Firing Pin Force

[gorlitsa]

How hard must a firing pin hit the primer to set it off? Say, for 22 (rim-fire, I know, but some concept), 38 and 45? I just can't get over how this dinky little bit of metal in the gun starts a reaction that creates an incredible amount of force. Maybe if I get some hard numbers I can reconcile the phyisics in my mind.

Thanks

 

[kurt IA.]

They run from 18lb to 30lb for centerfire, and around 18lb to 23 lb on 22 rimfires.

 

[Art Eatman]

Gorlitsa, think about it for a moment: The firing pin isn't "doing"; it's "being done to" by the hammer, which in turn is moved by the hammer spring.

Sorta like you taking a hammer and hitting a chisel. Your arm serves as the spring; a hammer is a hammer is a hammer; and the chisel does about the same thing as a firing pin.

How strong is your arm?

, Art

 

[4V50 Gary]

But wait a minute Art. What if it's striker fired like on the P-08 Luger or Glock? There's no hammer involved in either gun - so that partially invalidates the view advocated by you. Maybe we can rephrase the question: How much energy transfer is required to cause the primer to ignite?

 

[C.R.Sam]

Energy stored in spring is released through hammer/striker to deform primer enough to squeeze the priming material between cup and anvil. Hence causing ignition of the priming compound which in turn ignites the propellant charge.

Takes very little to start the fire.

Sam

 

[James K]

It seems not to take much, but the primer compound, unlike the propellant powder, is a high explosive. It is only because there is so little of it that the force can be contained. If, somehow, primer compound were loaded into the cartridge case, it would take only a few grains to blow the rifle apart.

The actual firing progression is usually not recognized. When the firing pin hits the primer, it first drives the primer and case forward to the extent it can, depending on the amount of headspace. Then the primer, driven by the force of the exploding primer compound, backs out of the case as far as the headspace will allow.

Then the powder charge ignites, and presses the thinner forward case walls against the chamber walls. The neck is expanded and the bullet floats on gas as it is driven into the leade of the barrel.

Meanwhile, the head (rear) of the case is backing up under pressure as far as headspace will allow, backing over the primer. Since the front end of the case is pressed against the chamber wall, the rearward movement will stretch the case, again as much as the headspace allows.

The propellant gas pressure coming back through the flash hole pushes the primer back around the firing pin. This is why a misfired primer has only a shallow indent, while a fired primer appears to have a very deep indent. If the pressure is sufficient, it will cause the primer to expand into the beveled area around it, causing the flattened primers we associate with high pressure.

When the bullet leaves the muzzle and pressure drops, the elasticity of the case causes it to spring back from the chamber walls. It can then be extracted. If, in an automatic or autoloading firearm, the breech opens before the pressure drops, the case head can be torn off the case, since the front end of the case is still pressed against the chamber wall.

The required elasticity is found in only a few metals cheap enough to be used for cartridge cases, and the best is brass, which is why cases made of other materials have not been widely used in recent times except under special conditions.

Jim