testing case expansion


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lilquiz
February 11, 2011, 06:16 AM
There is a method used to check over-pressure by measuring the base of the cartridge with a micrometer.Does anyone use this method and what are the tolerances.

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243winxb
February 11, 2011, 09:25 AM
Bottle neck cartridges like 243 , 30-06 are measured on the web area. There should be 0 (zero) expansion. Its hard to get a correct measurement. Expansion over .001" is easy to see, but you are already too hot/over pressure. The primer pocket will become loose at the same time. If your hot load only expands the web/pocket by .0001" at each firing, it will take a few firings for the primer pocket to get loose. I would guess other types of rounds can be measured on the web, but expansion seems to be common with 40 S&W, and no one seems worried about it. Maybe they should??:confused:

MEHavey
February 11, 2011, 11:06 AM
Problem is, many commercial loads easily produce measureable case head expansion. And therefore, "...case head expansion over that of commercial loads in tha brass..." became the reference point.

~~~~~~~~~~~~~~

Then in a a 24-Hour Campfire article 10 years ago, Ken Howell noted that:
http://www.24hourcampfire.com/ubbthreads/ubbthreads.php/topics/19115/Measuring_case_head_expansion

For decades, I defended the notion of miking cases to get an idea of the internal pressures being developed. I'm the editor who originally published both Bob Hagel's and Ken Waters' articles on how they do it. I also published Waters' "Pet Loads" article as a supplement for his Pet Loads book.
At the time, Waters and I both considered Hagel's method extremely risky. I still do (because it IS!). I supported Waters' more moderate approach. I've since learned how foolish and unreliable any variation of this basic technique is.
• Many cases don't expand enough, even at 80,000 lb/sq in., to warn of risky or excessive pressures.
• Catastrophic failures of overloaded rifles may occur with either the first over-hot round, or they may occur only after years of repeated use of over-hot loads. In the latter type of failure, the rifle has appeared "safe" with these loads, clear up until the time one round "caused" the failure "for no apparent reason."
• Cases work-harden in use. Repeated use makes them become brittle in the crucial portion exposed in the breech — typically 0.200 inch of the head of the case. Cases already too hard to show "excessive" expansion here (some, even at 80,000 lb/sq in.) are especially likely to become brittle in repeated firings and reloadings, and spew wild gas and bits of brass into a shooter's face. I have on hand now a rifle utterly demolished when half the head of the case blew back through the action. The rest of the case is still in the apparently unharmed barrel, but the receiver is in many pieces, and my friend still has one piece of brass in his face (a larger chunk was surgically removed).
• The maximum safe limit for many rifles and cartridges is well below the level of peak pressures that many cases can handle without any discernible or measurable indication of excess.
• The less experienced you are in the use of this method, the greater is the certainty that miking your cases will inevitably lead you to accept dangerously high pressures as "safe."
Careful lab tests of many typical "pet" loads, developed by attention to traditionally accepted "signs" of pressure, have shown their peak pressures to be 70,000 to 75,000 lb/sq in. The highest SAAMI "safe" pressure I know of, for any cartridge or rifle, is 65,000 lb/sq in. Most are lower. Many are much lower.
Some carefully lab-tested loads, developed by miking case rims, webs, and expansion rings, have developed 80,000 lb/sq in. without measurable expansion.
Stay well below the maximum charges listed in the manuals, and you'll be worlds safer without significant sacrifice in down-range performance. No micrometer is a reliable pressure gauge.

~~~~~~~~~~~~~~~~~~

The most useful article I’ve ever read came from Denton Bramwell.
www.shootingsoftware.com/ftp/dbramwell%20july%2019%2004.pdf

In it he argues rather persuasively (data is always persuasive) that expansion measurement IS correlated to pressure, but that the data/pressure correlation is all over the place to the point of not having any real clue as to the actual pressures you’re looking at -- even when completely safe.

His conclusions?
1) Use commercial ammunition.
2) Reload, and stick to the bookloads.
3) Reload, study the books, and get a strain gage.

(His) Bottom Line: “…both the PRE and CHE methods should be retired, and “Rest In Peace”.

~~~~~~~~~~~~~~~~~~~

For myself, I look at (many) Books (most of which disagree); I look at QuickLoad; I look at the Chronograph. That usually gives me enough data to predict/extrapolate a bullet/case/load/powder-lot combination to continue for a half-dozen reloads while watching the primer pocket effects. If the pocket fit is not appreciably affected by that point, I consider myself to be in dependably-useful territory with that bullet/case/powder/velocity combination. Then when changing powder lots, I drop back a couple of grains, adjust back to the targeted velocity from before, and standardize again.

fguffey
February 11, 2011, 11:07 AM
'There is a method used to check over-pressure by measuring the base of the cartridge with a micrometer.Does anyone use this method and what are the tolerances"

There is no method, reloaders do not measure before and again after. There is no standard, is there 'tolerances' the tolerance would change when the case being tested changed and then there is the work hardening of brass, if you overload a new piece of brass you get immediate gratification because the case head diameter increases and is not difficult to measure, after the case head is work hardened an overload can split the case head or blow a chunk out of it but that is the way things go with things that give the appearance of being stronger but at the same time become brittle.

To develop a method: You must start with new/unfired cases, you must measure the diameter of the case head with a micrometer than is capable of measurements of .0001. Because of rounds and tappers a micrometer with flat surfaces struggle to be accurate, so, bladed micrometers are used then there is the difficulty most have with sit-ups as in measuring the case at the same place every time.
Some start with kits, kits as in a new box of 20 over the counter commercially loaded rounds like Remington, Winchester Federal etc., start by measuring the case heads and record each measurement and record, after measuring (for standard) go to the range and fire and measure again, the difference between the measurements, new and once fired, is the standard meaning the case head is expected to increase in diameter each time it is fired if the load is the same, when increasing the weight of the powder or bullet changes are expected in the diameter of the case head, change powder? Start over. Again there will come a point the case head becomes work hardened and will quit expanding, there are those that will continue to use the test cases because the become 'so tough' they can not be compressed when fired, I believe the case that got tougher got more brittle, in the real world when I am told about the high numbers of firings a shooter gets with his cases, I smile, look for a reason to leave and wonder to my self "Did they get to #10 then skip the the next 30 and go straight to #40.

Again a shooter handed me a 35 Whelen case at the Market Hall Gun Show, I examined the case and ask him if that was the only case he owned, I ask him if he loaded that one case over and and over and over etc., he said his rifle had head space problems because the case was coming apart, his case body was .0025 thick, I offered to form 200 cases for him but he did not understand the question.

F. Guffey


To understand what is going on when a case is fired: The body of the case locks onto the chamber but not all of the case is supported by the chamber, some of the case is unsupported as in case head protrusion, the case head is supported by the bolt face meaning when fired the bolt face gets hammered by the case head, the hammering effect on the case head is measured in yield determined by diameter, psi, etc., .7854 of the diameter squared divided into PSI. unless the case and or chamber has been lubed like the slide and glide shooters, they thought they invented a sure fire method of eliminating case head unspent separation and or case stretch, most of those believed/think the firing pin drives the case forward to the shoulder of the chamber, like Hatcher they thought/think the case runs to the front of the chamber, locks onto the chamber then STRETCHES, SO THEY LUBED THE CASE/CHAMBER, some claim it (the method) worked, I say at the expense of the bolt lugs and receiver.

Once you start and find the case head expanded .00025 the first time you can expect the .00025 is the standard of case head upset for a normal load, if you load a case, fire it and discover the case head expanded .0005 thousands you have received the only warning you will may get,, firing that case with that load 10 times could cause a case head expanison of .005 thousands. As in relative?

*as in relative, if the case head expands .005 thousands, the primer pocket expands, the flash hole increases in diameter and the case head is crushed/hammered/suset and shortens and for those that can keep up, the case is locked to the chamber, the case head shortens and as a results the case stretches the only place it can that would be the area between the case body and head of the case EVEN WITHOUT HEAD SPACE being present.

918v
February 11, 2011, 12:03 PM
I use case body expansion to gauge my 9mm loads. 9mm chambers are conical as are the cases. The higher the pressure, the more the case expands. Typically, a 9mm case expands to .391", a +P to .393", and a +P+ to .394"+. I keep my loads to .390" or less.

• Cases work-harden in use. Repeated use makes them become brittle in the crucial portion exposed in the breech — typically 0.200 inch of the head of the case.

Maybe so, but I recently did a test where I reloaded a single 9mm case 30 times in a three hour period, measuring case growth and expansion. You would think the case would work harden, shrink, and expand less. It did not. Case expansion remained between .390" and .391" and the case grew in length by .003".

denton
February 22, 2011, 06:40 PM
I have just registered for the board.

As was stated earlier, both Pressure Ring Expansion and Case Head Expansion do contain some information, but there is so much random noise in the data that excessively large samples are required to provide useful information.

The methods do provide a convincing illusion of providing an answer.

Anyone who wants to test my conclusion can do so for themselves, with no special equipment other than basic reloading equipment and a micrometer. Here is the test:

1. Prepare about six matched pairs of cartridges, with powder charges in about .5 to .75 grain increments, within the safe region allowed by a competent reloading manual.

2. Prepare a piece of graph paper with grains of charge on the horizontal axis and your choice of PRE or CHE on the vertical axis.

3. Warm your rifle by firing about three shots through it. Try to make all the subsequent shots at the same chamber temperature.

4. Fire the 12 cartridges in random order.

5. Place a point on the graph for each of the cartridges, showing your choice of PRE or CHE plus the powder charge for all 12 cartridges.

If you get pairs of points closely grouping around an obvious trend line, PRE/CHE works very well and I'm a monkey's uncle. In the case that your pairs of cartridges are perfectly identical and your choice of measurement system has no error, each pair of dots will occur at the same place, i.e., one on top of another. If you get widely scattered points, with some low charges producing high PRE/CHE and some high charges producing low PRE/CHE, then you have the same result as I got. In that case, you have to ask, what is the smallest change in powder charge that PRE/CHE can reliably detect? The answer to that will permanently disabuse you of using either system.

HTH.

Ol` Joe
February 22, 2011, 09:54 PM
http://www.shootingsoftware.com/ftp/dbramwell%20july%2019%2004.pdf

kelbro
February 23, 2011, 07:27 AM
Anyone who wants to test my conclusion can do so for themselves, with no special equipment other than basic reloading equipment and a micrometer. Here is the test:

1. Prepare about six matched pairs of cartridges, with powder charges in about .5 to .75 grain increments, within the safe region allowed by a competent reloading manual.

2. Prepare a piece of graph paper with grains of charge on the horizontal axis and your choice of PRE or CHE on the vertical axis.

3. Warm your rifle by firing about three shots through it. Try to make all the subsequent shots at the same chamber temperature.

4. Fire the 12 cartridges in random order.

5. Place a point on the graph for each of the cartridges, showing your choice of PRE or CHE plus the powder charge for all 12 cartridges.

If you get pairs of points closely grouping around an obvious trend line, PRE/CHE works very well and I'm a monkey's uncle. In the case that your pairs of cartridges are perfectly identical and your choice of measurement system has no error, each pair of dots will occur at the same place, i.e., one on top of another. If you get widely scattered points, with some low charges producing high PRE/CHE and some high charges producing low PRE/CHE, then you have the same result as I got. In that case, you have to ask, what is the smallest change in powder charge that PRE/CHE can reliably detect? The answer to that will permanently disabuse you of using either system.


I performed just this sort of test in 2009:

http://forums.accuratereloading.com/eve/forums/a/tpc/f/2511043/m/9111088411?r=9111088411#9111088411

Another related post.

http://forums.accuratereloading.com/eve/forums/a/tpc/f/2511043/m/2661028211?r=2661028211#2661028211

As you can see, there are strong opinions around this method. For me, it's just another tool to try to keep from blowing my actions apart.

denton
February 23, 2011, 01:55 PM
Good! You're taking data and analyzing results.

I didn't read closely enough to pick up how the test estimated pressure. If it's from QuickLoad, I'm a little squirmy about how much error that might introduce into the system.

However, you did measure MV and over a normal operating range for a load, MV is highly correlated with pressure. So you can use MV as a substitute for pressure (or powder charge).

If you have MV data for a range of loads you can plot MV on the horizontal axis and PRE or some other estimate of pressure on the vertical axis. The R^2 value for the plot will give a pretty good sense of of the sum of all the random errors in the system. Since chronographs are very highly repeatable, practically all the random error should be the PRE/CHE error.

If you want to see how the data look, I'll be happy to run it through QuikSigma for everyone to see.

243winxb
February 23, 2011, 03:20 PM
Saw this online, found it interesting. modulus of elasticity- Cartridge Brass-
Material is 70 copper/30 zinc with trace amounts of lead & iron , called C26000. Material starts to yield at 15,000 PSI when soft (annealed), and 63,000 PSI when hard.
Material yields, but continues to get stronger up to 47,000 PSI when soft, and 76,000 PSI
when work hardened. Modulus of Elasticity is 16,000,000 PSI. This means to pull a 1.000 inch long strip to 1.001 inch long induces a 16,000 PSI stress.
So if you pull a 1.000 inch strip to 1.005 inch long, you get about 76,000 PSI, which is the max obtainable.

Then Olin info on Cartridge brass here. > http://www.olinbrass.com/companies/fineweld/Literature/Documents/Alloy%20C260%20Data%20Sheet.pdf :scrutiny:

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