasions. I got lucky.

Play it safe and best wishes.

 

[RegionRat]

Universal Test Receiver and test stand. Roughly $8500 and shotgun pressure test barrel (less the piezo transducer) runs about $1200.

 

[JASmith]

Kudos!

Not many of can do what you did!

I had a similar “can’t install gauge over the chamber” problem with a TC Encore and did much the same. I calibrated the Pressure Trace results against kniwn load data.

I thnk the calibration helps one learn a lot plus gives a feel for actual safety standards in load data.

 

[CharlieNC]

Keep up the nice work but be very careful!

 

[MagicPaddler]

I have learned some things.

The faster accurate 20 gauge slugs have higher pressure late in the barrel than some of the others.

I think that what effects early verses late pressure is how fast the power burns, not to be confused with powder burn rate.

With like projectiles I have found ( in ascending order) what effects early to late barrel pressures.

Area the gas can expand into without accelerating the projectile (wadding)

Primer wads and primer wad thickness.

Primer type. I have tested Cheddite, CCI_109, and Winchester black powder 209

With the same amount of powder and same slug the largest effect on peak pressure in the primer.

Primer wads seam to do the same thing as going to a cooler primer.

 

[Dave M.]

Just IME, I tend to lean toward the faster powder when given the choice, especially if the area under the curve is suitable. I THINK most of my good results are related to lower muzzle pressures at equal velocities. There seems to be a correlation. Another correlation is the ability of very efficient cartridges to utilize fastter powders as compared to similar cartridges. Someone here, there is an overbore comparison chart. The most accurate cartridges ever developed also seem to be very near the least overbore...or very efficient, which lends well to faster powders. Frankly, lots I don't know or understand in this regard but there are some clear patterns that nearly must be more than coincidence..just too much evidence to be luck, or chance. Something to it. I'm just not smart enough to put all the pieces together well enough...So, I just do what works.