Do Small (0.062") Flash Holes Change the Primer Flash?

[GermanS1]

I've been wondering about this topic for a while. My curiosity was centered around the Lapua Palma .308 case with it's small primer pocket and 0.062" flash hole as compared to the old Remington BR .308 case with a small primer pocket and a standard 0.080" flash hole.

I set up my primer flash test rig and photo gear and ran a test using the two case types and two primer types. The results are here:
http://riflemansjournal.blogspot.com/2011/03/primers-large-vs-small-flash-hole-test.html

 

[BoydAllen]

German,

....interesting, as always. Now here's one that you might also try. Compare the velocities using the Lapua cases, by drilling half of a small set to .080, with the same load, that is carefully weighed. My definition of a hotter primer is all about velocity.

Boyd

 

[Heavies]

Very nice test.

I don't know though. It may be that the photo timing was a little different, but, to me, the small hole case looks like the flash is more concentrated near the base, most noticeably with the 7 1/2's. The large hole case looks like a flaming volcano, but the smaller flash hole looks like a hot ball of gas.
Like you said you needed multiple shots to get the photos, so it could be that.
Other wise I'm thinking that it's opposite the nozzle effect. Perhaps the small flash hole makes hot primers seem more mild. Like your study of mild primers being more accurate than hotter primers.

 

[GermanS1]

The photo technique is to open the shutter in a darkened room and then fire the primer. The light from the primer will be enough for the shutter to close. The problem comes not from timing the shot, but from getting focused in the dark as the auto-focus feature is always struggling to focus on the steel ruler that you see behind the flash (and which is the ideal focus point). If I were doing this with a more sophisticated camera with manual focus, this could probably be avoided.

Boyd, I agree that MV with different flash hole sizes would be an interesting test, I'll try to get that done.

 

[LRTom]

German, This was one of the discussion points with Lapua when I first proposed the possiblity of producing a SP 308 case. Since they already were setup with small flashhole capabilities with the BR cases it was decided to go that way initially as we could drill out the hole to 0.080 during testing to compare. When I received the first test cases I ran some tests doing just this. The velocity differences between the large (0.080") and small (0.059") flashhole was statistically insignificant. However, I did notice that there were differences in the pressure signs on the primers. To further explore this I loaded both style cases with Wolf SMR primers (i.e. soft cup) rather than magnum or BR primers. Results: The small flashhole casess showed significant cratering but held the pressure (10 shots). With all 10 of the large flashhole cases the primers blew (pierced). This basically tells me that using the small flashhole creates a better pressure vessel in comparison to the large flashhole version. This is inline with accepted BR practices.


I have done fairly extensive testing with these cases and have fired approximately 35,000 rounds (by primer count) using the Remingtom BR case (in 308 Win) in the past 25 years. I'd be happy to try to answer any questions people have about this subject.

Tom Whitaker

 

[BoydAllen]

Whoo Wee!!! Hard data Thanks

 

[GermanS1]

Tom, thank for sharing that experience! It's a very interesting area to explore and it's ncie to have some options available for our application.

I shot a lot of the small primer Remington brass too, although not as much as you have. Last week I sold my last 500 unfired cases to Bob Jones. The cartridge I'd like to try a small primer in is the 6XC, maybe one day I'll make some from 6.5 x 47 brass, the neck will be a bit short, but for experimentation purposes that'll be OK.

Thanks again!

 

[dennisinaz]

I am amazed at the difference between the Wolf and Rem primer.

 

[LRTom]

That's why I decided to play around with the 6/6.5x47 Lapua rather than the 6 XC. (which is a great cartridge, BTW)

Slightly off the topic, the reason I approached Lapua about the SRP design was not to find some super new miracle case. A good load in the 308 is good be it LRP or SRP primers. I just found over the years that I didn't have to test through several lots of primers with SRP to get a competitive load. I could use just about any brand and lot of SRP and have a competitive load, i.e. SD < 8; ES < 20; whereas with the LRP I had to test several lots of the best "match" LR primers to find that load and then hope to be able to buy enough to last for a while. More than once I just went down to the local gun shop and bought 1000 of whatever SRP they had on the shelf and went with it. Bottomline is that it was one less thing to worry about.

Tom