Large vs. Small Flash Holes in .308 Win Brass
Conventional .308 Winchester brass has a large primer pocket with a large, 0.080″-diameter flash hole. In 2010, Lapua began producing special edition .308 Win “Palma” brass that has a small primer pocket and a small flash hole, sized 1.5mm (.059″) in diameter. Tests by U.S. Palma Team members showed that the small-flash-hole .308 brass possibly delivers lower Extreme Spread (ES) and Standard Deviation (SD) with some bullet/powder/primer combinations. All things being equal, a lower ES should reduce vertical dispersion at long range.
Why Might a Small Flash Hole Work Better?
The performance of the small-flash-hole .308 brass caused some folks to speculate why ES/SD might be improved with a smaller flash hole. One theory (and it’s just a theory) is that the small flash hole creates more of a “jet” effect when the primer fires. German Salazar (Rifleman’s Journal Editor) sought to find out, experimentally, whether this theory is correct. German explained: “During one of the many internet forum discussions of these cases, Al Matson (AlinWA) opined that the small flash hole might cause the primer flash to be propagated forward more vigorously. In his words, it should be like shooting a volume of water through a smaller nozzle, resulting in a flash that reaches further up the case. Now that kind of comment really sparked my curiosity, so I decided to see what I could see.”
More Primer Testing by Salazar
You can read more about this test and other primer experiments on RiflemansJournal.com.Salazar Primer Tests: Small Rifle Primer Study | Large Rifle Primer Study
Large and Small Flash Hole .308 Cases — But Both with Small Primer Pockets
To isolate the effect of flash hole diameter alone, German set up a test with the two types of .308 case that have a small primer pocket: Remington BR brass with a 0.080″ flash hole and Lapua Palma brass with a 0.062″ flash hole. NOTE: German reamed the Lapua brass to 0.062″ with a Sinclair uniforming tool, so it was slightly larger than the 0.059″ factory spec. The Remington brass has a .22 BR headstamp as this brass was actually meant to be re-formed into .22 BR or 6 BR before there was factory brass available for those cartridges.
German set up his primer testing fixture, and took photos in low light so you can see the propagation of the primer “blast” easily. He first tested the Remington 7 1/2 primer, a primer known for giving a large flame front. German notes: “I thought that if there was a ‘nozzle effect’ from the small flash hole, this primer would show it best. As you can see from the photos, there might be a little bit of a flash reduction effect with this primer and the small flash hole, the opposite of what we expected, but it doesn’t appear to be of a significant order of magnitude.”
Remington BR case, 0.080″ Flash Hole, Remington 7.5 Primer.
Lapua Palma case, 0.062″ Flash Hole, Remington 7.5 Primer.
Next German tested the Wolf .223 primer, an unplated version of the Small Rifle Magnum that so many shooters use. German notes: “This is a reduced flame-front (low flash) primer which has proven itself to be very accurate and will likely see a lot of use in the Lapua cases. With this primer, I couldn’t detect any difference in the flash produced by the small flash hole versus the large flash hole”.
Remington BR case, 0.080″ Flash Hole, Wolf .223 Primer.
Palma case, 0.062″ Flash Hole, Wolf 223 Primer.
German tells us: “I fired five or six of each primer to get these images, and while there is always a bit of variance, these are an accurate representation of each primer type and case type. You can draw your own conclusions from all this, I’m just presenting the data for you. I don’t necessarily draw any conclusions as to how any combination will shoot based on the pictures.”
Results of Testing
Overall, looking at German’s results, one might say that the smaller diameter of the small flash hole does not seem to have significantly changed the length or size of the primer flame front. There is no discernible increased “jet effect”.
Similar Posts:
- .308 Winchester — Large vs. Small Flash Hole Test
- .308 Lapua Palma — Lapua to Introduce a Small Primer .308 Winchester Variant
- Lapua .308 Win Brass on Sale at Bullets.com
- Lapua 6.5 Creedmoor Brass Has Arrived at Graf & Sons
- New "Outside-In" Flash Hole Reamer for Large Flash Holes
Tags: .308 Win, Flash Hole, German Salazar, Lapua, Palma, Primer
What ever happened to German’s web site? Last I heard, he had shoulder surgery and was going to take a break from shooting. That was a couple of years ago. I hope he’s doing well, his articles were very informative.
Tony
Back in the 1980’s, before the cartridge’s dimensions were standardized in 1989, I used a batch of .308 brass that was intended to be reformed to the 6mm Remington BR. Remington produced the brass, head stamped, “R-P Rem BR”, that had especially thinned shoulder and body thickness, and used a small rifle primer pocket, although with a .080″ flash hole. Formed full length into 7mm IHMSA, loaded with my 140 grain jacketed open point bullet, H-322 powder and a Remington 7-1/2 primer, it was phenomenally accurate in a bolt action Wichita Pistol.
Interesting how old forgotten things become the next great thing decades later. Still, worth experimenting with a few to see what I can come up with.
I have necked down 308 Palma brass to 260 so I could go to higher pressures without loose primer pockets, ala 6mmBR.
The best primer for that is the CCI 450 small magnum rifle primer.
That primer cannot set off 40 gr of CFE behind a 120 gr bullet, but will not set off 42 gr.
What does it all mean?
Small primer pockets are good for high pressure.
Large primers are good for igniting powder.
thank you heaps, i have been thinking about the possible benifits from a larger burn front through a certain size flash hole for some time . primer type /powder type / flash hole size and case diameter .thank you for your article .
I use the Lapua Palma .308 brass with a CCI 41 primer for Palma with the 155. Works very well. SD in the single digits. The brass is tough and has a lot of mass in the head. Weighs a average of 172 grams and all has been + or – a gram even from different lots. Makes three nice batches. Lasts forever. Primers are cheaper and easier to get. It has a PPC type small flash hole so the dies primer pin has to be changed on all .308 dies to deprime and avoid ruining the case. Important point not usually mentioned. A bit of a hassle when you get into collect dies that have to be turned down at the pin and so called universal decappers. Still I wouldn’t go back,,,good stuff.
I shoot f class. I shot ftr 308 and f open 284 win 6 BR also. Last year I started having pressure problems with the BR. I had to drop 2 full grains in powder to cope with it. What it ended up being was my new dies had the large decapping pin and was enlarging the primer hole. Only after much head scratching did I finally realize the core of the problem. I learned a lot about primers and flash hole size affecting accuracy and pressure. All said I much prefer small hole and primers for accuracy.
Excellent read . As the author concludes , there appears to be little to no difference in result , regardless of which primer used . Personally ; I suspect variations in SD / ES have more to do with slight variations in particular loading practices ,simply because of the nature of reloading . Types of powder ; temperature , and altitude could all have more effect on SD and ES than the primer , when comparing the photo’s shown above .
But the bottom line is still the same . Do what works best in your rifle …
So in theory, the narrow flash hole results in the flash igniting more powder faster than a normal size flash hole does.
Would the result be the same with a faster burning powder and a normal size flash hole?
on the contrary small flash hole makes burning slower and that is the benefit, the opposite of writing