Ruger M77 firing pin protrusion

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steveNChunter

New member
A friend of mine brought me his tang-safety M77 .25-06 and wanted me to figure out why he is experiencing an occasional misfire. I took the bolt apart and found some rust and junk inside the bolt body and on the firing pin spring, which was most likely the trouble. I also wanted to check the firing pin protrusion to make sure that wasn't an issue. I'm using a dial caliper to measure which I know isn't the best way, but I don't have the special measurement tool. I'm getting a measurement of about .043"

What is the acceptable range?
 
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.050" to .060" is about normal.

If the rifle chamber's headspace is at the maximum and he reloads cases sized back to minimum, there'll be at least .010" head clearance between bolt face and case head when the firing pin starts denting the primer flush with the case head with its .043" protrusion from the bolt face. That means the primers will be dented only .033" at most. And some primers will misfire with such shallow dimples in them.

If his firing pin spring's weakened too much (they all do with age), that compounds the issue.

Good stuff on pin protrusion....

http://www.brownells.com/aspx/learn/learndetail.aspx?lid=10379
 
I don't own a go gauge for a .25-06 but I'm pretty confident I found the problem when I took the bolt apart. I cleaned it all up and oiled it, if he still has misfires I will get a new firing pin spring before doing anything else. I just wanted to be sure the firing pin itself was ok. Is there a general minimum/maximum rule of thumb measurement for firing pin protrusion?
 
So what I'm getting from that article is that you need at least about .050" protrusion to rule out any issues. I also don't know for sure that I'm getting an accurate reading with my caliper. I guess I'll just have to let him try it out and see how it does.
 
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It sounds like there might be some corrosion left in the FP channel, inside the bolthead, that's limiting the FP from moving fully forward.


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My 77 from that era had the same issue. I only noticed it when shooting some Canadian '06 I had picked up. It was linked for MG use.

I replaced the mainspring with a heavier one...can't recall the brand of spring at the moment. After the spring change no more problems.
 
It sounds like there might be some corrosion left in the FP channel, inside the bolthead, that's limiting the FP from moving fully forward.
Click to expand...

I highly doubt it. I sprayed the inside of the bolt body full of brake cleaner, blew it out with an air hose, filled it with oil, turned it up and let it drain out, blew it out with air again, and then lightly oiled the firing pin spring after cleaning it with a wire brush wheel on a grinder. I looked inside the bolt with a light and it was clean as could be.

My current thoughts on the issue is that the FP spring is just barely long enough and any little thing such as corrosion in the FP channel is magnified because of the semi-short firing pin. I gave him the rifle back awhile ago and told him to try the same ammo in it and let me know how it does. Worst case scenario I'll get a firing pin and spring and put in it.

I measured my M77 in 6mm and the firing pin protrudes only .047" and it has NEVER misfired. Maybe that last .004" is doing all the work?
 
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