marushin mp40

my receiver falls apart when firing. It locks back into place but then falls apart again. any ideas? no cracks found.

Romagnus wrote:

It locks back into place but then falls apart again. any ideas?

"Falls apart" = upper separates from lower?
Does upper correctly lock back?
There are (at least) three pieces that should keep it locked: end of receiver (goes inside end cap), sear (goes into hole in the upper) and locking bolt (goes into the hole in upper). I found that sometimes locking bolt seems to be locked, but only falls fully into place after a bit of twisting of the upper receiver.

Hey Romagnus, it's great to hear from you, how's the military treating you???

Take a look at this: https://mp40modelguns.forumotion.net/modelgun-problems-f5/marushin-mp40-falling-apart-t1051.htm

Smootik's covered the main points, check the take down pin isn't protruding into the upper reciever tube too far. If it is, the bolt can hit it, knocking it out of place. This can seperate both reciever halves during firing.
Put the bolt in Safety, peer in through the slot in the reciever and you'll see the circular pin. It should be just flush with the tube.
If it sticks up just a little, it will be struck by the bolt.

Reciever locking lugs are next on the check list. These do wear, as did Claymore's. Fortunately, new reciever sets are available from Francky.

Hope all that helps my friend

yep, it will be the locking lugs on the rear of the upper and lower receiver they wear out you need to change both upper and lower receiver to cure the problem i am afraid changing just one wont work. however do what cerwyn said first before you buy new parts as its worth checking.

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Thanks everyone, I will be checking these things out and keep you update.