Basic Parts Inspection Guide... Marushin MP40

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Basic Parts Inspection Guide... Marushin MP40 (ABS)



Field Stripping and Cleaning have already been covered in previous Step by Step Guides so this guide begins with your MP40 already stripped into its component parts.

Re-assembly is explained in a separate Guide.

1. Detonator Pin and Chamber.


The metal Detonator Pin can be prone to corrosion damage and occasionally being knocked out of shape by miss fed cartridges.


Light corrosion can be removed with Wire Wool and / or Emery paper (Wet’n’\Dry)

The Chamber is moulded in ABS and will wear out in time. Below is a photo of a well used and new chamber side by side for comparison.


Check the base of the chamber to ensure the locating hole for the Pin is undamaged.



Check the entrance into the Chamber for damage.



Any rough edges here might interfere with your cartridges feeding into place. Any light nicks and burrs can be carefully smoothed out by a sharp blade or abrasive paper.
Check the inside of the chamber for dirt and deposits. Build up of dirt or gummed up old oil will cause cartridges to stick and jam inside

2. Bolt and Extractor

Bolt condition is vital to the firing performance of your model gun.
Starting at the bolt face, this photo shows a well used and a new example for comparison.




The Bolt strips each round from the magazine as it travels forwards. The two protruding “prongs” for want of a better term ( ! ) seen here directly above the circular recess in the bolt face centre, scoops the round up and pushes it forwards towards the Detonator Chamber.
As this is the first point of contact, any wear or chips to the “prongs” can result in miss feeds and a non firing gun. Notice how the older bolt’s (on the right of the photo) prongs have already been partly flattened through wear.

The round is pushed home into the Detonator Chamber and its’ rim settles in the circular recess in the Bolt Face Centre.
These photos show how the round is held. You can see the three means of securing the round…





i.e. Circular Recess, Extractor Claw and the prongs…
Any wear or chips to the recess edges can result in cartridges dropping off the bolt prematurely. If any of these 3 are damaged the cartridge will more than likely drop off the bolt. Generally this results in rounds firing, but failing to extract and eject cleanly from the chamber and port.


The extractor claw is spring loaded and clips over the cartridge rim to hold it securely.
Check that the Extractor pivots smoothly against spring pressure…



Check the extractor’s claw for dents or chips. Any slight dents can be cleaned up by careful use of a fine file. Aim to get a sharp and even edge to the claw.


Check the groove underneath the Bolt that rests against the Sear when the gun is Cocked. Wear here may result in the bolt releasing too early, i.e. before the trigger is pulled or failure to Cock securely.

3. Bolt Carrier and Cocking handle.

Check the cocking handle’s safety toggle for smooth and secure movement.
Ensure the outer diameter of the Carrier is free from burrs and dirt. This part slides against the inside of the Receiver tube so must be smooth.

4. Trigger and Sear.

The Sear moves down into the Lower Receiver when the trigger is squeezed. Ensure it drops into it’s recess cleanly and that it returns smoothly when the trigger is released. Problems here may lead to the gun firing accidentally and uncontrollably or the bolt refusing to move forward when the trigger is pulled.



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Guide updated...

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