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”.