I have an M261 unit, the firing pin appears to be almost new, the overall unit in very good condition.  The unit will not fire a chambered cartridge.  I have tried two different rifles, each with stock milspec triggers.  I have sequentially: completely cleaned the unit, tried different ammo, clipped 3 coils off the firing pin spring, polished the firing pin and recoil spring guide rod, beveled the firing pin to 45 degrees, and run the unit pretty wet.  Is my next option cutting more firing pin spring coils?  The primer strikes are so light as to leave little or no mark.  The firing pin protrusion appears too short to me (the pin face was not touched when it was ground), but I have no way to measure it.  To figure out if it is pin protrusion or an overly strong firing pin spring, I was thinking of VERY carefully loading a round without the spring, and seeing if it will fire.
Any advice appreciated.

UPDATE:  After trying everything, I am almost positive the firing pin is too short.  Firing without the spring I still only get the faintest hint of a strike (wouldn't see if not looking for it).  The protrusion is so slight I cannot measure it, a dental pick will barely snag on it.  I tried Google for a new firing pin.  Sarco was a no-go, I thought Compass Lake had them but could not find one on website.  Anyone have a link to someone selling them.  Oly arms is a no-go too.

Check to make sure the bolt is fully in battery. Check to see if pushing on the firing pin from the back of the bolt makes the pin protrude into the bolt face at all. The pin could be defective (short) or hanging on something inside the bolt. Make sure the hammer is striking the back of the bolt at the firing pin and not missing it off to one side.

Have checked the above, all seems normal other than very little firing pin protrusion past the bolt face.

Quoted: Have checked the above, all seems normal other than very little firing pin protrusion past the bolt face.

Buy a new firing pin before you bugger up something. At the least, measure it and post the length.

Try removing the firing pin spring altogether.  Also scrub the firing pin channel and verify that there's no gunk impeding firing pin travel.

The fp protrusion on my 261 appears to be about .045 although I'm using a spark plug gauge so it may be .005 off either way.    I had problems with mine as well, beveling the FP did wonders.  I'd echo the thoughts of others, check the protrusion and get another FP if necessary.  If the protrusion is sufficient, check to see if it’s going into battery. If it’s not, this would keep it from firing a round even if the protrusion is sufficient. Replacing the main spring may be necessary if it’s not.  I also did some polishing of the edges between the bore insert ( not sure of the tech. name) and the bolt because mine would occasionally get caught up and not seat fully.  I just kind of took the sharp edge off.  

As always, ymmv

My stock M261 also has ~0.045" of firing pin protrusion.  Conversely, the firing pin sticks out almost as far as the forward most face of the bolt (~0.015" recessed.)

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Can you measure the overall length of your firing pin?  My (functional) M261 firing pin looks to be 2.878" long.

If yours is any shorter you could try setting back the pin's shoulder, allowing the firing pin to move further forward relative to the bolt.  It's an easy task with a lathe, or you could try chucking the firing pin into an electric drill and holding it against a file.  You could also take a dremel to it.


As a last resort, any machinist or gunsmith should be able to whittle a new firing pin out of some bar stock in 15 minutes.

Quoted:
Can you measure the overall length of your firing pin?  My (functional) M261 firing pin looks to be 2.878" long.

My unused (new) FP from my spare parts kit is a measured 2.875" - just about identical to heart_of_darkness's FP.

Email me your address and I'll mail you one. I have a few and I'll check to make sure it falls in spec with what the others have said.

If you do need to get one made up in a machine shop here is a diagram, may be useful (I'm on my 3rd firing pin)

http://dansanderson.co.uk/m261.jpg