Pressure Trace

Thanks for that very useful information nick. When you say "carefully form a cylinder of metal" do you mean machine a level area on the taper cut so that the strain gage can be applied there and lies level (parallel) to the bore?
 
Yes. If you look up Hatcher's experiments bursting guns, you will see he could not burst a Springfield '03 barrel in .30-06 until he got the steel about 1/16" thick over the chamber and bore. You can almost always take a little metal off a modern contour without a pressure consequence. Accuracy consequences are another matter, however. I would not expect Hatchers thinned barrel to have been a tack driver, even before he made it thin enough to pop. Steel is pretty strong stuff.
 
Everything just arrived today from RSI! I just opened up the box and now my whole house smells like an ashtray full of cigar butts. :eek::D
 
Heh, heh, heh! Isn't it funny how tobacco smells are all becoming strange to us now? I'll have to break out a cigar. Been a year or so.
 
I never was a smoker--the cigarettes and odd joint or two I tried in high school to look cool never caught my fancy--thank God.:eek::D
 
Nick--do you do actual castings to get your chamber measurements--or just extrapolate from fired brass (and does it make that much difference)? I think I will beta test a 6mm creedmoor build I did.

thanks!
 
I've done both. The brass forward of the pressure ring is what expands to fill the chamber and it seems to spring back about 0.001"-0.002"; the larger number when the chamber is generous compared to the size of the case. The error varies with the size of the case, but it's about half a grain of water capacity in a .308 Winchester fired in a chamber 0.002" bigger than the case in length and diameter. In a tight match chamber the diameter will change less and a quarter of a grain becomes more likely.
 
I think I "heard" that in many cases cartridge designs are submitted--and tested--under the unrealistic assumption they need to default towards higher pressures, and therefor the test chamber is likely to be exceedingly tight.
 
Successfully initiated the system, connected it to laptop and did a strain gage test. It's funny --the manual has a picture of the test gage traces with a trace with a curve on it--so I spent 10 minutes flicking the gage trying to get the trace to develop a curve like the one in the manual. lol

Ended up completely disassembling my 6mm creedmoor upper, I figure I might as well clean all the components and get a cast done of the chamber while the gun is apart.

It looks like the only way I can attach a gage is directly in front of the case mouth--I assume that when you enter the measurements for the chamber OD--you use that portion in front of the case mouth? Problem is--isn't that tapered--and therefore does not comply with the .25" consistent thickness to either side of the gage requirement?
 
This is like being a kid and getting that brand new bike for Christmas when there is two feet of snow outside and ice covered roads. :)

I just dug out an AR 10 takeoff barrel and looking at the barrel, floated handguard and gas tube it would be just about impossible to mount a strain gauge perpendicular to the barrel axis at the mid cartridge point. On the bright side:
For this application the ideal placement of a strain
gage is as close to the center of the largest part of the
cartridge body as possible. This is typically an area
without taper so the gage will glue perpendicular to
the bore line and is not affected by metal of different
thickness.
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Unfortunately, placement in this location is not always possible if the action is equipped with a barrel nut
(Savage or AR with tube) and/or the cartridge is so short it barely
protrudes from the receiver. Satisfactory results can be achieved even if
the gage is glued over the case mouth but it should be at least ¼ inch
(7mm) away from metal of different thickness.
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As long as you can find a spot which is not in a transition I would expect reasonable results. I would also have thought you should have seen something with the flick test? While not a pretty pressure curve "flicking" the strain gauge as they suggest should have triggered and yielded some bumps on the trace. You may want to go over that again.

Ron
 
The flick test did work and it was gathering bumpy lines at the bottom of the screen--my problem is that I was following the directions from the manual--which for some reason has an actual shot trace superimposed over all the test flicks--so, not being the sharpest pencil in the box, I kept flicking at the sensor thinking I was supposed to get the bell curve. lol

The only question I have about putting the gage at the case mouth is that presumably there will be a step down from the diameter of the neck area to the free-bore area--and the gage will be sitting atop that. I could back the gage over the neck--but then it would be immediately adjacent to the shoulder.

PS--our roads and my driveway are ice-skating rinks after multiple storms--no fun!
 
PS--our roads and my driveway are ice-skating rinks after multiple storms--no fun!
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Same here. Somehow a trip to the range is not quite the fun it is when the range is not snow and ice covered. Like the driveway and everything else around here. :) Yesterday I went out to the garage and started my bike just to hear it run. About this time of the year cabin fever really sets in.

Guessing Unclenick will have some real good thoughts on this.

Ron
 
Well--I got impatient and just wanted to see if the thing recorded atrae or not--so I hooked a gage up to my 260 rem AR10 build which had just enough room for a strain gage to be attached over the case mouth. Configured the software and away I went. Although you don't have to have one--it's a big advantage to use a labradar with the Pressure Trace--for reasons I'll mention in a bit.

I did my handloads since I didn't have any reference ammo on hand. Reference ammo being one that is already tested and has a known psi profile which you can use to calibrate Pressure Trace with--or check to see if your readings "are in the ballpark". I only got 6 shots off, 4 of which returned useable traces (a piece of debris had lodged in the chamber neck area and thus skewed a couple of the readings The good traces were off by about 5,000 psi compared to what QL "forecasted" but upon calling Jim at RSI to talk about my first "toe dipping" he said my results were probably pretty close to accurate for the type of charge and load--even though I had done no baseline calibration.

I've cleaned the chamber thoroughly--and in addition a box of factory ammo with a known baseline pressure (yes, I know it's going to vary due to variations in weapon and conditions) has arrived so I'll give that a go next.

Back to labradar--it's obvious that the folks at RSI saw the potential of using the large amount of projectile trajectory it records and have made good use of it in their stand-alone RSI lab software. It's taking me a bit of time figuring out the idiosyncrasies of navigating and in-putting into the program--but it is quite remarkable and I would say it's worth it's price as a stand-alone product even if you don't want to buy pressure trace. The software allows you to really drill down into labradar's information and do some very detailed analysis of each of labradar's shots--including analyzing anomalies and making statistically-based decisions on whether or not to "toss" the shot analysis. You can even do a "real world" ballistic coefficient calculation after normalizing your data and inputting environmental variables.

It's still a bit over my head at this point--but there are advanced features for analyzing the PT traces which give you a better snapshot of "the best harmonics" for your barrel and ammo that goes beyond simply shooting accurate groups and looking for pressure signs.

Tghe only real downside I've experienced so far is the difficulty of setting the system up for recording from AR's--though that's no fault of the unit itself.
 
It is good to see another person using the Pressuretrace program. Below is a chart I made of my results. Please do not use this information as load data because it can be very incorrect.

I am using a custom made shooting platform. Using an MGM 20" barrel that is 1 1/4" in diameter.

unclenick, feel free to chime in any time!!!!!



Here is a link to one of my videos of the platform
https://youtube/6CiUFqhsFcg
 
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I want to thank those of you keeping the thread alive and adding some really interesting information. I always wondered just how good of a baseline (calibration) could be had with Pressure Trace.

Thanks
Ron
 
hmmmmm, not sure why, I can see it.
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I'm a bit curious about the these results. I'm not disparaging the author or attacking his results, just have some issues based on my limited experience with the PT.

Even RSI will tell you that it's common to have a "standard deviation" in results of approximately 3000 to 4000 psi +/- due to things like variations in gauge attachment points relative to chamber or expansion characteristics of the particular steel, so within the context of the very narrow range of his pressure readings between "acceptable" and "overpressure" I think it's too small of range to be relied upon (7,000 to 12000 psi). Also, I would use published results from a reloading manual as a "second generation" pressure estimate vs. measurements directly from an ammo manufacturer which has a vested interest in maintaining lot-to-lot consistent results. I can only see two of his actual trace results--the rest are blocked by his "corrected" windows. The two traces shown both show one shot with an erratic pressure return--when I see something like that I usually have a connection or gauge attachment issue.
 
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I'm a bit curious about the these results.
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Stagpanther, I assume you are referring to the direct link I posted?

If so, good questions but my experience is way more limited than yours!!! I am still trying to digest a little of what you are asking.

I think it's too small of range to be relied upon (7,000 to 12000 psi).
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If you are referring to testing only a few, maybe 5 cartridges at a time, I have to agree at this point. Al though I don't see your exact reference, I usually test ten at a time and even that may be too few but it is a start.

I would use published results from a reloading manual
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Hmmmm, I will let this digest for a while because I was thinking the opposite...especially after what I see after my results. After talking with Jim for a while, I figured I was on the right track!!

Buffalo Bore was my "control" because of it's consistency. Although I do not know their exact pressures, I used them because they claim to be "below" SAAMI max pressures. There is a total difference in night and day between Buffalo Bore and Winchester's Super-X hunting loads. They are also eager to express concern in not using their ammo in old worn out firearms or in firearms with tight chambers. My test barrel has a tight chamber. If I recall correctly, the highest deviation I got from a 10 shot group was 2,565psi.

As you can see from the handloading manual results, this is where I see my loads as being inconsistent but is expected until I can load them more consistent...which is why I used Buffalo Bore as the control, followed by Winchester's Super-X hunting loads AND (not shown) Magtech's so called Sporting loads....lower than Winchester results at 6,000psi

My MGM "test" barrel is 1 1/4" diameter and is the max allowed by the program. Jim told me this is more than likely the reason for the jagged lines.
when I see something like that I usually have a connection or gauge attachment issue
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Yes, the line "drops" Jim told me was probably connection interference but we could not find the problem. From photos I sent him, he said I shouldn't have any problems from my set-up. He told me that the drop should not interfere with the initial pressure readings.

There are photos on the webpage that show the strain gauge placement and was "acceptable" by Jim at RSI.

I hope to learn more as I go but I really have no desire to use this program for anything other than initial pressures, load consistency and primer dependability since it is not attached to a particular firearm. I have experienced several WLP delayed primer ignitions.

I would like to see some more folks reply to your post...maybe we can get some good answers...sorry I couldn't help....cause I just don't know. :-)
 
OK that's good information to know. I'm not an expert--don't claim to be--especially with the cartridge you're testing. My point is this--as far as I can tell you are trying to figure out whether the buffalo bore is or is not safe to use in the cartridge that has a very small "pressure tolerance" band between roughly 7,000 and 12,000 psi according to the figures you are using. When you are "correcting" the deviation from what is expected to what is returned by the instrument--it's very important to know what the accuracy of your baseline measurement is. That's why I would call an ammo manufacturer and (being really nice, because they probably have lawyers telling them to be careful for liability reasons) ask them what their pressure baseline is for an off-the-shelf retail cartridge is.

All of the cartridges I have tested for generally are in the range of 17000 to 65000 psi. A deviation of 3 to 4000 psi is not going to be a big deal as long as I know when I'm in the vicinity of the upper end of the safe pressure limits. Your pressure limits I'm guessing are probably dictated by an older cartridge originally made for a black-powder, low pressure tolerance old rifle/revolver. Your weapon's "tolerance" for pressure range is so small that I would be very reluctant to rely on the instrument's output alone unless you had a very high confidence in the pressure rating you use as a baseline for calibration/correction.

Don't tell anyone I said this, but I would be looking at one of the top ammo manufacturers like one that begins with an H and ends with a y to see if they are nice enough to help you with this.:D

As an aside--it gets really dodgy when using the instrument for testing a new wildcat for obvious reasons.

I also use "predictive" aids like labradar and quick load to see if they get me in ballpark with velocity and pressure ratings reasonably close to what I'm expecting from the pressure trace.

PS--another thing is that the ammo manufacturers can use a powder blend that is made to their specifications so your chances of matching their results for a given velocity and a given pressure with off-the-shelf consumer powders may not be a reasonable expectation. Those clever engineers at Hornady are particular notorious for achieving velocity results at lower pressures that I have been generally been unable to match in my handloads.
 
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