MP40 - Problem with Receiver Rotation

[heavy artillery]

I have an issue that I wanted to share for your feedback/suggestions:

 

While firing my MP40, I have begun to experience an issue while holding the magwell during firing. The magwell seems to cant to the left just slightly (obviously the receiver along with it) as if it's starting to disassemble. This can create an alignment problem where the sear doesn't catch the bolt (runaway). This issue doesn't occur if I put my support hand on the bakelite area (not turning the upper).

 

It appears that my hand pressure on the magwell turns the upper at the point that the bolt is fully closing. It appears the momentum allows the upper to jump just slightly out of the lower and rotate away from the disassembly catch. I can induce this without live rounds by retracting the bolt then pulling the trigger while holding the magwell. At the point that the bolt closes, the magwell rotates in my hand.

 

The disassembly catch looks fine and the receiver doesn't look like it's bent in the cutout area for holding the catch. I do see a scratch away from the hole which supports the theory that the upper is twisting away from the lower. While this area doesn't look deformed, it's logical that the area around this scratch has been absorbing any stress/damage from this issue (by the takedown catch jumping out of the normal position and creating a groove). Verifying and adjusting this area to the appropriate dimensions is my current area of focus.

 

Thoughts? Has anyone else experienced this issue? What was the resolution?

 

Thanks for your feedback.

Edited February 8, 2021 by heavy artillery

[huggytree]

Mine had that issue. I think its common if the guns been used heavily. I had the catch hole reworked. Also had to reform the body slightly to tighten things up

 

Your gonna need a gun smith.

 

I had runaways when it rotated

 

My gun is now rock solid tight.

[TSMGguy]

Right, sounds like the receiver is rotating on the frame. For whatever reason the take down pin is not engaging its slot in the receiver, or the slot is too big, allowing too much movement. In an extreme case the sear might no longer be lining up with the sear slot on the receiver, resulting in runaways. It's talented gunsmith time.

 

A little side to side play between the receiver and frame is normal with the gun assembled. This makes it easy to find the sweet spot for locking the take down pin into the receiver.

 

There should be some interference drag between the flange on the front of the frame and the corresponding slot in the mag well. This interface should not be noticeably loose with the gun assembled.

[MG08]

Mine had that issue. I think its common if the guns been used heavily. I had the catch hole reworked. Also had to reform the body slightly to tighten things up

 

Your gonna need a gun smith.

 

I had runaways when it rotated

 

My gun is now rock solid tight.

 

Yup - this right here - I have a nice one saw cut post sample that does this on occasion . On my Post sample gun, I just have to be careful to align the take down pin "correctly" with the receiver tube and ensure I get full engagement of the rectangular block into the corresponding hole in the receiver tube. If your receiver hole is wallowed out a little or peened in some to allow the block to ride out of the hole you will need to rework the receiver tube - or just see if careful alignment and not twisting to hard on the magazine will solve it for you.