Pierced Primer

[killer_chicken]

So I was at the range the other day, shooting my No.4 Mk I. During one shot something seemed to 'bite' my thumb. It hurt, but I couldn't really see anything wrong, so I didn't really think much of it (maybe I should have, in hindsight). The next shot misfired, it looked like the firing pin didn't strike the primer hard enough, this got me looking at the rest of the primers. Comparing the strikes, I noticed the brass from the previous shot (the bite) looked strange.



It's hard to get a clear view, but it looks as if the firing pin pierced the primer, which exploded back, onto the firing pin. I figure the 'bite' that I felt was the cocking piece hitting my thumb from the explosion, does this sound correct?

I checked the rest of my brass, about 100 rounds, and this is the only primer that has been pierced. Also, this was the first misfire I've experienced with this rifle. I've fired another 20-30 since without an issue. Should I consider this an isolated incident with a bad round, or do you think it's a problem with my firing pin / spring? I'm tempted to leave it until it happens again, but I'm not sure it's safe to do so?

Information

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[killer_chicken]

Sorry, I found some other threads on this topic, it looks like this has been covered before. I don't seem to be able to edit or delete my thread, so I'd appreciate it if a mod could delete this one.

[Parashooter]

Although it appears a defective primer is most likely here, it wouldn't hurt to inspect and polish the firing pin tip - especially since the recent piercing event undoubtedly added a bit more roughness to the pin's surface texture.

[Bruce_in_Oz]

Lookoing at the photo:

The case second from the left in the top row appears to have a bit of a crater "rim" around the striker mark.

This would indicate either:

Excess pressure with a "normal" primer, or

Softness of the primer cup.

Check for the fit of the striker tip where it passes through the front of the bolt head. Any more than a couple of thou' can start problems, especially with "soft" or brittle primers.

The big problem is that the gas-cutting that occurs every time a primer fails in this way will eat your bolt head and striker. The roughness that even minor gas-cutting of the striker tip causes is enough to cause subsequent primers to rupture, thus causing even more damage to the striker tip AND to the bolt head.

Strikers that are not concentric with the hole in the bolt head (particularlr through bending) will wear themselves and the striker hole eccentric. This is also "not a good thing".

The worst rifles I have seen for this problem were WW2-built Arisakas: Not too hard to fix by inserting a small, close-fitting bush in the bolt face, but still a fiddle.

Excessive striker protrusion will also work-harden and overstress the brass in the primer cup upon impact of the striker.

[killer_chicken]

Thanks, I'll try my hand at polishing the firing pin for sure.

I did inspect the firing pin in the bolt head when it occurred, and while I didn't check for smoothness, I did note that it was tight and centered.

[jmoore]

You may also want to flush out the bolt and give the bolt head an internal cleaning in case the biece of the primer blanked out is still in the works. That's likely what caused the misfire. It may also explain the inconsistant hits on the other primers.

[Peter Laidler]

I would treat it as an isolated incident. But I'd say that while you're thinking about it, do all the maintenance that the others have suggested such as clean out the bolt head and rinse the bolt through in degreaser

[killer_chicken]

You may also want to flush out the bolt and give the bolt head an internal cleaning in case the biece of the primer blanked out is still in the works. That's likely what caused the misfire. It may also explain the inconsistant hits on the other primers.

This is a good point too, I did take the bolt head off while at the range, but I wasn't looking for anything specific. I'll take a closer look, and flush it out, regardless.

[killer_chicken]

Although it appears a defective primer is most likely here, it wouldn't hurt to inspect and polish the firing pin tip - especially since the recent piercing event undoubtedly added a bit more roughness to the pin's surface texture.

As you can see below, my firing pin definitely needs some sandpaper treatment.

Check for the fit of the striker tip where it passes through the front of the bolt head. Any more than a couple of thou' can start problems, especially with "soft" or brittle primers.

Here is a photo of the firing pin protruding from the bolt head, I think it looks good, but I'm not sure what a "good" one looks like...

You may also want to flush out the bolt and give the bolt head an internal cleaning in case the biece of the primer blanked out is still in the works. That's likely what caused the misfire. It may also explain the inconsistant hits on the other primers.

When I took the bolt head off the bolt, I could see some brass looking material inside the bolt, on the piece that holds the firing pin inside. I also found a few flecks in the cone inside the bolt head, I carefully scraped it off and blew it out, and will rinse it out with some solvent.

Thanks for the info guys!

[Parashooter]

Normal pin protrusion looks like what you can see at "A" in this image. It should be between .040" and .050". Note that the protrusion is controlled by the striker flange contacting the rear of the bolt head at "B". It looks like your pin tip is so badly damaged it may end up short after the correct contour and surface finish is restored.