TNW Suomi M31 review (hammer fired version)

I recently acquired a newer TNW M31 Suomi in 9mm.  Having several cases of postwar British Sten ammunition and nothing I would want trash it with, the idea of having a pistol caliber weapon that weighed more than a loaded M1 Garand sounded like a great idea.

The first thing I noticed was how heavy and overbuilt this thing was.  Keep in mind this is a semi-auto reproduction of a first generation submachinegun.  These were typical labor intensive to build and mostly overbuilt for the calibers they were chambered in.  In fact the rear sight still bears the original patent date of 1932.

The way TNW makes these into BATF-legal semi-autos from a full auto parts kit is to construct a new reduced diameter receiver and then convert the method of fire from open bolt to closed bolt.  The earlier renditions of these incorporated a striker-fired closed bolt assembly with the trigger pack converted to semi-auto.  There have been a few reports of those earlier striker fired versions failing to ignite hard primers in addition to excessively heavy triggers.  The later version I acquired resolves some of these issues by using a hammer fired bolt assembly.  This not only reduces the trigger pull, but allows greater force (actually impulse) to be imparted on the firing pin.  Although I haven't tested the British surplus ammunition with this, I have no doubt it will ignite just fine as it has cratered the primers on all the commercial 9mm ammo I have thus far fed it.

So far, I have put approximately 200 rounds through it since new and it runs flawlessly with 36 rd stick magazines.  I also have three drums.  Those are an entirely different story as none of them will feed more than 10 rounds without a failure to feed.  I have since adjusted the spring tension on the drums and anxiously await another chance to test them.

Accuracy is minute of 6" dueling tree plate at 80 yards.  Recoil is nil. My friend did manage to pull a shoulder muscle shooting this and I somehow injured my gluteus during the whole ordeal (this thing weighs a ton).

Cleaning is a breeze due to TNW retaining the original quick detach barrel assembly.  The only detractor from the field stripping is removal of the rear cap to access the bolt assembly.  This is done by retracting the charging handle all the way to the rear and unscrewing the cap while holding said handle.  The older TNW versions had a set of nested springs (one for the bolt and one for the striker) that were known to fly across the room.  Fortunately, the newer hammer fired version has only a single operating spring to cycle the bolt and stays put relatively well.

To sum it up, this is a great buy if you like historical firearms and don't mind a pistol caliber rifle that weighs more than a loaded M1 Garand.  It's definitely a keeper for me.

Pictures of the bolt assembly:

Trigger Group