Pierced primer!!

[Jim K]

While a very sharp and pointy firing pin can cause a true "pierced" primer, what we call a "pierced primer" is not the result of "piercing" at all; it is almost always the result of a weak firing pin blow.

The combination of the firing pin spring and the firing pin mass must be sufficient not only to fire the primer but also to prevent the internal pressure from pushing the firing pin back and forcing a tiny disc of primer metal into the firing pin hole, causing the "piercing."

Some guns have a mechanical block that keeps the firing pin from being pushed back, but most bolt guns, like the Lee-Enfield, do not. Also note that since that little disc of brass has to go someplace, it ends up inside the bolt. A single "pierced" primer will rarely cause a problem, but if there are enough, those tiny pieces of primer metal can block the firing pin and the rifle will not fire at all.

Incidentally, firing pin protrusion is never the problem, since no matter what the protrusion the firing pin will stop in the primer. No firing pin spring ever put in a gun has enough power to actually punch a hole in the primer. Firing pin protrusion needs to be controlled for other reasons, but not to prevent pierced primers. (Using a modified rifle, and under test conditions, I have created a firing pin protrusion of almost 1/4 inch, with no problems.)

Jim