I have one of these that I built from the kit last year. It's been a good gun with a few quirks. I know a lot of people who bought finished guns from TNW had multiple issues with them. Hopefully I can clear up some of those issues for people that I have solved and maybe someone can offer me some help with the issues I'm still working on resolving.

I'd first like to make not of the proper way to remove the rear cap, as I have read some extremely dumb things on the internet regarding it. To remove it, you pull the bolt handle back till it clears the slot in the cap enough to allow you to rotate it. Loosen the cap six turns (assuming the threads are clean and it was on all the way), till the slot lines back up with the operating handle and allows you to leave the operating handle return to it's closed position. If you loosened it enough turns you will notice that the cap is now loose enough that the op handle no longer blocks the cap from turning but the cap is still retaining the spring. The cap can now be removed the last turn without having to hold the operating handle back (and without having to hold the spring in extreme compression!). Installation is reverse... you insert the spring and thread the cap on one turn till the slot lines up, retract the op handle and thread the cap on the rest of the way.

A note on lubrication. You will often hear that the original specs required the gun to be run dry. The reason for this was because in the subzero temperatures it was intended to be operated in any lubrication would be prone to freeze and gum up the bolt, which would result in the bolt not traveling back far enough to catch the sear, causing the gun to "run away" and fire till the magazine was empty (72 rounds!) or till the lubricant warmed. This is not a concern on the closed bolt semi-auto conversions, only the original open bolt design. So don't be afraid to wipe down your bolt with light oil. Do NOT lubricate the firing pin in cold weather though, as the same condition can result (this applies to many other guns too of course).

If you have an parkerized model, it's extremely important to smooth the coating on the bolt and all of the internal moving and sliding surfaces. TNW went a little nuts with the parkerizing and it causes things to stick. When I got my kit the only parkerized parts in it were the bolt and barrel, and I took a wire brush to the entire bolt till it was smooth like a blued part. The barrel is OK to let be.

A few issues I noticed with this gun are related to the fact that it was originally designed as an open bolt firearm with a fixed firing pin. To those uninitiated, that means the bolt stays open all the time, and when the trigger is pulled the bolt falls closed, pushing a cartridge into the chamber and firing it, then immediately popping back open and locking open in waiting for another trigger pull to drop it again. On this design, the bolt never actually fully closes because the cartridge fires before the bolt shoulder ever touches the front of the receiver (on this gun the bolt face doesn't touch the breech face as it does on many other designs and instead the bolt actually stops when the bolt shoulder reaches the front of the receiver tube).

One thing that some people notice with these is that once you chamber a round, it is often impossible to get it out of the chamber without firing it. The reason for this is the chamber on the original open bolt barrel (and any barrel designed to be a replacement for a full auto KP31) is not cut to the same specs as the chamber in a closed bolt firearm. The chamber is instead shorter and the front is tapered to slow the cartridge and allow the open bolt firing pin to press into the primer before the cartridge is fully chambered, so that the cartridge has time to ignite and develop pressure before the bolt reaches the bolt stop (remember, it is not supposed to reach it). The problem is, when converting the gun to closed bolt, you no longer have the firing pin igniting the cartridge before it fully chambers and so the bolt jams the cartridge into the chamber taper all the way till the bolt shoulder bottoms out. If you inspect your fired brass you can see the effect it has. It severely taper crimps the brass. In a normal gun, this would result in excessive chamber pressure and would be avoided at any cost as to avoid blowing up the gun. In a kp31, this is tolerable, as the gun was designed (overbuilt) purposefully to handle higher than normal chamber pressures. In fact, according to "SUOMI-KONEPISTOOLI", published in 1942 by O/Y TIKKAKOSKI AB, an acceptable and safe method to clear a squib from the bore is to shoot it out with another cartridge. Yikes. I have seen reference to several people running a normal SAAMI spec 9mm Parabellum reamer into their barrel to correct the chamber to closed bolt specs, but after much consideration, I don't think this is a good idea. Here's why. The bolt was never designed to slam completely closed with each shot, and the resulting shock of the bolt shoulder crashing into the receiver will eventually have a work hardening effect on it, and will eventually result in the bolt fracturing in half where the bolt head meets the bolt body. It's the same effect as we have seen with the firing pins in improperly dry fired CZ VZ52 pistols (which look remarkably like a tiny KP31 bolt). There MUST be something in front of the tip of the firing pin (or in our case the bolt head) to cushion it at the end of it's travel, and the tapered open bolt chamber design accomplishes this, even if it does pose a risk of overpressure and mangles your brass beyond reuse. If the effect of this cushioning is enough to prevent eventual bolt breakage remains to be seen however, and won't be known till someone has several thousand rounds through a semi-auto closed bolt KP31 to see what happens.

One other problem and side effect of the bolt closing to it's stop is that I have had issues with the safety turning itself back on. I have determined there is nothing wrong with my safety to cause this. The detent for the safety is very strong as it is, and the safety does not move easily, but the shock of the bolt snapping shut is enough to cause the safety to pop on. I have only noticed it with the drum magazines though. My theory on that is that the drum magazine feeds straight into the chamber from the center, and does not slow the bolt speed as much as the offset feeding of the 36 round double stack stick magazine. I don't see an easy fix to this, as installing a stronger safety detent spring does not seem like a viable solution because the safety is already overly stiff.

I plan to add more to this later... I'm out of time right now though. Hopefully this is some benefit or interest to someone and maybe someone else has some tips and tricks to share.

My kit during it's build...

Currently....

Good article my friend, I did a kit last year as well. Lots of polishing to get the trigger a little better and some  minor fit issues. but  the drums are the biggest issue for me I had to re tension them for the gun. No biggie but I have over 12 drums! This kit is a super fun project and fun to shoot.

Any way you could expand on that? Mine shoots 100% with sticks but no go with the 4 drums I own. Good write up BTW.