A Few Questions Regarding Brass and Chambers

[LA50SHOOTER]

75kpsi is true in the annealed state - but the barrels we use are heat treated & stress relieved.

 

[Richard Coody]

where does 75 ksi come from ? or even 75Kpsi ?
I'm quite sure that loads have ran (probably most cases not on purpose) higher than 75Kpsi, and I'm not talking about the threaded portion of the barrel in the receiver ring. - There are some manufacturers that "proof" firearms to 1.25 to 1.5 times the spec'd industry standard. - For a regular .308 Winchester it would be above 75Kpsi.

the yield strength of the steel the barrel was made from. 75,000 pounds per square inch.

3.14 x D x L = crossectional area

lets say 4.5 sq in which equals roughly 300 to 350 ksi

 

[Richard Coody]

75kpsi is true in the annealed state - but the barrels we use are heat treated & stress relieved.

View attachment 1058857

cool did not know that. still leaves plenty of safety factor with the crossectional area we are dealing with.

 

[rammac]

Recently, a discussion came up regarding the details of brass/chamber relationship and causes of difficult bolt lift. There wasn't a clear answer and it caused me to investigate. I didn't find anything that made it clear to me, so I'll ask here, hoping to get some hard, detailed facts.

Thanks in advance.

1. What is the relationship between the brass and the chamber when firing? They both grow under pressure and they both shrink back, but at different degrees? In particular, how does over-pressure change this?
2. Why doesn't virgin brass always grow to full fire formed dimension in one firing?
3. What are other causes of difficult bolt lift besides over-pressure?

Any other thoughts or sources of information will be greatly appreciated.

Thanks,
Richard

1. Cartridge brass and chamber steel expand at different rates, given the same amount of pressure and heat the brass expands more than the steel but both expand to some degree. When the pressure is removed, and the pressure didn't exceed the elasticity capability of the metal, they will both return to their original size but the steel returns to it's original size quicker than the brass. This is a design feature so that the brass seals the chamber until the bullet has exited the bore. If the pressure is high enough to exceed the elasticity of the brass then the brass will not return to it's exact size, it will be larger than it was originally. If the brass is expanded enough, due to the high pressures, then it will stay in contact with the steel after the pressure is removed and you will experience hard bolt lift.

2. Virgin brass can grow to full fire formed dimensions in one firing if you know how to calculate the required pressures and how much powder is required to achieve those pressures without over-stressing the cartridge brass. The standard fire forming procedure is a generally accepted process that works across the board for all fire forming, it's not specifically suited to any particular fire forming requirement.

3. All tight bolt lift is due to either a poorly tuned action or mismatched fit between the brass and the chamber. An improperly tuned bolt cam or improper locking lug contact will cause hard bolt lift. Anything that causes the brass to exceed it's elasticity capability will cause the brass to contact the chamber steel and that will create friction that will in turn make the bolt harder to lift. A scored chamber can cause hard bolt lift.

 

[dcali]

Just one thing to clarify - the propellant gas pressure is not the same as the stress on the steel, even though they are both in units of psi. It's a common point of confusion, but they're apples and oranges. I'm too lazy to go calculate it, but if you look, you can find a decent approximation if you google "thick walled pressure vessel hoop stress", or something along those lines.

 

[mike06]

Not to hijack this thread but it looks like I can get a answer here. I have 3 308 rifles doing the same thing. 2 Rem 700 Milspec 5r Gen 2 and a Ruger Precision. With new unfired Lapua and Peterson brass I check with a RCBS case mic gauge the cases read zero on the gauge. After firing they all come out at a negative .002 on the gauge. I then will rechamber the fired case and they will cycle just fine. I have never had a rifle that has done that. Thanks Mike

 

[Richard Coody]

Not to hijack this thread but it looks like I can get a answer here. I have 3 308 rifles doing the same thing. 2 Rem 700 Milspec 5r Gen 2 and a Ruger Precision. With new unfired Lapua and Peterson brass I check with a RCBS case mic gauge the cases read zero on the gauge. After firing they all come out at a negative .002 on the gauge. I then will rechamber the fired case and they will cycle just fine. I have never had a rifle that has done that. Thanks Mike

i think what you are seeing is the new brass body expanding to the chamber dimension shortening the overall case length. don't set up a shoulder bump yet. repeated firings should move the case shoulder forward. monitor and let us know.

 

[mike06]

i think what you are seeing is the new brass body expanding to the chamber dimension shortening the overall case length. don't set up a shoulder bump yet. repeated firings should move the case shoulder forward. monitor and let us know.

Richard thanks I have not had to bump the shoulder yet after 15 firings on the Peterson on a few I have used my Redding body die to resize and not touch the shoulder after the resize my readings will go back to 0 on the case mic then back to -.002 after firing again.

 

[riflewoman]

I’m here....

Recently, a discussion came up regarding the details of brass/chamber relationship and causes of difficult bolt lift. There wasn't a clear answer and it caused me to investigate. I didn't find anything that made it clear to me, so I'll ask here, hoping to get some hard, detailed facts.

Thanks in advance.

1. What is the relationship between the brass and the chamber when firing? They both grow under pressure and they both shrink back, but at different degrees? In particular, how does over-pressure change this?
2. Why doesn't virgin brass always grow to full fire formed dimension in one firing?
3. What are other causes of difficult bolt lift besides over-pressure?

Any other thoughts or sources of information will be greatly appreciated.

Thanks,
Richard

The relationship is the steel chamber supports the brass against the pressure in the case internal. Brass has an expansion of 2x that of steel. This means that for a given stress brass will stretch twice as far. According to this https://www.meadmetals.com/metal-products/brass/physical-chemical-properties And, based on the hardness of cartridge case heads and lower bodies being 85 to 95 Rockwell B, the tensile strength is 90000 psi. The yield strength is 60000 psi. So at normal pressures, the case head is barely at yield. The duration is so short it takes a few firings to get much flow.

At excessive pressures the brass has more time above the yield point and therefore more time to flow. The result is heavy bolt lift.

Heavy bolt lift can be caused by steel on steel galling and heavy friction too but if that is happening your action needs to be reworked or replaced.

 

[riflewoman]

Your typical stainless steel barrel is not in the annealed condition it has been hardened to about 26-30 Rockwell C. This makes the yield about 115,000 psi. However, the analysis method must use the thick-walled tube analysis technique and, for reasons way beyond the scope of this discussion, the cartridge pressure reauired to yield the barrel is slightly lower than that.

Now there comes the saving grace in that the pressure where the gun barrel yields the brass cartridge case has started to flow into every nook and cranny it can. When you see a belt form where there wasn’t one before I’d suggest that the pressures are a touch (or several) too high.

 

[mike a]

Expansion of the chamber is significant if it happens. That's why small shank savage guns have trouble with large diameter cartridges like WSM. Not enough steel there to keep expansion down, the chamber swells and brass along with it. Now the barrel shrinks and the brass is tight. It only has to be .001or less. Matt

Small shank vs WSM. That's very good info. Thanx Mike

 

[rckendall]

I’m here....



The relationship is the steel chamber supports the brass against the pressure in the case internal. Brass has an expansion of 2x that of steel. This means that for a given stress brass will stretch twice as far. According to this https://www.meadmetals.com/metal-products/brass/physical-chemical-properties And, based on the hardness of cartridge case heads and lower bodies being 85 to 95 Rockwell B, the tensile strength is 90000 psi. The yield strength is 60000 psi. So at normal pressures, the case head is barely at yield. The duration is so short it takes a few firings to get much flow.

At excessive pressures the brass has more time above the yield point and therefore more time to flow. The result is heavy bolt lift.

Heavy bolt lift can be caused by steel on steel galling and heavy friction too but if that is happening your action needs to be reworked or replaced.

Thank you so much for that explanation. If we can look a bit closer now to the length of the brass, I may learn even more!

What differences would there be between a case that has been neck sized only, one with shoulder bumped back .001, and another bumped back .002? Will there be a difference in bolt lift required, and does the bolt receive the same pressure in all three situations, under normal pressures?

Thanks,
Richard

 

[riflewoman]

Thank you so much for that explanation. If we can look a bit closer now to the length of the brass, I may learn even more!

What differences would there be between a case that has been neck sized only, one with shoulder bumped back .001, and another bumped back .002? Will there be a difference in bolt lift required, and does the bolt receive the same pressure in all three situations, under normal pressures?

Thanks,
Richard

Within the measurement error, the bolt thrust is identical. Cases that have been neck sized only get a stiff bolt lift because the brass flow from firing begins to accumulate. After 5 or so firings bolt lift becomes hard. Been there, done that.