A Few Questions Regarding Brass and Chambers

[rckendall]

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

 

[dave@aDave]

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

For question # 2, I believe it is because of the Hysteresis loop characteristics of the particular alloy of the brass

 

[dkhunt14]

1- overpressure allows the barrel and tenon to swell and that allows the brass to grow extra big. When the tenon goes back the brass is now too tight in the chamber.

2- because the dimension is too great to allow the brass fill the space. It takes more then one firing to fill in the space.

3- the dimension of case at .200 line is not sized or sized enough or the shoulder is not bumped. Matt

 

[hawkeyeshoooer]

Different rifle=different chamber. The cases may be a touch too long for the chamber neck and not releasing the bullet correctly. Or the loaded catridge may be too close to the lands. Loading for one rifle doesn't necessarily mean that you can shoot that load in another rifle.

 

[Richard Coody]

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

well when the rifle is fired the brass goes plastic and adheres to the chamber wall. no way brass can contain that kind of pressure. once the pressure is relieved the brass springs away from the chamber wall. with overpressure loads that springback is reduced and the case continues to grip the walls of the chamber causing difficult extraction.

expansion of the chamber is insignificant compared with what is going on with the brass.

fireforming for headspace might take several firings because the case does grip the chamber walls. what is really happening is the case is staying in place and a new shoulder is being formed . a small portion of the new brass shoulder becomes case wall and and a portion of the new brass shoulder becomes neck. now this is just a few thousands. whatever the difference between the new brass shoulder and the location of the chamber shoulder. takes several firings to get it right.

timing of primary extraction can lead to difficult extraction even with acceptable pressures.

 

[dkhunt14]

well when fired the brass goes plastic and adheres to the chamber wall. no way brass can contain that kind of pressure. once the pressure is relieved the brass springs away from the chamber wall. with overpressure loads that springback is reduced and the case continues to grip the walls of the chamber causing difficult extraction.

expansion of the chamber is insignificant compared with what is going on with the brass.

fireforming for headspace might take several firings because the case does grip the chamber walls. what is really hapening is the case is staying in place and a new shoulder is being formed . a small portion of the new brass shoulder becomes case wall and and a portion of the new brass neckbecomes shoulder. now this is just a few thousands. whatever the difference between the new brass shoulder and the location of the chamber shoulder. takes several firings to get it right.

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

 

[Richard Coody]

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 .001. Matt

that is why savage no longer makes magnum barrels in small shank. overall chamber expansion is insignificant

 

[JFrank]

Gents
Here is where RifleWoman and Webster need to do the math for us.
R W you there girl?
J

 

[rckendall]

Thanks to everyone who replied, and I hope the discussion continues.

I seem to remember that the case does grip the chamber, as mentioned, but don't know how that affects everything or anything.

Richard

 

[Richard Coody]

Thanks to everyone who replied, and I hope the discussion continues.

I seem to remember that the case does grip the chamber, as mentioned, but don't know how that affects everything or anything.

Richard

well leave a.little oil in your chamber and see what happens.

 

[rckendall]

well leave a.little oil in your chamber and see what happens.

I think I have made all the mistakes possible, but I don't remember that one.

If a virgin case doesn't fully expand in the first firing, how much pressure does the bolt see? Is it normal for the bolt to see less than full chamber pressure?

 

[Richard Coody]

Gents
Here is where RifleWoman and Webster need to do the math for us.
R W you there girl?
J

any of the guys on board with the pressure trace equipment can tell us how much that barrel chamber deflects. that is really what the strain gauge is measuring to calculate the pressures involved.

yield strength of stainless barrel steel is about 75 ksi

 

[270WinDude]

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

For the cause of sticky brass though I'd say the explanation Richard gave, pointing to happenings with the brass, is more significant then the expansion of the chamber. So IMO both are correct, just one over the other for explanation of brass sticking. I've also noticed some brass of mine will grow to a good fit for my chamber in 1 firing while some take a lot longer to get the headspace dimension right, presumably for the reasons mentioned. I think pressure on the bolt has a lot of variables too depending on case type. I do believe it's possible to see practically no pressure on the bolt in certain circumstances. I'd imagine the bolt would see no less or more pressure given not fully formed brass or fully formed and I'd imagine that due to the force of the pressure mostly against the case walls and out with the bullet. Really short fat cases may be different?

 

[Milo 2.0]

I think I have made all the mistakes possible, but I don't remember that one.

If a virgin case doesn't fully expand in the first firing, how much pressure does the bolt see? Is it normal for the bolt to see less than full chamber pressure?

I feel your bolt gets hammered harder on new undersize brass, new brass is really expanding and it is not all going forward the way we like. On new brass, flatter primers do not bother me.

 

[Richard Coody]

I think I have made all the mistakes possible, but I don't remember that one.

If a virgin case doesn't fully expand in the first firing, how much pressure does the bolt see? Is it normal for the bolt to see less than full chamber pressure?

some do lightly lubricate their chambers for fireforming. what i hear is they can fireform a case with one firing because the oil does not allow the brass to grip the chamber wall. i would certainly not recommend this. it greatly increases the pressure on the bolt face. to each his own. whatever works for you.

 

[dmoran]

In my experience, it takes 3-cycles before brass settles in. After the 2nd firings there close, but still subject to further expansion. Attaching below specifications from 5-firings to Norma 6Dasher brass, for an example of what one may expect for brass stretch and capacity variation between cycles:

 

[270WinDude]

some do lightly lubricate their chambers for fireforming. what i hear is they can fireform a case with one firing because the oil does not allow the brass to grip the chamber wall. i would certainly not recommend this. it greatly increases the pressure on the bolt face. to each his own. whatever works for you.

I'm going to have to dig up this article I read a long time ago. You make a good point and it's completely in-line with the tests I read about. Lube did greatly increase pressure on bolt face. On the other hand when the brass was very tight the tester may or may not have fired a rifle without the bolt... If I find it I will post info about the tests. Certainly makes me think twice about accidently leaving my cases a bit slick from lube.

 

[LA50SHOOTER]

yield strength of stainless barrel steel is about 75 ksi

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.

 

[Dusty Stevens]

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.

And a 6ppc on match day is above that too

 

[LA50SHOOTER]

And I'm sure the diameter of the barrel, especially at the chamber area (where pressure extreme would be greatest) will have a lot to do with how much a barrel will take.