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FNB spring housing option in MP38/40


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#1 Black River Militaria CII

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Posted 09 March 2019 - 02:45 PM

Substituting the FNB bolt and spring hiusing is an accepted practice to preserve the orignal parts in the MP38/40 SMGs. In my experience some guns will run with the unaltered housing but many wont even with SMG ammo. The FNB recoil spring is a bit stiffer than the standard German SMG spring and the buffer spring is also stiffer. If a gun short recoils and runs away, there are a couple options to use the FNB housing and recoil spring. Replace the FNB springs with Wolff MP38/40 springs, which works well usually but again, in some guns those springs still a bit stiff. A second option is to remove the FNB buffer spring and plunger and use only the main recoil spring. The travel of the bolt is sufficiently buffered by the FNB recoil spring so the housing doesnt telescope completely and it wont bottom out against the end of the buffer tube. Below is a description of what is inside the FNB shring housing and how to disassemble and reassemble it for either approach to using the housing.

The FNB bolt has a fixed firing pin so the front end of the recoil spring housing is different than the standard MP40 which has the fring pin in a boss on the threaded housing cap. The firing pin is not numbered, but the, firing pin boss on the cap is numbered to the gun.
The FNB cap is flat with two holes for a pin wrench. The holes are about .130 +/-. It is best to make up a couple short lengths of rod, 1/2 or so so the fit is tight in the holes when using them to unscrew the cap. My practkce is to mount the housing in a lathe chuck, lock it, and use an open ended wrench across the pins to unscrew the cap. The cap is staked so start very slowly until the stake is broken. Retighten it and then unscrew further, retighten a bit and unscrew further and ease the cap out slowly refersiing until the rotation feels unrestrained. The metal is soft and will strip easily.
The cap has the buffer tube attached to it which is pinned into the back of the cap. Drift out the pin. The tube is a tight fit but is not threaded so does not unscrew. Anchor the tube and rotate the cap slightly back and forth to eas it out. There is a small centering lug in the inside of the cap that fits into the rear end of the tube and the crosspin goes through that lug also.
With the buffer tube out, remove the spring, and the piston and the keep track of the small lug to use in the reassembly of the tube in the cap. If no buffer spring is used in the tube, the empty tube should be the reassembled to the cap. The tube acts as a spring guide so still has a function.
Reassembly of the tube to the cap requires the small lug to be located in the reqr end of the tube so the pin can go through it. A small dab of grease on the lug can be used to temporarily keep it correctly aligned as the tube is pushed back into the cap.
My practice for reassembly of the cap to the tube is to tighten the pins in the cap in a vise with the threads on top and lower the housing with the spring on the tube onto the threads so I can make sure the housing is correctly aligned to thread properly onto the cap threads.
Remember to clean the cap threads and the inside threads of the housing. Once the cap is tight, restake it at the joint of cap and housing. Lube the spring lightly through the air holes in the side of the housing.

The sliding safety is identical to the German type and unless the ball dentent spring is broken or missing the handle will stay pulled out during firing and will not abrade the side of the receiver. If when cocking the bolt, the handle is grasped so that no pressure is exerted to move it sideways towards the lock position, it will never contact the receiver.
All I can think of.
Comments and suggestions welcomed.

Edited by Black River Militaria CII, 09 March 2019 - 03:35 PM.

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#2 timkel

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Posted 09 March 2019 - 02:53 PM

I learned something new today. Thanks


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#3 Petroleum 1

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Posted 09 March 2019 - 09:52 PM

Very cool thanks Bob!!!


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#4 Chopper28

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Posted 10 March 2019 - 11:08 AM

I bought a FNB bolt assembly from IMA years ago. Put it in my Wilson MP40  tube gun just to see what would happen. Gun never missed a beat and ran perfectly. I just keep the FNB bolt as a back up. FWIW


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#5 Haenelistklasse

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Posted 10 March 2019 - 12:26 PM

That is funny you should say that because I also bought an FNB bolt years ago and tried it on my Erb tube gun. It seemed that the bolt was too loose if memory serves and it would not function at all. Do others have problems like this? Just very curious...


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#6 Uncle Dudley

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Posted 10 March 2019 - 01:52 PM

http://www.machinegu...showtopic=20377 I thought it was the FBP submachinegun.

https://www.ima-usa....interchangeable

Edited by Uncle Dudley, 10 March 2019 - 01:53 PM.

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#7 Black River Militaria CII

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Posted 10 March 2019 - 03:27 PM

Dudley is, of course, correct and sorry for the error in writing "FNB" instead of "FBP'. The post above was an impromptu endeavor off the top of my head from some comments on a site about MP40 firing pins being numbered to the guns and then turned into an FBP post. Been meaning to write about the FBP parts for a long time and never did. Earlier in the week I had been posting to a customer about the FNB43 submachine guns and that stuck for some reason. The use of FBP conversion parts is so common that at least people knew what I was addressing! 


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#8 Uncle Dudley

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Posted 11 March 2019 - 11:45 AM

I'll have to look at my original firing pin, but I don't think the pin itself is numbered, but the assembly is numbered to the gun.  I bought a FBP assembly for my MP40 to save the original and it worked fine.  I then found another complete MP40 bolt and recoil assembly, which I have been using.  I also have a spare firing pin.  The pin is pinned and peened to the recoil assembly.

 

U D


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