Firing Pins: Size Matters

Handloaders often use the appearance of a fired primer as one of the signals of elevated chamber pressure. A common sign of increased pressure is the rounded edge of a primer flattened against the primer pocket. A slight increase in headspace, however, can also cause this appearance on a primer. Another telltale signal of elevated pressures can be observed as primer cratering—even with a perfectly normal rifle. But, again, various conditions can be present that will produce false readings.

The firing-pin hole in the bolt face or breechblock may be too large or the firing pin may be too small. Also, the shape of the nose of the firing pin may be incorrect, as well as the hammer or striker-spring force. A Means of Correcting Primer-Cratering Faults This article resulted from a case where the tolerance between the firing-pin nose diameter and the firing-pin hole in the breechblock of a single-shot rifle were poorly matched.

A firing pin of the proper size and shape can correct primer-cratering faults, thus permitting a more accurate interpretation of fired primers. The original hole in the breechblock of this rifle appeared even and smooth in diameter, and the firing-pin nose diameter appeared smooth and uniform in diameter. However, the difference between the two sizes was excessive. This resulted in a bizarre cratering of fired primers even with normal-pressure handloads.

I then decided that the solution was to make anew firing pin with an oversize nose diameter to fit the breechblock hole. As described in the text, the new firing pin was made somewhat longer and to a diameter that fit the breech-block orifice. Below center: The partially protruding firing pin shows a nose diameter that fully fills the hole. On the left we see the result of normal cratering around the firing-pin indentation.

The cartridge on the right shows abnormal cratering before the properly sized firing pin was installed. Making the New Firing Pin Making up the new firing pin was done with the part in the lathe. I used apiece of drill-bit shank that had some malleability, yet was hard enough to maintain its shape during use. The steel was also very machinable. Using the old firing pin as a pattern, I increased the diameter of the nose by 0.008 inch and length of the nose by 0.010.

I also increased the body diameter to better fill out the existing hole in the breechblock. This amounted to about 0.004 inch. In addition, I lengthened the whole firing pin a few thousandths. The

nose point configuration was formed to a spherical shape. The retaining-screw slot was milled out the same depth with a flat spot midway on the part allowing unrestricted travel of the firing pin. Conclusion The new firing pin eliminated the extrusion of metal around the nose of the pin as normal-pressure loads were fired. A defective or worn firing pin can give a false signal of impending pressure problems, and thus should be replaced.

In this case, making anew firing pin was the clear solution, and will allow the gunowner to get a much more reliable interpretation of fired primers. ■