Primer Hole Prepping

We learned in another review that prepping brass actually does matter. But what about something as trivial as prepping flash holes? Does it matter? As you can see all over the site, there are sub 1/2 Inch groups out of an AR15 without prepping flash holes. So how CAN it matter that much?

Going into this test, there are probably hundreds of thousands of reloaders out there thinking "PLEASE, PLEASE, show that prepping primer holes does nothing, or almost nothing because I am already spending a TON of time prepping my brass as it is!! Not another step!!" Right?

Lets see what we can learn here. We did 10 shot groups, and that is not close to enough, but lets take a look using Nammo 5.56 Brass.

10 Shot groups: 100 yards

Close to 25 grains of N133 is very good with many 50 grain bullets. This is a Sierra Blitzking 50 grain @ 2.255.

Here is the KEY:

Normal = The brass flash hole is not touched, as it came from factory. Its near 80k, some are slightly less.

81k = The brass flash hole was drilled to 81k uniform

100k = The brass flash hole was drilled to 100k uniform

10 shot groups usually give good indicators of patters, especially when you are doing 2 different loads. But in this case, its not definitive how much it matters, just looking at the target. That being said, the stats tell a story.

The 2 groups that were drilled to 81k and were all uniform, were the 2 best groups. But barely. Also, the 100k groups were both the worst, but one could argue by 1-2 shots out of 10, because the main clump of 7 shots was good.

What is conclusion?

Conclusion is....some evidence?

You be the judge folks. But let us offer this possible explanation.

By drilling out the primer hole to 100k, this put more space in the case, so pressure went down, and the statistics support this theory. Notice the Velocity was 16-25 FPS lower with the 24.8 charge, and it was 12-16 lower with the 24.9 charge. So we can conclude since both were lower, its good evidence. BUT, the theory of why those groups were worst, could be that if we increase the charge by .1 to make up for the velocity loss of the larger drill hole, will that make it more accurate and help? You see how complicated this is?

We cannot just run a test, drill out a flash hole, and say "oh, that is the reason". Becasue when you change the flash hole, you are possibly changing pressing and velocity.

so all it is is evidence. One thing interesting, is that the SD and ES was dramatically lower in the 100k lots. BUT, if we played with seating depth and charge to lower velocity and pressure to equal that, would we get lower SD and ES too?

You be the judge. We will do some more follow up testing in future, to help answer this age old question in AR15's.

Despite all this, you now know a great Varmint load.