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