Light Firing Pin Strikes

(I wrote this for the Taurusarmed.net forum.)

Light Firing Pin Strikes

If you get failures to fire they may be due to light firing pin strikes. The indicator after the hammer or striker falls would be an unfired cartridge with a shallower than normal indentation on the primer. A light firing pin strike could be caused by insufficient energy transmitted from the firing pin to the primer, insufficient depth of the primer strike, or a combination of the two.

Insufficient depth of the strike is caused by one or more of the mechanical elements being out of specification. Excess headspace can cause light hits. Excess headspace can be caused by an out of spec firearm or out of spec ammunition. The best way to test headspace on the pistol is with a headspace gauge. A cartridge gauge can be used to check your ammunition. A quick check on semi-autos can be performed by removing the barrel and inserting a round of ammunition until it is fully forward. The head of the cartridge should come out even with the rear of the barrel where it comes in contact with the breachface. If the head of the cartridge is too far forward, you can see that the firing pin might have difficulty coming in full contact with the primer.

Insufficient firing pin protrusion can cause light hits. With the hammer or striker fully forward, measure the firing pin protrusion and compare that to the factory standard. Centerfire rifles and handguns usually measure between .050” and .060” but you should check with the manufacturer to get the correct tolerances for your firearm. Note that some firearms, such as the 1911, use an inertial firing pin that does not contact the primer when the hammer or striker is fully forward. On a 1911 with the slide and barrel removed, use a punch to push the firing pin fully forward and ensure that you get at least the minimum protrusion.

The energy transmitted to the primer is a combination of the mass and velocity of the hammer or striker. Mass is generally fixed at the time of manufacture but modifications may change this. If you bob the hammer on a revolver, you decrease the mass and may cause light hits. You would either have to increase the mass of the hammer (perhaps with a tungsten insert) or increase the velocity to correct this. You would increase the velocity by using a heavier mainspring or decreasing the friction of the moving parts.

The velocity of the striker or hammer is determined by the strength of the mainspring and friction between the parts. Lack of lubrication, incorrect type of lube, dirt, or machine marks can all slow the mechanism. Too much lubrication can act as a shock absorber and slow the mechanism. Striker fired firearms are particularly susceptible to too much lube. Lubricant that is too viscous can cause problems.

In very cold conditions many perfectly good lubricants can become too viscous. One of my most common jobs during hunting season in northern Wisconsin was degreasing rifle actions that were experiencing light strikes. The oil or grease in the striker channel or on the mainspring would turn into tar and slow the mechanism. Typically I would disassemble the bolt, flush the mechanism with solvent, and then lightly oil with Rem-Oil. I could often do this while the customer waited and get them back out and hunting. I found Rem-Oil performed very well in temperatures well below zero. Most light synthetics or a dry lube, like dry Teflon or molybdenum disulfide, would work as well.

Even if the mechanism is operating correctly, something can absorb the energy before it can be transmitted to the primer. If a cartridge isn’t seated all the way forward, energy may be lost when it is used to push the cartridge into position. This is common with revolvers where the cartridge is not sized correctly or the bullet hits the chamber throat before the cartridge rim hits the back of the cylinder. Generally, your ammunition should drop all the way into the revolver’s chambers easily and even rattle a bit when the firearm is shaken. Fouling can cause a cartridge not to chamber fully. Also a primer that is not seated all the way into the primer pocket can absorb energy.

Military ammunition typically has harder primers than sporting ammo to prevent slam-fires in automatic weapons. Sporting ammunition primers can vary in hardness. The harder the primer, the more energy it will take to set it off. In order to get a light double-action pull, it was very common with PPC target revolvers to put in light mainsprings. The lighter mainsprings would often limit the shooter to one brand of primer to get reliable ignition. This would be unacceptable for a self-defense arm.

I got one batch of the early Ruger 77 mark II rifles that had a manufacturing flaw in the striker mechanism. The rifles were giving intermittent light strikes. In the cold weather, I assumed it was due to the lubrication but, as spring and summer wore on, some of my customers were still getting light firing pin hits. I finally figured out that the firing pin had been machined with a small step on it. This step would sometimes- but not always- catch on a similar step in the firing pin channel. This slight catch absorbed enough energy to prevent ignition. It was simple enough to bevel both steps and later production runs didn’t seem to have this flaw. Similar manufacturing flaws can occur in other makes and models.