I took some friends to my gun club today and had some fun. But, my friend's mosin slam-fired on him. He had previously fired approximately 40 rounds of 147 gr Russian surplus without incident.  While cycling the bolt action, the loaded round discharged without a trigger pull. It fired just when then bolt locked in place. Nobody was injured, thankfully.
I too own a Mosin 91/30. I have fired hundreds of rounds with only a thrilling recoil and boom. Never have I heard or read about a bolt action slam-firing like this. I will disassemble the bolt assembly tonight and see if anything is out of spec.
 Any ideas from the hive?


ETA: Firing pin protrusion and attempt to rapidly load and fire seem to be the problem. I used the included field tool to verify the setting on the firing pin, and adjusted it. BTW, found a chunk of brass in the bolt assembly when I took it apart.     Thanks for the help guys.

Check for gunk under the sear/trigger spring (can't remember what its called..). Also for chipped/worn sear on the cocking piece. Was this rifle thouroughly cleaned before it was fired? As in the action removed from the stock, bolt dis-assembled  and all cosmoline removed?

I agree with KB7DX as to giving it a good cleaning. A bolt action can "slam fire" if there is too little sear engagement and the bolt is closed with vigor. This however is not typical of Nagants with their heavy pull unless someone tried to smooth out the trigger or it's really worn out. After a good cleaning, work the bolt vigorously with an empty magazine––the striker must hold otherwise it's unsafe.

Thanks for the input. I'll help him out and clean it thoroughly.

Use the combination tool to check the firing pin protrusion.
Somebody likely turned the screw on the back of the bolt and set the firing pin to extend too far forward or it turned in the bolt during firing.
After it is set properly, use a center punch to lightly stake the back of the bolt into the screw slot to hold the firing pin in proper position. HTH.

I've had that happen to me with my 39 Tula before. If I work the bolt really hard the cocking piece will not engage sometimes. Also when the bolt is in the cocked position and the bolt is slapped upwards, the rifle will decock itself, i've never seen that before. I've had my rifle completly apart and everything seems to be in good order. I'm going to replace the sear and see if that corrects the problem.

I thought you had to work the bolt hard on a Mosin.

I'm with this, sounds like a FP protrusion problem.

i would work the bolt on an empty magazine a bunch of times... slam it shut, etc,

see if the rear cocking knob "safety handle" moves forward. if it does, you have a sear problem.

if the safety handle doesnt fly forward, it would seem like the firing pin is just too far forward.

I have a 1942 M91/30 that fires on closing, also. I'm not sure what exactly to call it, but the bottom part of the cocking knob that slides over the sear has a groove worn into it. I replaced the trigger and sear parts, so the only thing I can think of is that cocking knob needs to be replaced.

If he fired 40 rounds before this happened, I highly doubt that it's a firing pin protrusion issue. The sear is the weakest link in this design. They are prone to bending or breaking, especially if they've been thinned out before. It could be the cocking piece, but I would put money on the sear being the issue. I would inspect it to see if it needs to be bent back to a safe tolerance, or replaced.

check rifle as mentioned above and:

Remove the action form the stock.  Check the screw that holds the trigger spring in place.  If that is loose, it can cause a slam fire.  Also check to see if it is shimmed.  Even a very thin shim has a big effect on trigger pull.  Too much shim and you will get a slam fire.

What if the bolt is loose in the receiver? Could that cause it the jump the sear? If I carefully close the bolt, I can jiggle the coccking knob and cause it to "fire".

Then the sear is not sitting high enough, or the edge/s of the sear and/or cocking piece are rounded off. There is some play in all the bolts, but if it's able to fire without pulling the trigger, those items need to be inspected and repaired/replaced.

I fixed mine. The problem I determined, was that the sear was not preotruding up into the reciever far enough to stop the cocking knob from moving forward. The solution was to take the sear out, and Dremel the underside of the reciever where the sear rests on it. After a little material had been ground out, the sear went farther up and now there are zero slamfires.

toss said rifle in the trash, buy another. they're cheap. If you spend 2 hours and $30 plus driving 20 miles or so to buy parts how much have you saved?