Firing pin protrusion

bewsher500

Well-Known Member
Whilst becoming an internet expert on rifle rebuilding (sic) I discovered a Midway video where Potter measures the amount of pin that protrudes on firing.

having just rebuilt a rifle and test fired it on a empty cartridge with a fired primer I was concerned about the depth of the strike in the primer.

I have just compared the .270 parker hale bolt to my .243 parker hale bolt and they do have quite different pin protrusions.

Is this likely to be a problem?

.270 - 1.48mm and 1.7mm (manually pushed out)
.243 - 0.9mm and 1.16mm (manually pushed out)

1.7mm seems an awful lot to protrude

013-2.jpg
 
Not being an expert (and I'm sure there are some on here) you said you test fired it? I think the only issue would be if it pierced the primer - other than that I doubt there would be that much of a problem...
 
I test fired it into an already fired primer in an empty.
It made a considerably deeper dent than the rifle that had fired the round.

I appreciate there is no charge to work against the pin but it just looks a lot

IMAG0874.jpg
 
I'm surprised no one has responded to this.... Oh well. I tried this on an already fired cartridge. Same rifle and you can see the one struck again was much deeper and had bent the primer in. I dont think this is a good test to see if your pin protrudes too much. I think if it isn't piercing the primer its not much to worry about.
223 primer.jpg
 
Think about it, the fired case will have expanded fully to the chamber, this may then result in it not sitting as far into the chamber as a resized round. this in turn will lead to the deeper strike you have noticed.
I'm sure there will be a technical manual available some where that gives the min and max tolerances for any part of the rifle and action, ask someone like Fultons at Bisley.
 
it is not so much the dent size that concerned me but the comparison between two identical actions.

an extra 70% of pin protrusion is an awful lot when the 0.9-1.1mm on the compared action already leaves a normal strike on the .243
 
I'm surprised no one has responded to this.... Oh well. I tried this on an already fired cartridge. Same rifle and you can see the one struck again was much deeper and had bent the primer in. I dont think this is a good test to see if your pin protrudes too much. I think if it isn't piercing the primer its not much to worry about.

When you fire a rifle, it takes just a few milliseconds from when the firing pin is released to the bullet exiting the barrel. A lot happens in those 4-5ms.

The sequence is this: the firing pin strikes the primer thus crushing the primer compound against the anvil; the primer compound, which is a percussion- and friction-sensitive high explosive, explodes; the explosion generates gas at high pressure within the primer cap; the high pressure forces the primer cap to back out of the primer pocket, the primer cap is stopped by bolt face; the firing pin spring holds the firing pin protruding out through the firing pin tunnel; the main charge of propellant ignites and creates high pressure within the cartridge body; the friction between the bullet and the cartridge case neck is overcome and the bullet moves forwards; in response, the cartridge case moves rearward and, in doing so, re-seats the primer cap; the cartridge case presses against the bolt face; the primer cap deforms against the bolt face and firing pin. This is why the indentation appears to be small - the primer cap is moulded against the firing pin and bolt face.

The apparently larger firing pin impression on a fired case is the result of there being no high pressure involved. A misfire also has an apparently large dent. An analogy might be this. Throw you body against a van's rear door, the indentaiton will be large. Now take a hammer to the door. The indentation will over a small area.

--JMS
 
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