What does small-primer 308 brass do for me?

[Ned Ludd]

This is an excerpt from an email I sent to a friend some time ago on the topic of different kinds of Palma brass and the flashholes they use. I've posted the letter portion from Alpha Munitions previously, but it's pertinent to this thread so some may find it of interest. The take home message is that there is some room for experimentation, even within the relatively narrow category of Palma brass:

We've previously discussed the idea using the smaller flashhole on .223 Rem brass. In fact, Monte Milanuk currently has a thread at A.S. asking that very question. A relative "newcomer" to the brass manufacturing arena, Alpha Munitions has chosen to do the exact opposite with their .308 Palma Brass. They actually use the larger flashhole. I have recently contacted both Alpha Munitions and Peterson, who also makes .308 Palma brass [http://alphamunitions.com, https://www.petersoncartridge.com]. Their "average" case volumes are 54.8 gr and 57.3 gr, respectively. My values for Lapua Palma brass case volume have been right at 56.5 gr. Clearly there are other difference between these three case manufacturer's "Palma Brass" besides just the flashhole. However, this letter from Alpha Munitions explaining why they chose to use the larger flashhole was posted earlier at A.S. I thought you might find it interesting:

First off thank you for purchasing our 308 SRP brass, and for the question concerning our small primer pocket large flash hole setup. There has been a lot of debate back and forth concerning small vs large flash holes, and most of it is just that, debate. It is hard to find any kind of confirmed study. This answer may get a little long, but its a question deserving of such an answer.

First, we have only performed testing on larger volume 308 cases, being the 6.5 creedmoor, 260 rem. and 308 win. There is a lot of history from benchrest shooters who have looked at flash hole size within the PPC family. The results of each case (ppc vs 308) individually cannot be assumed true for the other size cases. Due to gas pressure being highly sensitive to volume (PV=znRT, ideal gas law), and many other factors, it is not valid to assume the results for one particular case must transfer to another case. That said, I'll discuss more what we have seen and found.

Lets hit some background first. People speak of the "jet affect" which was assumed in Percussion Primers, Design Requirements by E.R. Lake, 1970 where he states "as the size of the flash hole is decreased, then the pressure of the [primer] products and the length of their "spit" is increased". Simply put, the smaller the flash hole the greater the length of the jet the primer creates. Going through this report there is zero data backing up the jet affect for our specific application (rifle ammo). A further study, most popular and known in the shoooting industry was performed by German Salazar, 2011 Large vs. Small Flash Hole Test, which presents photographic results which may contradict the "jet affect". Salazar shows images of a large flash hole primer pocket appearing to visually produce more "spit" or "flash" in comparison to the small flash hole. Again, this is all visual results with no quantitative test data.

The reason I wanted to hit on the past research or common knowledge in the shooting industry is that people believe this stuff and speak like it is the 100% undeniable truth, when there is no true data to back it up. We started looking into the large vs small flash holes and have shown the affects of the flash hole on the end result, being the consistency of velocity. Our tests have taken identical cases, except for the flash hole size, and observed the performance characteristics of both. There is a lot of detail I could go into for the setup, but I'll save that for the technical article. We first found a velocity node to shoot well then loaded the same load into the test cases. These were then shot out of the same gun, and recorded using the same chrono (lab radar setup).

In standard conditions we found to have slightly better results (velocity ES and SD) with the small primer pocket large flash hole setup, now is it a huge difference that would blow everyone away, no. However, testing at extreme low temp conditions we did show the velocity ES difference between the large and small flash holes opens wider.

It is my opinion that there is a balance of both primer and flash hole size to produce the best velocity ES. I think for optimal results you want to use the smallest primer possible which effectively ignites the powder charge. There is too much variance in primer ignition pressures that, I think, leads to much of the pressure variance in firing, consequently affecting the bullet velocities. It is possible (my THEORY.. just a theory...) that the jet affect could actually worsen the inconsistencies of the primer ignition creating greater inconsistencies in the powder ignition.

Anyway, really appreciate the question and I hope this helps!

Thanks,

Andrew Rixon

Director of Engineering and Manufacturing
Alpha Munitions

 

[riflewoman]

An actual test.

this test didn’t show a statistically valid difference. But he alludes to what would be needed to determine the difference if any.

 

[Murray Brook]

Good test. Would be nice to see it conducted with a different primer. No doubt the small S&B primer with H335 isn't up to the task. Things may have been different using the CCI 41 for example.

 

[Laurie]

I just got a routine update from Peterson Cartridge about new models and having made their 22 Creedmoor case-heads harder. It gives a cautionary tale about a 22 Creedmoor user's loads as follows:

But here is the thing to watch out for.

Recently, we received a call from a customer who was shooting our new .22 Creedmoor casings. He called for advice because he was struggling to find a load he liked. He explained he was loading them with 43 grains of H4350 and a 77 grain Berger VLD bullet, but he was starting to see signs of overpressure before he could find an accuracy node.

That sounded like an awfully high-pressure load to us. He also said he didn’t have access to velocity screens, and that there isn’t a lot of published and verified load data for .22 Creedmoor which includes velocities and their correlating pressures. With that said, his approach was to keep adding powder until he saw signs of overpressure.

The thing is, because the head hardness is so high on our new .22 Creedmoor casings, they don’t show signs of overpressure until they are way overpressure.

Peterson Cartridge is committed to the .22 Creedmoor caliber, so we made the investment to purchase the pressure testing equipment for this caliber. We are also committed to being as helpful to our customers as possible, so we volunteered to test the load he was using.

To say we were surprised at what we found, when we tested his load, would be an understatement. The load he was using created 85,000 lbs of pressure. The max safe pressure for the Creedmoor family is about 62,000 psi according to SAAMI(Sporting Arms and Ammunition Manufacturers Institute). At 66,000 psi, which is 4,000 psi above SAAMI max pressure for the Creedmoor family, the average velocity was 3,330 fps. Our velocity screens reported 3,651 fps for his load which produced 85,000 psi..

And only at this high pressure did he start to experience heavy bolt lift and see extractor marks on the case head.

I wouldn't be surprised if the same couldn't be said for some of the loadings and recorded MVs I've seen in 308 Win SRP F/TR over the years (and for 6.5X47 Lapua too if some of the MV claims on forums are anywhere near accurate). Well, maybe not 85,000 psi in 308, but well above the 62,000 ceiling. These SRP cases don't show pressure signs until it's really high.

 

[Laurie]

In standard conditions we found to have slightly better results (velocity ES and SD) with the small primer pocket large flash hole setup, now is it a huge difference that would blow everyone away, no. However, testing at extreme low temp conditions we did show the velocity ES difference between the large and small flash holes opens wider.

Very interesting indeed. I assume Mr Rixon's final sentence (low temperature conditions) is saying that the ES improvement obtained from the use of a larger diameter flash-hole becomes greater in these conditions, and that finding doesn't surprise me at all. A small improvement in normal temperatures is food for thought however.

How have you got on with your Alpha brass?

 

[AckleymanII]

So, it is obvious that many of us are shooting very high pressure loads, Large and small primer brass. This brings up another issue on how various actions, bolt heads take pressure, you rarely if ever hear anyone talk about this issue.

 

[riflewoman]

So, it is obvious that many of us are shooting very high pressure loads, Large and small primer brass. This brings up another issue on how various actions, bolt heads take pressure, you rarely if ever hear anyone talk about this issue.

In modern actions, like Remington style and even Winchester “push feed” style, the limiting factor is still the cartridge case. Even bolt setback doesn’t occur until the cartridge case is beginning to ooze into crevices. For the “controlled feed” kind like the Mauser, Springfield, Enfield and the like, the case design and manufacture (head hardness) are the limiting factors. The case hardened actions will get setback in very high pressure and big magnum cases but by that time the case is ready to pop.

 

[AckleymanII]

I wonder if a lot of the custom actions have bolt heads that are rated at what the Remington and Winchester bolt heads are rated?
I worry about metal fatigue from shooting out many barrels.

 

[SPQR70AD]

What does SRP brass do for me?....Make you buy small rifle primers?

 

[riflewoman]

I wonder if a lot of the custom actions have bolt heads that are rated at what the Remington and Winchester bolt heads are rated?
I worry about metal fatigue from shooting out many barrels.

From an engineering perspective, it isn’t metal fatigue. Fatigue implies a variable stress at a moderate load rate. A typical gun failure is from a non-variable stress applied rapidly ( but not an impact in the classic sense). These load cases are normally analyzed using fracture mechanics. Way way too complex to discuss here. But rifle actions today are built using the best steels for fracture toughness there are. They’re also kept out of heat treat levels where fracture toughness is diminished.

 

[AckleymanII]

So, do you think discussions of NOT using the 338 Lapua on Remington 700's is invalid?

 

[J F Johannes]

I asked my wife about tighter. She thought tighter was better but she didn’t want to get surgery or a divorce. The good news is my girlfriend is not complaining. Not sure what the final cost might be.
Ben

You drinking again Ben?

 

[riflewoman]

So, do you think discussions of NOT using the 338 Lapua on Remington 700's is invalid?

No. Not necessarily because of the bolt thrust. The 700 bolt nose is too thin when opened up to accept the 338 case. With the bolt nose so altered if a case does rupture, the venting gas will likely cause grievous harm. Opening up the locking lug bolt way to accept a larger bolt nose reduces the bearing area to the point where bolt thrust can become a problem. That’s why Remington made their 416 Rem Mag on a regular belted Mag case and then upped pressures to match 416 Rigby ballistics.
If you want to use a 700 action just chamber it for a 340:Wby or 338 RUM. Any difference in performance will be hard to discern. Ammo is scarcer this.

 

[BenPerfected]

I asked my wife about tighter. She thought tighter was better but she didn’t want to get surgery or a divorce. The good news is my girlfriend is not complaining. Not sure what the final cost might be.
Ben

Just a little Cab with dinner

 

[Ned Ludd]

Very interesting indeed. I assume Mr Rixon's final sentence (low temperature conditions) is saying that the ES improvement obtained from the use of a larger diameter flash-hole becomes greater in these conditions, and that finding doesn't surprise me at all. A small improvement in normal temperatures is food for thought however.

How have you got on with your Alpha brass?

Actually, I've always used Lapua brass. I simply contacted Alpha to get more information about the flash hole diameter issue, just to try and stay on top of things. I know a few shooters that have used Alph Munitions Brass and they have been extremely happy with it. When the response from Alpha is taken together with the flash hole diameter test video information that @riflewoman posted above, in my mind it becomes questionable whether large or small diameter flash holes can be definitively stated to be "better". It probably depends on the given situation (i.e. temp, cartridge, powder, etc.), and must therefore be tested by the end user.

 

[AckleymanII]

Thanks Riflewoman!