Worth replacing Mosin Nagant trigger with Finn 39

[mbogo3]

Would there be any point in replacing the trigger on a Russian 91/30 sniper with a Finn 39 one.The trigger pull on my sniper is outrageous.

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[Patrick Chadwick]

The short answer is "no", the reasoning is a little more complex.

Generally, the trigger pull on rifles is a result of the interaction of the trigger, sear, cocking ledge/notch and their engagement, plus mainspring (firing pin spring) and trigger return spring. And it can be DANGEROUS to fiddle with all this if you don't know exactly what you are doing. I'm glad you posed the question, as there are plenty of contributions on the forums to help you.

If you look up keywords such as "sear" and "engagement" and "angle" you will find a lot of material. And look for "Slam fire" and "safety-off fire" for a couple of serious danger points. And please read up as much as you can before doing anything!

Springs determine a force that must be overcome. The action of a service rifle is intended to function within a certain range of trigger pull. The minimum that can be reliably and safely achieved is perhaps 3 pounds, which is actually prescribed as a lower limit by, for instance, the BDMP.

1) Weakening the mainspring THIS CAN BE VERY DANGEROUS. Springs weaken anyway with age, they never get stronger.
Weakening the spring reduces the force that holds the sear safely in the cocking notch. This can lead to the dangerous condition known as slam fire, where a cocked rifle discharges if it is simply shaken or banged on the ground - or, when the rifle is cocked and you simply take you hand off the bolt - or, the ultimate, the rifle cannot be cocked at all, since the sear is not held in the cocking notch.

2) Reducing the sear engagement. This is also DANGEROUS. The bolt has a certain sideways play in the receiver body. If you look at the back end of the bolt while slowly depressing the trigger you can often see how the bolt body rises a bit before the sear starts to pull out of the cocking notch, producing the effect of a "first pressure" which is actually just taking up the slop in the action. Reducing the engagement increases the likelihood of "slam fire".

3) Altering the sear/notch engagement faces.
You will find references to "polishing the engagement surfaces to reduce friction". This is possible if done expertly. But, yet again, it can be DANGEROUS. The engagement faces are often only case-hardened. Filing will often cut through the case-hardening, producing a trigger system that will wear very rapidly, and ending in the same trouble as before. Which may well have been caused by the case-hardening being worn through!

4) Altering the engagement angle. VERY DANGEROUS. The engagement angle determines the "hook" effect of the ratchet formed by the sear and cocking notch. Again, if you look carefully at the bolt while depressing the trigger, you see that not only will the bolt body shift, because of the slop in the action, as in 2), but the bolt itself moves very slightly backwards as the increasing trigger pressure forces the sear up and out of the notch. A steeper angle thus increased the "hook" effect and increases trigger pull. A flatter angle reduces the hook effect and reduces trigger pressure. It is terribly easy to reduce the angle to the point where it is even slightly negative, which means that the mainspring is trying to lever the sear out of the notch instead of holding it in!

5) Removing burrs. This certainly can be done - using a diamond lap with the requisite care. But it is all to easy to round the edges of the sear and cocking notch ledge in the process, producing a mushy trigger release and, by effectively reduing the engagement angle AND depth, all the hazards listed above.


So now you have an idea why simply swapping the trigger is not going to be the answer! A perfect new trigger will not help much if the cocking notch depth and edge are damaged or worn.

[WarPig1976]

Here's an option.

http://www.timneytriggers.com/shop/t...t-trigger.aspx

[Josh Smith]

Hello,

A two-stage Mosin trigger and sear setup usually has an increased sear engagement for safety (never decreased if done properly) and a slight positive sear engagement.

An M39 trigger must be fitted to the sear.

A properly fit M39 two-stage sear/trigger should move the cocking piece back slightly, but should never catch or move it down. I made a demonstration once:

https //www.youtube.com/watch?v=zdO8M4-idqE

I played with thinning the sear springs, and the Finns did it a lot, but I wouldn't recommend doing so. They got fragile and started sticking at times. Just not a good idea.

Regards,

Josh