case head expansion

[TackDriv3r]

I was bored and decided to do some measuring, measured the case head above the belt on the virgin Norma 300 Win Mag brass and on the fired brass. Difference on fired brass grew .005" on the case head above the belt, so I full sized with the bushing and measured again and its .001" less than the measurement on the fired cases. Is this measurement reasonable?

Measured the belt on virgin and fired cases, .003" expansion. Reasonable?

 

[fguffey]

I was bored and decided to do some measuring, measured the case head above the belt on the virgin Norma 300 Win Mag brass and on the fired brass. Difference on fired brass grew .005" on the case head above the belt, so I full sized with the bushing and measured again and its .001" less than the measurement on the fired cases. Is this measurement reasonable?

You have proven the case can be sized down to the belt; I have belted cases that have increased in diameter .017", if I had an interest in reducing the case diameter above the belt I would have to use a collet in my lathe. The case expansion above the belt was caused by heavy loads and non support of the case in front of the belt.

F. Guffey

 

[ireload2]

Your measurements are affected by pressure, the strength of the brass and the size of the chamber.

The fired brass will expand less or more if the chamber supporting the brass is smaller (close fit ahead of the belt) or if the chamber is larger (sloppy fit with lots of clearance ahead of the belt). When the chamber is large the brass just expands until it is stopped by the chamber walls. This expansion happens at standard pressures.
(It is even worse with excessive pressures.)
Just ahead of the belt, the thin case wall joins the solid head. At this junction the case will expand easily if the chamber is larger than the unfired case.
I would prefer minimal clearance on the order of .002 to .003. This minimizes case expansion at that junction. However cases require manufacturing tolerance for economical production. Chambers also require tolerance to keep the costs down. A maximum case must always fit a minimum chamber. So the usual clearance can seem to be huge to accuracy shooters.

Your numbers are reasonable to me compared to belted chambers I have experience with.
There are other chambers such as those on the 6.5 Arisaka and the .303 British that are at least .010 larger than the brass. They produce much worse expansion than what you have even with low pressures.
You can have the opposite effect too. A close fitting chamber and brass combination will show very little expansion because the chamber supports the case really well.

 

[fguffey]

The case expansion above the belt was caused by heavy loads and non support of the case in front of the belt.

And I ask: If the case head spaced on the belt of the case how can the case in front of the belt be unsupported? And then there should be a question about where I got the ideal the case in front of the belt was not supported and then I could ask: "How could that be?"

F. Guffey

 

[ireload2]

You always have questions but no answers. How could that be?

And I ask: If the case head spaced on the belt of the case how can the case in front of the belt be unsupported? And then there should be a question about where I got the ideal the case in front of the belt was not supported and then I could ask: "How could that be?"

F. Guffey

 

[SheepDog]

Sure its reasonable. You are measuring the amount that the case expands minus the retraction of the brass after firing.
That measurement can tell you a bit about the resiliency of your brass and less about the size of your chamber and virtually nothing about the chamber pressure.
Measuring the "pressure ring" of brass is done at the middle of the web that the flash hole goes through. It's the thickest brass on the case and the measurements are taken in .0001" inches. It is said that if the diameter of the brass at that point expands more than a certain amount (.0003"?) you have excessive pressure. The truth is it's a lousy way to test for pressure because the hardness of the brass varies enough that you can be well over 70000 psi and get very little expansion or you can get a lot of expansion at pressures below 60000 psi. The best test I know of to check for excessive pressure is to load your brass until the primer pocket expands. If that happens in under ten loads you have excessive pressure for your gun and that cartridge. You can change cartridges and run the test again over and over until you find a cartridge that lasts more than ten rounds or you can drop your powder charge to reduce the pressure or back away from the lands a bit to reduce pressure.
What you're doing has no real meaning but .003" expansion sounds like a reasonable amount of expansion at that point. You might get more or less at different points on the brass but as long as the brass isn't splitting it's all reasonable.

 

[fguffey]

Sure its reasonable. You are measuring the amount that the case expands minus the retraction of the brass after firing.
That measurement can tell you a bit about the resiliency of your brass and less about the size of your chamber and virtually nothing about the chamber pressure.
Measuring the "pressure ring" of brass is done at the middle of the web that the flash hole goes through. It's the thickest brass on the case and the measurements are taken in .0001" inches. It is said that if the diameter of the brass at that point expands more than a certain amount (.0003"?) you have excessive pressure. The truth is it's a lousy way to test for pressure because the hardness of the brass varies enough that you can be well over 70000 psi and get very little expansion or you can get a lot of expansion at pressures below 60000 psi. The best test I know of to check for excessive pressure is to load your brass until the primer pocket expands. If that happens in under ten loads you have excessive pressure for your gun and that cartridge. You can change cartridges and run the test again over and over until you find a cartridge that lasts more than ten rounds or you can drop your powder charge to reduce the pressure or back away from the lands a bit to reduce pressure.

SheepDog, I appreciate all the work and effort you put into your response; and hope I did not contribute to the confusion. Case head expansion is measured on factory, new over the counter ammo. Case head expansion measurements are used for comparison. In the old days case head expansion with factory loads was .00025", when a reloader wanted to know how his reloads compared to factory ammo he loaded the once fired cases and fired them; after firing he would compare the case head expansion again. By this time the case head has expanded .0005". Because sizing and firing the case works the brass no one continued to measure case head expansion after the 2 firing.

But as always there were reloaders that thought the case head would expand .001" after firing the case 4 times. Then there is the expansion of the primer pocket, if the primer pocket expands/increases in diameter the flash hole diameter increases and the case head shortens from the cup above the web to the case head; and that is the reason the case head expands or increases in diameter.

F. Guffey