Ask Foghorn: Snap Caps?

Texas Deputy asks:

I know that some makers explicitly state that dry firing is safe, and will not damage the firing pin and related assemblies, but others strongly recommend snap caps for repeated dry firing.

What is your stance on snap caps?

This is actually a pretty easy one, so let’s jump right in. . .

Dry firing is the practice of pulling the trigger and letting the hammer hit the firing pin in your firearm without having a live round in the chamber. Because there’s nothing in the gun (the gun is “dry”) nothing goes bang, but everything else about the gun (the trigger pull, ergonomics, etc) is the same as if it were loaded. Dry firing is a great way to practice sight alignment and trigger pulls on firearms without spending a single penny on ammo.

I follow a 70/30 rule on firearms practice — 70% dry fire, 30% live fire. Dry fire practice is a great way to develop the fundamentals but at the end of the day, you really need to put rounds downrange to test your skills.

The problem: dry firing has the potential to ruin your gun.

Firearms are designed to be used live and loaded. When a designer is figuring out how the firing pin and hammer assemblies work in a gun, they assume that there’s a live round in the chamber. That means the firing pin meets resistance in the form of a primer as it moves forward. The energy of the hammer falling is transferred through the firing pin and on to the primer where it ignites the primary charge and sets off the round.

When the firearm is dry – as in empty – that primer isn’t there to absorb the extra energy from the hammer. So instead of most of the energy being transferred to an external component designed to use that energy, it needs to be transferred to something else in order to stop the firing pin and keep it from flying into the chamber.

Designers typically include some sort of system in the firing pin channel or on the firing pin itself that stops or arrests the forward movement of the firing pin, absorbing that extra energy as it brings the firing pin to a stop.

On an AR-15, that arresting system comes in the form of the chunky flange on the rear of the firing pin, shown here disgustingly coated with carbon. When the firing pin moves forward, that flange impacts the rear of the firing pin channel in the bolt carrier and keeps it from moving any farther forward. The flange is so chunky because it needs to absorb all of the extra energy from the gun’s hammer and transfer it to the bolt carrier (a chunkier piece of metal more capable of absorbing the energy).

The act of arresting a firing pin puts a tremendous amount of strain on the metal due to the magnitude of the forces in play. It may not seem like a lot of energy, but we’re talking about tiny pieces and it’s certainly enough to crack or break the metal over time.

That’s why many of the modern firearms have easily replaceable firing pins — especially the M1 Garand, M1 carbine and M16. All three of these weapons use an arresting system that eventually wears out the firing pin and can cause it to break. As a result, they were designed to make those parts easily replaceable by the soldier. In a time when every single part of a military firearm had proof marks and a serial number, the M1 Garand and M1 carbine had no such markings on their firing pins and extractors for this very reason.

Most centerfire rifles and handguns have an adequate arresting system and easily replaceable firing pins, but rimfire firearms are another matter. Their components are much smaller than their centerfire counterparts and therefore are much more likely to break, even with an adequate arresting system. They need something in the chamber that will allow them to safely transfer the forward energy of the firing pin to a more robust piece of metal. That’s where snap caps come in.

Snap caps emulate the primer of a live round, whether it’s a centerfire or rimfire cartridge, and allow the firing pin to dissipate its energy in a safer manner.

You physics fans out there will remember that force equals mass multiplied by acceleration. So the more time something takes to slow down (or lose energy) the less force will have acted upon it. Or something like that. Anyway, the key concept here is that by distributing the application of force over a longer period of time, firing pin undergoes less stress.

Snap caps allow the firing pin to dissipate its energy over a longer period of time due to their design — specifically the placement of some malleable material where the primer should be. So rather than the firing pin striking the hard surface of the chamber or some other arresting mechanism designed to immediately stop the firing pin (think flying egg versus brick wall) the snap cap will slow the firing pin down before bringing it to a complete stop (think flying egg versus a soft fluffy wall of pillows…or Snooki).

Personally, I can’t think of a reason not to use a snap cap when you’re dry firing. Even if you have a modern firearm made from the best materials, you’re still putting wear on those parts and that will eventually lead to them breaking. Admittedly it might be centuries before that happens but hey, it’s possible!

If you’re still too cheap not to practice with snap caps, here’s a list of firearms that should probably never be dry fired without a snap cap:

  • Rimfire firearms: The mechanical bits are just too small to stand up to much abuse, and if you don’t treat them right, they will eventually crack and need to be replaced. And trust me, replacing a rimfire firing pin is not a task to be taken lightly.
  • Older firearms: 1980 is the year I have in my mind for when this starts applying, and ESPECIALLY anything made during WWII. Older firearms may have been manufactured using substandard materials and therefore often have more brittle firing pins and related parts. The caveat here is that if your gun costs less than $100 (AKA Mosin Nagant) and has no sentimental value I wouldn’t worry about it too much.
  • Rare firearms: If there’s only three of your model gun ever made, then there’s a good chance that you’ll never be able to find replacement parts. You might want to baby it a little bit more than the rest of your guns.

Modern firearms (rifles especially) are typically good to go for dry firing, but I’d never chance it with any of those three categories.

Sorry for the delay for those of you still waiting for your question to be answered — I just settled into my new place here in San Antonio and I’ll be getting to work on that question queue ASAP. Really.

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