An idea: pressure sensitive cases for reloaders...


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Macchina
May 3, 2014, 10:08 PM
Just spit balling here, but how cool would it be if somebody was able to manufacturer a set of "pressure telling" casings we could reload? These cases would be loaded like any other brass case but set up to alert the reloader when a loaded round was fired at or near the max pressure for the given round. These cases would be sold in small numbers (10 or so) and one or two would be used during high-end load development to make sure a load is still safe.

I'm not sure how it would be done, however the case would have to be exactly identical in capacity to a standard case to ensure an accurate test.

Some possibilities would be a pressure sensitive paint on the inside of the case that was calibrated at the right pressure. Maybe also a small closed vessel in the head of the case (exposed to the powder of course) that would rupture at the max pressure.

I'm going to think on this a bit more, any thoughts? Would you be interested in something like this?

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WestKentucky
May 3, 2014, 10:14 PM
A burst disc would take up volume. Like the idea but not sure how it could be done.

rcmodel
May 3, 2014, 10:33 PM
It's been done for a century with new factory cases from the same lot number.

That's how the reloading manual publishers did it before pressure testing equipment became available, and affordable for small companies.

Not very accurate, but that's all they had to go by back then.

The procedure is to load in increments, then measure case head expansion in 10,000ths of an inch as the pressure goes up.

The problem with pressure sensitive paint cracks, etc, is.
Loose factory chambers.

What cracks in mine won't crack in yours if you have a tight match chamber that won't allow case body expansion to crack the paint.

What doesn't change however, is the brass case head hanging out of any chamber.
When the new brass case head starts to flow and expand, pressure is getting very high.
And it's time to stop and back off.

rc

Macchina
May 3, 2014, 11:15 PM
I hear you RC, but what about low pressure cartridges like .38 Special. This is the round I was thinking about. You are right though: in high pressure cartridges, this is a technique I could (should) be using if I want to push the envelope.

.38 Special doesn't really show any of the pressure signs we're used to because it's max pressure is so low. If you have to two cases out our are having excessive case head expansion in a .38, you're probably already WAY over pressure...

119er
May 3, 2014, 11:42 PM
Maybe something like a plasti-gage band at several locations with known case wall thicknesses and other properties. Basically a calibrated case. Maybe the manufacturer could supply a table to compare measured thickness at the multiple locations for an average result. This would keep the math minimal to the end user.

Plasti-gages are usually used in checking bearing clearances, but the case forces against the chamber, so this may not be viable.

Fun idea, but I'll stick to my proven reloading data sources and spens my money on more components!

amlevin
May 4, 2014, 09:45 AM
The "pressure indicating cases" would most likely be expensive. Also expensive on a continuing basis as one would need a set for just about every load they develop.

There's a better system on the market and in the long run far less expensive.

For about the price of a mid-range hunting rifle one can buy the RSI Pressure Trace II as well as the software to analyze all kinds of things that are happening in the barrel when the round is fired.

http://www.shootingsoftware.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=RSI&Category_Code=PT

Of course most of us "old timers" just increase the load until we hit either a super accurate load or the bolt lift starts to get a little difficult and then proceed accordingly. That method has also been used for almost 100 years:rolleyes::rolleyes:

Offfhand
May 4, 2014, 02:19 PM
Something somewhat similar to what you suggest has already been done and was, in fact, moderately successful. Several years ago John Nosler developed a simple pressure measuring system that utilized standard cases fired in standard rifles. Essentially, it worked by embedding small pressure specific discs in the outer case wall. The discs were calibrated for pressure zones and were accurate to within about 5000 psi, which gave fair warning when a load was approaching or exceeding max levels. I do not know if John patented the system, but even if he did it would have expired by now. Perhaps someone else will give it a try. Attached is a photo of three of the caliber specific dies used to prepare cases for the discs, also a prepped case.

Kernel
May 4, 2014, 06:21 PM
How about a gas check with some kind of pressure sensitive properties? Attach it to a lead bullet, shoot it into a sand berm, dig it out and observe.

Or a special primer that had an internal structure that would fail or deform above a certain pressure? Shoot the rounds, pop the primers, interpret the internal deformations. Primers are pretty simple, but they do have some "guts" that could be made to fail or deform in a calibrated way.

amlevin
May 7, 2014, 11:49 AM
How about a gas check with some kind of pressure sensitive properties? Attach it to a lead bullet, shoot it into a sand berm, dig it out and observe.

Or a special primer that had an internal structure that would fail or deform above a certain pressure? Shoot the rounds, pop the primers, interpret the internal deformations. Primers are pretty simple, but they do have some "guts" that could be made to fail or deform in a calibrated way.
Rifle bullets like those I shoot end up fragmented in the berm and are impossible to recover. No joy for that method in my case.

As for other methods that work similar to the "crusher method" where any device or object requires a mechanical measurement and then conversion to "pressure", the opportunity for error is too large.

I don't think there would be much support for a method that used primers, where you would increase and shoot loads until the primer "popped". Enough of us have had that happen already and the damage it does to bolt faces is not nice.

While typing this I did have a thought. A calibrated groove or hole cut in a case head that is reduced in depth when the case pressure limit is reached might be an option. Of course the method of accurately measuring and the consistency of he test case material would be critical.

I kind of used this method informally when shooting a .357 magnum. I reduced my loads by .5 gr when I saw the headstamp marks flattening out after firing a new case. :)

JRH6856
May 7, 2014, 12:09 PM
the case would have to be exactly identical in capacity to a standard case to ensure an accurate test.

I have yet to find a standard case that is "exactly identical" in capacity to a standard case. Well, some are and some aren't. Out of a 100 new unfired cases, the empty case weight may vary across the lot by several grains, and so does the case capacity (measured in grains of H20).

Robert101
May 7, 2014, 06:29 PM
I'm not sure why a dimple method could not be developed. We use dimpling to test lead hardness and using a case dimple (in a controlled circumstance) could be an effective means to generalize pressure. The dimple would not have to be large as to effect case volume. Just simply compare the degree of remaining dimple to a chart for comparative pressure results. Was this the intent of the Nosler cases?

spitballer
May 8, 2014, 02:40 PM
Yet another interesting post! Thanks to Offhand for sharing those intriguing images and the story that goes with them.

I, too, have always wondered about this. My first inclination would be a pressure-sensitive strip of some type. Probably couldn't be fitted very easily on fire-formed cases.

I'm beginning to like this crowd...

Kernel
May 9, 2014, 07:40 PM
until the primer "popped"

That's funny you'd take it that way. If you ever took a primer apart you'd see there is an internal support structure. A couple little tiny metal pieces. Every manufacturer has a design that's a little different. I was thinking you could design those supports to fail or deform at a certain pressure. It would be a go/no-go kind a thing. Shoot the round, remove the primer, take it apart to see if the little piece inside failed or is still intact. If it's intact the design pressure has not been exceeded. If it's failed, you exceeded the design pressure - which could still be well below max. The challenge and expense would be designing the little metal thingies to fail or deform in a calibrated and consistent way.

J.R.W.
May 9, 2014, 09:52 PM
I'd have to guess that the initial pressure caused by primer ignition in the pocket would exceed the overall case pressure from the powder burning. Also, variations in firing pin force and protrusion would probably skew the results too.

FastCut
May 9, 2014, 10:38 PM
http://www.sensorexpert.com/

We use this to check clamp ups at Lock Mart....as well as other similar media. I am not sure but you might be able to place a thin strip along brass and then fire which could yield some interesting information but it would have to be calibrated somehow since it is being used in a non-standard condition.

Kernel
May 9, 2014, 11:45 PM
Tactile Surface Pressure Indicating Film.

Sheesh, why's eveything gotta be called "tactical" these days. Oh, wait..........

All kidding aside, that stuff looks interesting.

JRH6856
May 10, 2014, 12:28 AM
With a max range of 43,200 psi, that film might be usable for gauging pressure in handguns, but max is too low for rifle cartridges. Not sure it could stand up to chamber temperatures though.

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