Check Your Flash Hole!

By Allen Schuh

Reloaders of centerfire cartridges agree on one main theme: the finished cartridge, made up of a case, primer, powder and projectile, should be as consistent as possible when we finish. If there is no consistency, there can be no accuracy.

I was cleaning my spent .30-06 Springfield brass when I noticed a bit of debris in the flash hole of one of them. The flash hole is simply the little hole in the back of the cartridge case where the fire of the primer gets to the powder in the casing and ignites it. I usually pop out the old primers almost mindlessly, then I use a bit of cotton on a stick to clean around where the new primer will be seated.

It is during this activity that I really look at the case. Usually, I am looking for cracks or any irregularity in the shape as a clue to retire the case. In all my years I had never checked the internal diameter of the flash hole. Nor do I remember seeing it mentioned in the various reloading manuals.

With one case, it was obvious that there was some debris in the flash hole. It was probably gunpowder and primer residues or part of the old primer. As a matter of curiosity, I tried various drill bits to clear the material. I used a 5/64 drill bit to clear it and started wondering about the other cases in the bucket, so I checked all of them. I found variations in size of the flash hole, even among those from one manufacturer, but purchased in different years and among those of different manufacturers, even purchased in the same year.

When you think of all the work we go to in order that every round is the same, we should not ignore the size of the flash hole. If the size of the hole varies, then the time to reach a higher pressure will vary. The typical flash hole was about 5/64 inch across, but there was variability from too tight to pass a 5/64 drill bit to rattle room.

I became interested in the size of the flash hole, specifically whether there is something important about the 5/64 inch diameter. If manufactures picked a number, such as 5/64 inch, they must have had a scientific reason. I looked all around and cannot find an explanation for this number. If it was just picked by convention, then why not try something different? I got to thinking: what would happen if I drilled the hole larger and used a magnum primer instead of a standard primer? (Substituting a magnum primer should raise the MAP, without any other change. -Editor)

If the purpose of the hole is to get the flash into the powder, should not a bigger hole ignite the powder faster and reach a higher pressure sooner? Would not a magnum primer be better? Around here the regular and magnum primers are about the same price. We are talking about just the time to drill the flash hole bigger, say 8/64 inch, rather than 5/64, and see what the results would be. I painted the backs of the brass red to remember that it was modified. I only did it to 10 cases to limit the risk if things did not go right in the experiment.

I am pretty careful about procedures. What I found was that the median impact at one hundred yards was one inch higher with the larger flash hole. I held everything constant except the size of the flash hole. This could signify a change in either velocity or the recoil impulse. It is also likely the result of a change in the pressure curve, which could be dangerous and almost certainly means the published reloading data is no longer valid. The bottom line: clean and (if necessary) deburr flash holes, but do not enlarge them!

Looking back, the first flash holes were incorporated in the design of muzzle loading firearms hundreds of years ago. The purpose of the flash hole then called a vent hole or sometimes referred to as a nipple or cone, or in cannons called the touchhole, was to channel a bit of powder from the external to the internal through a short tube or vent. The flash hole was shaped like a funnel, being relatively wide at the top, so it would better catch the powder sprinkled from the powder horn. Then it narrowed, so it was small at the end where the main powder charge waited.

If the flash hole was too wide, the burning gases would blow back into the shooter's face. By trial and error, designers tried different openings at the top and bottom ends of the flash hole.

What feedback would a modern centerfire shooter get? When a centerfire rifle is fired, the expanding powder gas wants to flow both ways. The expansion of gases pushes the bullet out the chamber and through the barrel, simultaneously pushing the primer and case back against the bolt, which can lead to some interesting problems if the pressure is too great. If the bolt is hard to open, it is often a sign of excessive pressure.

The main advantage of checking your flash hole while reloading is to insure consistency, which relates to increased accuracy.

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Copyright 2016 by Allen Schuh and/or All rights reserved.