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Old February 10, 2018, 11:34 AM   #26
stagpanther
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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!
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Old February 10, 2018, 01:03 PM   #27
Unclenick
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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.
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Old February 10, 2018, 01:27 PM   #28
stagpanther
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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.
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Old February 11, 2018, 07:42 AM   #29
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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?
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Old February 11, 2018, 11:05 AM   #30
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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:
Quote:
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.
Quote:
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 ΒΌ inch
(7mm) away from metal of different thickness.
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
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Old February 11, 2018, 11:17 AM   #31
stagpanther
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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!
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Old February 11, 2018, 12:07 PM   #32
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Quote:
PS--our roads and my driveway are ice-skating rinks after multiple storms--no fun!
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
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Old February 16, 2018, 08:06 PM   #33
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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.
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