A reader wrote:
We have looked at the SAAMI specs online and cannot find the specification for the diameter of the flash hole. This arises due to getting decapping pins stuck and broken. Interested in any info you folks have.
Flash hole and decap pin diameter issues

Regarding SAAMI specs, SAAMI does not have much official information published on flash holes (that I could find, at least), and I do not believe they are officially standardized, even though there are industry standards.

Large decapping pins are around 0.080, small decapping pins are around 0.060. In my experience, flash holes in 5.56/223 (and most flash holes in general) are right around 0.075-0.085, and it is usually safe to use large decapping pins, despite it being "22 caliber".

There are exceptions, such as "NPA" headstamped 5.56 brass, which has a microscopic flash hole somewhere around 0.050 (I usually throw this brass in the recycle bin). Some S&B brass seems to have smaller flash holes and primer pockets as well.

You will find a lot of variation even within the same caliber regarding flash hole size. It is not necessary to have a large flash hole for reliable ignition; bench rest shooters claim to see improved accuracy with brass that has smaller flash holes.

Other flash hole issues

Another common problem is to find off-center flash holes in certain runs of certain, usually lower quality, brass. In my experience, there is no one single headstamp that shows this problem all the time, but rather, this problem crops up as a sporadic quality control issue.

A single off-center flash hole can bend your pin slightly, such that the next piece of brass, with a normal flash hole, causes the pin to bend or break. Always check the last few pieces of brass after a broken pin incident, if it's not immediately clear what caused it to break.

Beyond decapping, there's only one problem a small flash hole can cause. On very light, low pressure loads, mostly only wax or plastic bullets or some subsonic rifle loads, the primer can back out upon firing if the flash hole is too small.

Large vs small decap pins

The calibers that pretty much always require a small decapping pin are as follows:

  • Lapua 6.5x47
  • 6mmBR
  • 220 Russian
  • Norma 6 PPC
  • Lapua .308 Palma
  • Most 22 hornet

Always try to use a large decapping pin if you can. On crimped primers, the large decapping pin will help avoid problems such as punched primers. As long as the flash hole is not excessively small, the decap pin will actually enlarge and standardize your flash holes by knocking off any burrs left from the punching process. Though, if your large pin is getting stuck in the flash hole often, and everything else is set right, it's time to switch to small.

Tip: Always prep your pins!

Before I install a new decapping pin, I always polish it on a wheel to remove any edges.

I also very slightly bevel one side of the tip like the end of a hypodermic needle, this seems to help with primer "suck back", preventing the primer from squarely wedging onto the decapping pin. Just don't make it too pointy or it will puncture primers. Everything should be mirror smooth and blunt with no edges.

Proper Pin-installation Prevents Poor Performance

When installing a non-headed decapping pin, it is sometimes helpful to use a vise to hold the rod so that you get enough leverage on the nut. Be careful to not clamp down on your resizing ball though, it's easy to crush a steel resizing ball, and the expensive carbide ones will just shatter. (Learned that one the hard way)

Whenever you loosen the die or the decapping assembly nut, take care that the pin is centered before you tighten it back down. The easiest way is to run an already sized and lightly lubed of brass carefully into the die so that the flash hole in the brass holds the die and decap assembly straight as you tighten things up. The threads on decap assemblies are very sloppy sometimes, so always make sure things are straight.

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