Flash Hole Size

[243winxb]

Flash Hole Size/Spit Hole testhttps://apps.dtic.mil/dtic/tr/fulltext/u2/a599210.pdf

From Link- For this experiment the spit-hole diameter was reduced from the mil-spec 0.078 in to 0.055 in.

4. Conclusion A 7.62-mm primer force measurement breech design was proven out with standard M80 and M80 ‟A” cartridges. A subsequent experiment evaluated the influence of reducing the spit-hole area by 50.3% on primer pocket pressure and early motion. The experiments were successful in proving the primer force breech’s efficacy in evaluating early time interior ballistics. Through the course of the experiments the M80’s no. 34 primer produces very little initial force output. This is contrary to what has been previously observed with the no. 41 primer used in the M855 cartridge. However, when the spit-hole area is reduced the force witnessed in the primer pocket is about three times that of a no. 41 primer and 20 times that of a no. 34 primer with standard spit-hole areas. Similar results were observed in fully loaded cartridges. The reduction in spit-hole area caused a localized increase in primer cup pressure but did not significantly affect the overall interior ballistic cycle. The decreased area resulted in the propellant charge burning sooner and the projectile moving slightly earlier when compared to the baseline M80, but not by a significant amount. The M80 ‟A” projectile motion occurred significantly sooner when compared to both M80 variants due to its quicker burning propellant formulation. Finally, variations associated with the initial engraving process are a potential source of deviations in the 7.62-mm cartridge performance. It is at this point in the ballistic cycle where differences in performance are seen in the short barrel data. After the initial engraving process is completed, the performances measured in the short barrel experiments are consistent.

Do you find a smaller flash hole is better? What cartridge?

 

[cw308]

I reload 308 for benchrest shooting only . I make sure all the flash holes are the same size , off the top of my head I think it's a #45 drill bit that is the correct size . I read Federal is using small primers in the 45 Auto cases making the case base stronger and also a 50 fps in velocity using the same load . Haven't read anything on rifle cases . For now Im happy with CCI & Federal benchrest primers .

 

[dminn1]

I use small flash hole in .308, .243, and 6.5 Creedmoor. I get better numbers in all of these than with their large primer large flash hole variations. How much is due to the flash hole I can't say but I like the small flash numbers.

 

[243winxb]

The spit-hole diameter was reduced from the mil-spec 0.078 in to 0.055 in.

When the spit-hole area is reduced, the force witnessed in the primer pocket is about three times that of a no. 41 primer and 20 times that of a no. 34 primer with standard spit-hole areas.

The pressure in the primer pocket is calculated to be approximately 23,890 psi( #41 primer.)

The "approximately" or guess on the testers part, seems on the high side to me. But i am getting info from 2 different links and cartridges. 5.56 & 7.62

 

[spclark]

I reload 308 for benchrest shooting only... I think it's a #45 drill bit that is the correct size.

#45 = 0.082” dia., that’d be for large rifle primers.

#53 = 0.0595”, works with the original 6BR & PPC flash holes, may be too small for cases like Lapua’s 308 Palma brass. I shoot that, uniform flash holes w/ K&M small primer tool @ 0.0625” or 1/16”.

 

[rjtfroggy]

I open my 6br & 30x47 up to 0.073 and that is still small according to some who are more experienced than me.

 

[Macropod]

Why does 223 brass have a large flash hole?

At least the Lapua stuff I have does.

 

[243winxb]

Why does ( Lapua) 223 brass have a large flash hole?

A smaller flash hole would seem to increase pressure in the pocket and require a military type crimped primer? The #41 , if used , would seem to need a larger flash hole?

The standard dimension for Lapua 220 Russian and 6mmBR flash holes is 1.5mm or .0590″.

http://bulletin.accurateshooter.com/tag/flash-hole/

 

[dmoran]

A number of years ago, I tested Small Primer flash-hole diameters in .005" increments from .062 to .082" with two 6Dasher barrels and from three different powders (RL15, N140, Varget). At every increment going larger accuracy depleted a little worse, leaving me with an obvious conclusion that the .062" diameter was optimally the best, and the larger the flash-hole the worse the accuracy capability.

Can't speak to other cartridges or calibers, and everyone should test there own scenario's regardless, is my advise.

 

[mikeeg02]

Donovan,

Did you record velocity to see if there was a change ? That may of required tweaking the load to keep in tune? Just curious.

 

[Joe Maisto]

#45 = 0.082” dia., that’d be for large rifle primers.

#53 = 0.0595”, works with the original 6BR & PPC flash holes, may be too small for cases like Lapua’s 308 Palma brass. I shoot that, uniform flash holes w/ K&M small primer tool @ 0.0625” or 1/16”.

I have one of those great K&M flash hole tools as well. He did his homework...

 

[Rick300]

When I purchased Alpha Munitions brass a while ago, I asked them about this. Here is their response:

Rick,

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 shooting 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 (velcoity 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

 

[cw308]

I only shoot 200 yards , that's the maximum distance for the ranges in my neck of the woods . I only shoot 200 yards , l found a load that shoots that distance very accurately without the need for speed . When going different distances I could see finding a hotter load but for me my light load works well .

 

[243winxb]

Ammunition manufacturer Fiocchi of America solicited this research in order to potentially improve their manufacturing process as well as their product. By seeing the effects of flash hole variations on the end use of the cartridge, Fiocchi would be able to see if changes are needed.

Its a long read. Conclusion is on page 115. Powder used for both 223/308 was WC 749.

https://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=8415&context=masters_theses

Quotes - Pressure seems to climb to a peak and then fall as flash hole diameter increases in each caliber. This peak occurs at the 2.4mm flash hole on the .223 Remington plot and at the 2.8mm flash hole on the .308 Winchester plot.

It is shown that as flash hole diameter increases above 2.0mm, there is a decrease in group size (i.e. an improvement in precision). Both calibers have the 3.0mm flash hole having the best precision amongst the 4 other flash hole diameters tested. The 3.0mm flash hole values for each caliber also have relatively small standard deviations compared to the other diameters.

The 2.4mm and 2.8mm diameter peaks seen in the .223 and .308 calibers, respectively, could be the ideal flash hole diameters for each caliber.

For both .223 and .308 calibers, the 3mm flash hole diameter group size decreased by 20% when compared to the 2mm flash hole diameter control. This decrease was the greatest of all the other diameters and was accompanied with small standard deviations for the 3mm flash hole diameter values. Although there was a decrease in group size for flash hole diameters greater than 2mm, all but the 3mm diameter had large standard deviations. Further testing on the 3mm flash hole diameter would be required to confirm these findings.



Change powder and/or primer and my guess is, result may be different?

 

[Ccrider]

I thought Lapua 220 Russian and 6br brass flash holes were now .062?

 

[Texas10]

Reading this will start to answer some of your questions.http://forum.accurateshooter.com/attachments/arl-tr-7479-pdf.1092789/

In the short barrel study it was found that the primer unseats the bullet and starts the engraving process well before the burning powder charge begins to drive the bullet down the barrel. During this process the bullet is often kicked a bit off-axis. Graphs in the study show the degree of off-axis or tilt of the nose of the bullet.

Additionally, the primer force uses the powder charge to drive the bullet much like a piston, and that a cartridge that is 100% full of propellant is more consistent in doing so.

One might conclude that a full charge of propellant and a more gradual push from the primer will combine to engrave the bullet with less off-axis shift, and therefore fly straighter once it's left the barrel.

 

[Texas10]

I thought Lapua 220 Russian and 6br brass flash holes were now .062?

Lapua 6br brass as well as Peterson SRP brass are 1.5mm.

223 brass has an oversized flash/spit hole because it was found to work better in that particular case size. It is unique in that regard and one should not infer that larger cases will perform better with larger flash/spit hole diameters.

Edit: It appears that an article posted as I was writing this disproves what I just posted about larger flash/spit hole diameters.

 

[truck driver]

Been curious about something else with primer pockets and that is that bottom of the pocket is tapered toward the flash hole which gives the impression that the lash or flame from a primer is directed toward the flash hole but when one uniforms the primer pocket it flattens the taper. So just which is better uniformed or not?

Edit to continue post.

I understand the need for uniform depth just don't understand the taper.
I also checked the flash hole diameter on several different brands of magnum rifle brass and found it to be .1772" the same as the Redding flash hole uniformer.
Is this normal for magnum rifle brass?

 

[RegionRat]

@Texas10 , I don’t think that other paper was universal in the conclusion. It certainly begs some more questions, namely if that flash hole diameter alone will produce the same effect regardless of the other parameter combinations.

I can imagine the size of the DOE it would take to determine if the flash hole size is independent of the primer, powder, case, chamber, bullet, bbl, length, etc. in terms of achieving small ES/SD. It was good that they mentioned the effect of ambient temperatures. Mil-Spec environments include some high and low ends that most folks don’t even consider. Ball powders and stick powders don’t make life easy on this kind of testing once cold weather comes into the matrix.

My guess is that within a window you might find a generalized min-max window for large and small primers, but only time will tell if there is some universal improvement that could be applied across the board. The fact is that many different examples of folks achieving small ES/SD exist with as-is flash holes, but the question is would that have been a wider tuning window against temperature and other changes if the hole was bigger or smaller? Even the two batches of 41 primers showed a significant delta in the study, so the test matrix would have to be huge to see if a primer batch alone sways the results.

One thing I do miss about working, was the challenge of designing these tests and the instruments we needed to measure them. The folks at the arsenals were truly brilliant scientists and had a passion for their field. It was the politics in the middle 90’s that became a big turn-off for most of us. Then they came around and destroyed most of the committees and told us to go outside of government, so it pretty much marked a huge downturn in the quality of research in small arms. Most standards bodies are not interested in Mil-Spec research so the white paper counts have plummeted. The real loss to us all, was that political philosophy chased a lot of good young talent away from the field, and they were the ones who inherited the legacy from their predecessors.

I worked with heavier calibers, and the flash hole sizes on them were set to a standard, but not optimized to a loading. I’m not sure what would have happened if we had wide variation in bullets and powders for example, and then tried to determine each optimum and then specified flash holes specific to a loading.

The budgets and work it takes to standardize ammo to a spec was daunting. On the M1 (during the change from 105 to 120), we did get to see many loadings, but even more interesting were the differences between rifled and smooth bores. The Brits kept the rifling but we ended up with smooth bores. Those rigs have velocity and performance like they are just giant varmint guns...

Flash hole size has lots of work needing to be done, but my own opinion was that primer performance would need to become standardized first to prevent batch to batch variations from confounding the results. There were several instances of “bad” batches of primers during my tenure, and I don’t know that we have any guarantees we have seen the last of them.

There were lots of politics getting in the way of funding the research. It was one of the reasons I retired early, but they also lost young folks like Bryan Litz. It isn’t that the arsenals are dead, but the forums for peer review and influence on directing the research are not the same caliber. Forcing these and other topics into the category of “Small Business” or “Minority Preference” was a regrettable mistake. It takes big budgets and large resources to pull off these studies and the idea that grad students or small businesses will do this work has been a failure. I really hope the philosophy in the beltway changes some day, cause there is work that needs to be done.

 

[bozo699]

Sure a lot of 3 year old threads popping back up
Wayne