Ballistic testing- strain gauge

[LaPoint]

Thanks Dogchaser37 & Megasupermagnum for discussing the Pressure Trace system. I have looked at them but haven't been willing to spend the $$. I think that you may have pushed me over the edge. On a side note-- Dogchaser37 I have a sweet spot for GWPs. I had a small male for almost 15 yrs. He was the most versatile hunting dog I've ever hunted behind. He was a great family pet and too smart for his own good sometimes but we loved him dearly. He passed a little over a year ago and still miss him daily.

[megasupermagnum]

You don't need, or even want multiple rounds for calibration. You can read about the specifics in the PTII manual. What is very important is that you have your barrel inputs correct. The OD of the barrel is easy, just measure the exact spot you put the strain gauge. The ID of the chamber is a little tougher, but I simply slug it with a lead ball. Even being off .002" makes a difference, so you can't guess on this at all. If you do that right, calibration is more or less just checking to make sure it is reading right.

[Larry Gibson]

With a new test barrel or if a new strain gauge in installed I run a 10 shot test of the reference ammunition/load i have for that barrel/cartridge. Pressure tests, like chronograph tests, will give a different average psi even if tests are conducted back to back with the same ammunition. Also, the variation will and can seem a bit large as compared to average fps variation. If all checks out with the reference being close to the expected psi then we can compute a correction factor. I will occasionally then shoot a 3 or 5 shot test of the reference ammunition/load, especially if the test barrel hasn't been used in a while. If the average psi falls within the ES psi of the original 10 shot test, then the test barrel is good to go.

Just a couple days ago I pressure tested some 35 Rem loads in my M91 Mauser with Shilen barrel. I had not shot that rifle in well over 2 years, so I ran a six shot test out of the last box of Federal 35 rem PowerShok. The average psi was 1,800 psi over the previous test almost 5 years ago. I also tested a carefully duplicated load from Lyman's 50th manual with everything the same as in the manual except the 200 gr J bullet. Lyman used a 200 gr FTX which as seated quite deep, and I used a 200 gr Rem CorLok which was seated to the bottom of the case neck. Because of the small 35 Rem case i expected a slightly lower psi than the 32,400 psi Lyman listed with the FTX bullet. And so it was, my load with the Remington bullet ran 2,000 psi less. Also different conditions can make that much difference also. That's why it is also imperative to enter the conditions for each test into the computer program. You can do that with both the M43 PBL and the Pressure Trace II. Thus, the test barrel was reading what it should have within test parameters.

Now some may think that is an awful lot of difference and it does seem so. However, it is not. Anyone familiar with pressure testing and having enough experience at such will quickly understand that. It is why SAAMI lists not only a MAP for each cartridge but also a MPLM, a MPSM and MEV can be calculated. Let me add that pressure measurements are not as "precise" as many, if not most, think.

[megasupermagnum]

I was going to ask you how you deal with the winter temperatures Larry, but I forgot you are down in paradise. If the weather there is anything like the few times I've been to Yuma in the winter, it's probably a beautiful 70-75 there, might even need a long sleeve shirt. I guess a better question is how do you deal with your summer temperatures when it's 100 every day?

[Stockcarver]

Fun stuff. Photos from some 25 years ago. A 50 cal cartridge putting out about 5500 ft/lbs for a lever gun project. Hundreds of test rounds through the M98 test gun.













50 cal is the top rifle

[Larry Gibson]

I was going to ask you how you deal with the winter temperatures Larry, but I forgot you are down in paradise. If the weather there is anything like the few times I've been to Yuma in the winter, it's probably a beautiful 70-75 there, might even need a long sleeve shirt. I guess a better question is how do you deal with your summer temperatures when it's 100 every day?

In the summer with temps of 85 - 110 [it does get hotter, but I don't test on those days] I get to the range about 45+ minutes before shooting light and get set up. That way at first shooting light (when the Sky Screens will function) I'll shoot a test (usually 10 shots) with 2 minutes between shots. I have a small air pump I blow the hot gas out of the barrel with right after the test shot. Then I will shoot a second rifle off another bench for 10 minutes or so letting the test rifle "cool" a bit before shooting another test. I'll stop testing when the temp hits 100 in the shade. That usually gives me about 2 - 3 hours of test time.

Then I'm back home, clean the guns, print out the data sheets, put the equipment away, hit the pool, have a marguerita, take a nap, hit the pool, have a marguerita, take a nap, etc.......it's tough but I struggle through it.....

[toot]

Welcome.

[Larry Gibson]

stockcarver

Nice test fixture. should be easy to switch barrels and even bolts for different cartridges. Too bad we didn't get together for some testing and shooting when I lived up in Tacoma area.

[Dogchaser37]

Cool stuff, if I ever decide to take up centerfire shooting in earnest, I will come back to this forum. Thanks everyone!!

[megasupermagnum]

My first memory of Arizona was getting off a plane in Phoenix. We left Minneapolis when it was reasonably warm, maybe 15 degrees in December. I took one step out of the airport in Phoenix at midnight, and I was blasted with 80 degree hot air. Still it wasn't as bad as the time I went to Miami, and thought I was going to choke on the air. Actually Arizona is one of the most pleasant places I've been, although I could never handle that heat. South Dakota hit 107 this summer, and that was one of the most oppressive things I've ever experienced. I really feel for those in the humid NW that got to feel 115 in that tropic jungle.