Curious reloading problem

[Nivium]

So I need an expert, which I am not. I've noticed an unusual trend of case lengthening in a 308 based pretty squarely on an increase in the pressure of the fired round.

For example, starting case length on all brass (new lapua brass) was 2.006, a bit over minimum trim. I used twenty pieces of this brass, working up my load to find pressure (which I never found, though I ended up over the max powder load). I marked all the cases so I know the amount of powder used in each one, and have them ordered from lowest powder/pressure to highest.

Now, measuring the case length on the once-fired brass, I see that that the minimum load, low pressure ones are still 2.006/.65 after neck sizing, but as I move to the higher brass used for the hotter loads, the case length goes up commensurate with the increase in pressure. The middling loads yielded once-fired brass that is now around 2.009 in case length, and the hottest loads are now measuring 2.013 in case length, and so on.

So my question: is it just common knowledge that I'm somehow failed to acquire that more pressure extends the length of a piece of brass? Also, is there a range of growth in a piece of brass that indicates too much pressure? Since I never actually got any pressure signs other than flattened primers which I got even for the minimum loads, I'm wondering if this action just doesn't show pressure well and maybe I can use this increase in case length to gauge whether I should back off. Or is it completely unrelated to pressure?

Thanks in advance

 

[straightshooter1]

So I need an expert, which I am not. I've noticed an unusual trend of case lengthening in a 308 based pretty squarely on an increase in the pressure of the fired round.

For example, starting case length on all brass (new lapua brass) was 2.006, a bit over minimum trim. I used twenty pieces of this brass, working up my load to find pressure (which I never found, though I ended up over the max powder load). I marked all the cases so I know the amount of powder used in each one, and have them ordered from lowest powder/pressure to highest.

Now, measuring the case length on the once-fired brass, I see that that the minimum load, low pressure ones are still 2.006/.65 after neck sizing, but as I move to the higher brass used for the hotter loads, the case length goes up commensurate with the increase in pressure. The middling loads yielded once-fired brass that is now around 2.009 in case length, and the hottest loads are now measuring 2.013 in case length, and so on.

So my question: is it just common knowledge that I'm somehow failed to acquire that more pressure extends the length of a piece of brass? Also, is there a range of growth in a piece of brass that indicates too much pressure? Since I never actually got any pressure signs other than flattened primers which I got even for the minimum loads, I'm wondering if this action just doesn't show pressure well and maybe I can use this increase in case length to gauge whether I should back off. Or is it completely unrelated to pressure?

Thanks in advance

It sound like you're dealing with an issue of variances before your brass is fire formed, as what you're describing often happens with my .308 brass before they're fully fire formed and it not really the pressure at play so much. Though, pressure can certainly be a factor when fire forming as higher pressure will tend to more fully form the case over low pressure.

 

[iceng]

Brass grows when fired. Think "squeezing tooth paste from a tube". It does settle a bit over time, but does occur.

our hero P.O.Ackley improved shoulders to limit case growth. Its a thing, often why people trim every few firings.

 

[Nivium]

It sound like you're dealing with an issue of variances before your brass is fire formed, as what you're describing often happens with my .308 brass before they're fully fire formed and it not really the pressure at play so much. Though, pressure can certainly be a factor when fire forming as higher pressure will tend to more fully form the case over low pressure.

So you're saying that the increased pressure may be causing more complete fireforming, thus when neck sizing, there is more material to move around, thus more case length? That makes sense to me.

When you say it happens to your 308, are you also noticing that more pressure results in more case length when fire forming new brass?

 

[Nivium]

Brass grows when fired. Think "squeezing tooth paste from a tube". It does settle a bit over time, but does occur.

our hero P.O.Ackley improved shoulders to limit case growth. Its a thing, often why people trim every few firings.

I understand that it grows upon firing, but is it also a thing that more pressure when firing results in more growth?

If so, is there a way to extrapolate my pressure from the amount of growth? In other words, if my high-powder cases grew .006 in length after the first firing/neck size, does that mean overpressure?

 

[Aftermath]

I understand that it grows upon firing, but is it also a thing that more pressure when firing results in more growth?

If so, is there a way to extrapolate my pressure from the amount of growth? In other words, if my high-powder cases grew .006 in length after the first firing/neck size, does that mean overpressure?

Case growth (elongation) IS related to pressure. More PSI means more squeeze against the steel with the brass serving as a buffer. There is no place else for the brass to move except towards the muzzle. It's movement is as a fluid. It's a fluid dynamics "problem". As such, there is very likely an equation to use this elongation as a pressure indicator.
However, unlike other indicators such as heavy bolt lift, pierced primer...you are going to have to measure and measure very carefully to find that difference in length. Also, that elongation is dependent upon the brass mixture and that depends on lot number as well as manufacturer. The elongation also depends on case wall thickness and that depends on lot number as well as manufacturer.

Using case elongation as an indicator of too much pressure is a fools errand.

 

[JS/]

Don't worry about case length when using new unfired brass. First firing will expand the case to fit the chamber. For pressure testing keep tabs on the case head diameter. Overpressure will expand the brass there, but you don't want to ruin that nice Lapua brass so follow reloading book guidelines.

 

[straightshooter1]

Brass grows when fired. Think "squeezing tooth paste from a tube". It does settle a bit over time, but does occur.

our hero P.O.Ackley improved shoulders to limit case growth. Its a thing, often why people trim every few firings.

Your analogy for tooth paste doesn't really work for brass being fired since the pressure is from within rather than the outside. But, it does work when sizing the brass for the pressure being from the outside. And if fact, almost all the growth, in terms of OAL, comes from the sizing process and very little, if any, from the firing.

 

[straightshooter1]

So you're saying that the increased pressure may be causing more complete fireforming, thus when neck sizing, there is more material to move around, thus more case length? That makes sense to me.

When you say it happens to your 308, are you also noticing that more pressure results in more case length when fire forming new brass?

Because the head space dimensions on new brass is very liberal in being short, the first firing produces a the most movement. And just how much movement on that first firing is effected by the amount of pressure. It often takes two firings to fire form brass and sometimes three (some people say it can be more, but that might depend on the larger cases with more mass). As JS/ says, you really don't want to "overpressure" you brass just to, in effect, speed up the fire forming process.

I've never attempted to quantify how pressure effects case length as you've done. In the past, I've worked with forming various aircraft metal alloys and have precision reloaded for some time now and have a pretty good handle on how our brass is formed and reformed when fired and resized. And what I can tell you is that when brass is fired, whatever the pressure is, it's equal on all surfaces of the case, so there's no lower pressure area for the brass to be pushed into other than than the initial area for expansion to fit the chamber's dimensions. When we size out brass, we strive to minimize the amount of this space. If the size of the case is substantially less than the dimensions of the chamber, as virgin brass often is, then the material is going to be stretched (like the distance in the distance of the shoulder to the chamber's head space dimensions. . .less any springback). Once your cases are fully fire formed, the firing process, regardless of pressure, isn't going to have much effect on the case's OAL. What you do when sizing can have an immediate substantial effect on OAL (oversizing the brass can lead to OAL growth when being fired).

And, oh yes, if one "over anneals" their brass (meaning, fully annealed where it's soft), there can be a LOT of growth when fired and the expansion without any springback gets the case stuck in the chamber. You don't want that!

 

[JerkyFreak]

Pressure squeezing the brasscauses very little elongation. That you can quite easily see if you neck-size exclusively. After the 3rd firing case growth is very slow.

The real culprit is head space. The larger the head-space, the faster the growth. The growth is the result of the section above the web stretching. The amount of stretch is also pressure dependent. At very low pressure there will be no stretch and you might even see the fired primers standing slightly proud of the case head.

It can also depend on whether you jump or jam the bullets. Jamming the bullets reduces the distance the firing pin shoves the case into the chamber before it swells and grip the chamber and stretching to the rear.

 

[bax]

If pressure doesn't cause additional case length, where to donuts come from?

 

[JerkyFreak]

If pressure doesn't cause additional case length, where to donuts come from?

Not quite what I said. Very little elongation is caused by pressure squeezing the brass.

When you size a case and push the shoulder back, the excess brass moves into the shoulder, then the neck. It definitely does not go back to the area above the web.

 

[bax]

Not quite what I said. Very little elongation is caused by pressure squeezing the brass.

When you size a case and push the shoulder back, the excess brass moves into the shoulder, then the neck. It definitely does not go back to the area above the web.

I am trying to work it out, I am not trying to be disagreeable.

"The excess brass" ... where did the excess come from? Why is it "excess"?

I'm thinking about the standard model that I think we all share: firing pin pushes cartridge shoulder against the chamber, primer fires, brass neck and shoulder sticks to chamber walls, cartridge base is pushed back against the bolt, the thin-est part of the case stretches and gets thinner. When sizing, we shove the case shoulder toward the cartridge base.

In a perfect world, the thicker brass in the base and the sidewalls would somehow go back together in the thin area but that does not happen. Why does the donut form in the neck and not just above the area that is getting thinner?

When we size the shoulder back, is the thin area rigid enough to resist being shoved back together? Is all of the brass above the thin spot moving toward the shoulder and neck and the neck brass moving toward the case mouth? When I think about it, that seems most likely to me. And annealing would make that easier - the shoulder and neck are now softer and more ductile so they would flow better. That would explain differences experienced by people who do and do not anneal. Perhaps that was obvious to everyone else - it is sort of a new idea to me.

 

[JerkyFreak]

When sizing, the brass migrates to the shoulder and neck. You are not only swaging the brass towards the neck, you are also reducing the diameter. As the diameter reduces the brass has to become thicker. When the brass transitions into the neck it is thicker than when it started giving the dreaded donut.

Cases are designed and hardened to become progressively harder as you move from the neck down to the case head. So, yes, the brass moves to the shoulder and neck; path of least resistance. It doesn't go back to the base.