Change in Brass = Increased Pressure?

[daved]

I have a go-to 308 load using Lapua brass - Varget 43.5/-030 jump/BR2/175SMK - that yields 2700fps MV through my 22" Gradous barrel. SDs are usually 8-10 and 3 to 5 shot groups under 0.5". I've heard some good things about Alpha brass so I thought I'd try them. Before loading the new Alpha cases I compared them to twice fired fully prepped Lapua cases, and found essentially no difference in case weight, case volume, length and neck thickness - all things that might cause a difference in performance and pressure. So rather than do a formal or mini-OCW, I figured I could just use my normal load in the Alpha brass and make some minor adjustments from the initial group/chrono data.

So much for assumptions and shortcuts! The Alpha loads had a slight but noticeable increase in recoil and a couple had heavy bolt lift. LR measured MV was not much higher than Lapua loads fired at the same time - 2721 vs 2702. Two 5 shot test groups were slightly tighter and had lower SDs (both 8) than the Lapua loads. The cases however showed clear signs of increased pressure - extractor marking, mod primer flattening and one pierced primer.

This experience certainly reinforces the advice to always start low and work up when developing a load or changing components but I can't figure out what characteristic of the Alpha brass caused the increased pressure. This is NOT to say there is any problem with the brass itself - I would just like to know if anyone might have an explanation for the higher pressure.

Thanks for any input----

DaveD

 

[Gustav7]

Curious on this as well... very interesting if the numbers were the same.

could it have to do with neck tension? new brass may grip a little tighter? Can't see how that would cause that much of a difference.

 

[mudpig]

softer brass might explain the appearance of higher pressure, but the pierced primer doesn't fit that theory.

 

[daved]

I thought about neck tension - I use an expand/uniform mandrel on new brass prior to initial fire forming to ensure the same tension as on my reloaded brass. That said, I did note that more force was required to seat the SMKs in the Alpha than the Lapua. Since I brush the inside of the case necks when prepping fired brass I thought that a slightly smoother surface in the Lapua might be responsible for the difference.

I'm tempted to just reload a couple of the once-fired Alpha cases after normal prepping and see if the pressure signs persist.

 

[Gustav7]

I thought about neck tension - I use an expand/uniform mandrel on new brass prior to initial fire forming to ensure the same tension as on my reloaded brass. That said, I did note that more force was required to seat the SMKs in the Alpha than the Lapua. Since I brush the inside of the case necks when prepping fired brass I thought that a slightly smoother surface in the Lapua might be responsible for the difference.

I'm tempted to just reload a couple of the once-fired Alpha cases after normal prepping and see if the pressure signs persist.

I was also thinking what about headspace of the new cases. If the Alpha are a just a tad thinner, but are also a tad shorter from the shoulder, they may hold the same volume but would require more force to fit the chamber, raising pressures. Maybe re measure the volume with once fired cases?? or once fired resized? Idk...just a thought.

I think you should do the once fired Alpha over again... all the measurements, and then fire as well and see what pressure signs you get

 

[whatsupdoc]

Come on, no way you can tell me that you can feel the recoil difference of 21 FPS.
Case volume should be measured on cases that are fired in a particular rifles chamber and not resized.
Your original load may be too hot where 20 fps more pushed it over the edge.

 

[Greg Langelius *]

1) Neck thickness comparison. Pressure spikes change burn rate.

2) Case weight distribution. Is it concentrated in the case walls or in the case base? Things as innocuous as extractor groove depth can skew interior volume vs equal weights.

My brass is segregated from the start into batches that are always loaded together, based on loads for each rifle. Keeping loads away from max extends case life toward a goal that allows a single batch of brass to serve over the life of the specific barrel (maybe, but it's a worthy goal). Using a longer barrel permits charge weight reduction for the same velocity, extending both brass and barrel lifespans.

My handloading strategies always favor bore/brass life over higher velocities. When I forget this, I buy trouble.

Greg

 

[daved]

I'm thinking more that Gustav's may have something - the final pressure is important but so is the change in pressure over time. A more abrupt rise due to increased headspace could produce signs of overpressure with a relatively small incremental increase in peak pressure. Re-measuring etc in progress---

Whatsupdoc - you're right, there isn't much additional calculated recoil associated with an increase in MV of even 40 fps. But a more rapid rise to that peak would result in a more abrupt recoil impulse, ie a difference in felt recoil.

 

[natdscott]

@daved don't overthink this. Tilt the can less.

You have learned something here, and so far, it was a cheap lesson. Learn it well, and pass along your knowledge to somebody else when you get a chance.

 

[daved]

1) Neck thickness comparison. Pressure spikes change burn rate.

Neck thickness of the Lapua is actually .001" more than the Alpha.

Comments well taken---

 

[daved]

@daved don't overthink this. Tilt the can less.

You have learned something here, and so far, it was a cheap lesson. Learn it well.

No question as to solution - I'm just curious. Yes, I know what happened to that cat---

 

[Supersubes]

I'm thinking more that Gustav's may have something - the final pressure is important but so is the change in pressure over time. A more abrupt rise due to increased headspace could produce signs of overpressure with a relatively small incremental increase in peak pressure. Re-measuring etc in progress---

Whatsupdoc - you're right, there isn't much additional calculated recoil associated with an increase in MV of even 40 fps. But a more rapid rise to that peak would result in a more abrupt recoil impulse, ie a difference in felt recoil.

It’s actually the opposite. A short/small case lowers pressures. If you loaded the same brass again with the same load it will produce more pressure. Virgin brass is always slower and an more forgiving than fired brass is.

 

[Gustav7]

It’s actually the opposite. A short/small case lowers pressures. If you loaded the same brass again with the same load it will produce more pressure. Virgin brass is always slower and an more forgiving than fired brass is.

Interesting. Why does a shorter case produce lower pressure? ceteris paribus

 

[Supersubes]

Interesting. Why is that?

Some would say that the pressure is being bled off by forming the case, which affects overall peak pressure. Hard to say without looking at pressure trace data, but a change to the pressure curve downward is happening for sure. Easy to rationalize the opposite, but it never happens that way. Lots of data to support this.

How did you measure case volume? How many cases did you measure.

 

[918v]

Interesting. Why does a shorter case produce lower pressure? ceteris paribus

In factory new brass some energy is used to expand the case while most is used to propel the bullet. The direction of spring back is toward the case.

A reloaded case is partially expanded already and the direction of spring back after sizing is toward the chamber walls.

Therefore a reloaded case is happy to expand and hug the chamber. The amount of energy formerly consumed to expand the case is now used to propel the bullet.

Hard bolt lift is caused by brass pushing on the bolt face. To lighten bolt lift you can anneal the case after sizing. This will change the direction of spring back to mimic factory new brass.

 

[supercorndogs]

What is the shoulder measurement of the virgin alpha vs your fired brass? Could the brass be too short?

 

[Doom]

First off and just my opinion, I believe the Lapua load is already overpressure per SAMMI. You just haven’t exceeded the capability of any of the components. When you change brass you changed something. Flash hole diameter, hardness, volume, whatever? Enough to move the pressure slightly higher and beyond the capability of the brass.

I’m always suspicious of case volume measurement, especially if done with water due to the meniscus. If I were to speculate, i would guess the Alpha brass has slightly less volume and is softer than the Laupa.

 

[Gustav7]

In factory new brass some energy is used to expand the case while most is used to propel the bullet. The direction of spring back is toward the case.

A reloaded case is partially expanded already and the direction of spring back after sizing is toward the chamber walls.

Therefore a reloaded case is happy to expand and hug the chamber. The amount of energy formerly consumed to expand the case is now used to propel the bullet.

Hard bolt lift is caused by brass pushing on the bolt face. To lighten bolt lift you can anneal the case after sizing. This will change the direction of spring back to mimic factory new brass.

Makes sense. Thanks for educating

 

[daved]

What is the shoulder measurement of the virgin alpha vs your fired brass? Could the brass be too short?

The case lengths of both the new Alpha and resized Lapua are 2.005. I didn't measure the shoulder since I assumed that any significant difference there would be reflected in case volume.

It’s actually the opposite. A short/small case lowers pressures.

I ran a quick simulation in Quickload using my Lapua load parameters - one grain decrease in case volume increases pressure 2000PSI.

First off and just my opinion, I believe the Lapua load is already overpressure per SAMMI.

Same simulation gave pressure well under SAAMI. However it showed an estimated MV of 2625, 75 less than the measured MV. So I'm not certain what to make of that. You may well be correct b/c of a tighter chamber, etc that is not reflected in Quickload.

 

[Supersubes]

The case lengths of both the new Alpha and resized Lapua are 2.005. I didn't measure the shoulder since I assumed that any significant difference there would be reflected in case volume.


I ran a quick simulation in Quickload using my Lapua load parameters - one grain decrease in case volume increases pressure 2000PSI.


Same simulation gave pressure well under SAAMI. However it showed an estimated MV of 2625, 75 less than the measured MV. So I'm not certain what to make of that. You may well be correct b/c of a tighter chamber, etc that is not reflected in Quickload.

You’re missing something, and stumbling onto a shortcoming of quickload. The case is soft and squishy in the grand scheme of peak pressure, but still takes many thousands of pounds of pressure to form to the chamber walls. Quickload doesn’t take that into account. It’s merely simulating two different chamber sizes essentially, and obviously the smaller chamber with a same charge will make more pressure. 918v explained it pretty well above.

 

[supercorndogs]

Not case length. Shoulder length, headspace on new Alpha brass, could be excessive. That would explain the pierced primer.

43.5 of Varget in Lapau brass with a 175 SMK, leaves ejector marks on my 308 brass.

 

[BLKWLFK9]

Some would say that the pressure is being bled off by forming the case, which affects overall peak pressure. Hard to say without looking at pressure trace data, but a change to the pressure curve downward is happening for sure. Easy to rationalize the opposite, but it never happens that way. Lots of data to support this.

How did you measure case volume? How many cases did you measure.

This. I found that out from virgin alpha dasher brass and 1xf alpha brass. My 1xf load is .1 less but 30 fps more in velocity. I also found pressure sooner in charge weights with alpha over the lapua brass.

 

[Greg Langelius *]

My load for the 175 is 42.2gr of IMR-4064, and for the 168, it's 43.5.

At the time I started using them, I had them earmarked as Max loads; but over time, posts here and elsewhere began to show charges migrating upward.

I shook my head and stayed with my charges. I still think that my judgement about them being a reasonable set of max loads has been sound all along.

IMHO, there is no great (if any) benefit to hot loading. Bores and brass suffer alike, and unnecessarily so. Any accuracy advantages due to higher velocity are marginal at best; and regardless of what the velocity may be, all loads require wind compensation. Any differences are just small matter of degree.

I'll always prefer shooting a longer barrel with less powder than a shorter barrel with more powder. When the primers go flat, I'm doing it wrong.

Greg

 

[Steel head]

Changes in case volume and how much expansion definitely effects pressure.
Put my 260 load in lapua brass and you’ll blank every primer guaranteed.

I definitely see a difference in brand new or oversized vs once fired with minimal sizing.

 

[Doom]

My load for the 175 is 42.2gr of IMR-4064, and for the 168, it's 43.5.

At the time I started using them, I had them earmarked as Max loads; but over time, posts here and elsewhere began to show charges migrating upward.

I shook my head and stayed with my charges. I still think that my judgement about them being a reasonable set of max loads has been sound all along.

IMHO, there is no great (if any) benefit to hot loading. Bores and brass suffer alike, and unnecessarily so. Any accuracy advantages due to higher velocity are marginal at best; and regardless of what the velocity may be. All loads require wind compensation. Any differences are just small matter of degree.

I'll always prefer shooting a longer barrel with less powder than a shorter barrel with more powder. When the primers go flat, I'm doing it wrong.

Greg

I agree with you. My 168 SMK load is 43.2gr in Lapua and LC brass and I have been to 44.8 or so with no pressure signs in a Rem 700 and I tested to almost 46 gr in Win brass.

The increased velocity doesn’t justify the wear and tear.

 

[Greg Langelius *]

About a year or so back, I spun off the 30BR experimental barrel from the old Ghost Dancer and remounted the 28" 260 barrel.

In the process, I created a perfect storm of never-do's.

The headspace was too short. The 143gr ELD-X was a new bullet, and the charge weight was 2 full grains in excess, using the old 140 A-Max load.

First shot, flat primer. Second shot, blown primer. There was no third shot.

Remounted the barrel using an actual Go Gauge, reduced the charge by 3.8gr, and started over.

It was a perfect illustration of what's happening here in this topic.

Upshot; ELD-X development is back up on the shelf, and development is being started from scratch using 140gr Speer Gold Dot.

More to come on this, but it's one of many projects contemplated, and won't go at the top of list (308/168/IMR-4064, 6.5G/120/8208XBR, 223/62&75/Varget, 260/140/H-4350&H-4831SC, all Speer Gold Dots). I do not load to suit throat dimensions, all my loads are at magazine length. My 308/168/43.5gr/ IMR-4064 load is an FGMM clone load.

Greg