A while ago, in response to a thread here, I made a replacement muzzle nut for the AGM MP40.
Before I started work, I did some research to determine if a real-steel muzzle nut would fit (it won't BTW).
During that research, I came across this image, and was intrigued by the little piece of wire poking out the front of the slot in the foresight block.
I quickly realised it was there to stop the nut unscrewing.
This retainer spring works by gripping a knurl on the nut, resisting any tendency to rotate.
Having worked out what it is, and how it works, I decided it'd be a good idea to fit one to my MP40.
As it happens, AGM copied the design of the foresight block so closely, that it's possible to do this mod very easily, with minimal tools - the necessary slot and pin are part of the AGM design, so there's no need cut anything on the gun. 
Rather than reinventing the wheel and designing a spring myself, I had another rummage and found an image which contained this:
The short "tail" pushes on the top of the barrel, the "loop" provides a fulcrum and some extra springyness, and the long tail sits in the nut knurl. It's flat, so a 1:1 drawing or photo can be used to easily form it.
I printed that image so it was 1:1 scale, using the nut in the image as a scale reference. That was 95% scaling in Paint Shop Pro for my setup.
Using some 0.85mm (21SWG/20AWG) steel piano wire (also called music wire), and the scaled photo as a guide, I made a spring.
The exact wire diameter isn't too critical, but it must be able to fit in the groove, and around the pin. I know 0.85mm wire fits in my AGM gun.
The length of the short tail shouldn't be changed, as there's only so much room in the slot for it, and if you make it too long, you won't be able to get the foresight assembly back onto the barrel.
This is what I made:
To fit it, I used a punch to drift the forsight pin half-way out - just enough to get the "loop" of the wire over the pin, with the long, bent-up end towards the muzzle.
When the pin is refitted, the muzzle end of the spring is pushed down a bit to allow the barrel to pass through the assembly.
The nut is then screwed on while holding the end of the spring up.
When the nut is done up, the spring is released to sit in the end of one of the knurled grooves in the nut.
Having fitted the spring, my MP40 now looks like this:
Doing this mod was a 30-minute job the first time, and making a second spring took a fraction of the time of the first.
Tools used:
2 pairs of pliers to hold/bend the wire.
Screwdriver as a mandrel for forming the "loop".
Heavy-duty cutters for the wire.
Pin punch
Hammer
As you can see, no special tools, so just about anyone could do this.
Hopefully I'll not lose the nut now. In any case, it'll be slower to get loose.
Thanks for posting that. Might put one on my gun if I can find any suitable wire.
Piano wire is quite hard and slightly springy.
Any proper model shop should have it, usually in 1 metre or 1 yard lengths. There'd be enough for 30+ of these springs out of a piece that long.
Places like Hobbycraft might have it too.
If I make a replacement nut for someone, I'll include a spring. But for just the springs, they'd cost more to post than to make.
Nice
Incidently, the TOP MP40 comes with the nut retainer fitted and as AGM copied the TOP, that's why it's ready to accept the wire! It's a shame AGM couldn't be bothered with this detail in the first place!
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I just checked and my SRC MP40 comes with that spring but it seems to be too thin and in effective as I had to tighten the nut back on!
Can you measure the wire, ZB? It might not be stiff enough, or may need to be reshaped.
The 0.85mm piano wire I used is just right. I copied the shape of the original in the image as closely as I could, and it does the job, but is not too stiff.
The main thing is that it'll stop the nut unscrewing quickly if it gets loose. With a retainer in place, you should hopefully notice the loose front end before the nut comes off.
Tim
nice post , thanks
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I've got no way of measuring the wire and as I'm a bit ham fisted, I'm tempted to just put a dab of threadlock on there instead.