A couple years ago I purchased a “GSG MP-40 p” with the thought of someday I’ll bite the bullet and SBR it. As it became more and more apparent the ATF was getting increasingly “political”, I decided to keep away from this idea. A friend of mine with similar views about his AR pistol, decided to add a barrel extension (pinned and welded) and faux suppressor to convert his AR pistol into an AR carbine.
A quick “GOOGLE” search for “gsg mp40 barrel extension” brought me to MFI. Not only did they have the barrel extension I would need, but also a complete kit including slide on faux suppressor. They also have a replacement WWII period correct front sight hood.

GSG 9mm MP40 - MFI Barrel Shroud / Fake ...ant (mfiap.com) GSG 9mm MP40 - MFI Barrel Shroud / Fake Silencer / Faux Suppressor & Barrel Extension SET! Price includes S&H for USPS Priority Mail & Insurance. / 922r Compliant (mfiap.com)

MFI Replacement WWII Period Correct Fron...nce (mfiap.com) MFI Replacement WWII Period Correct Front Sight Hood for the GSG / ATI MP-40 / MP40 / MP38 / Price Includes S&H via 1st Class USPS + Insurance (mfiap.com)


Original front sight hood & muzzle cap (circled), Replacement sight hood & barrel extension (arrow).

* * * NOTE * * * : If you decide to follow this path and have anything PERMANENTLY attached to the muzzle, make sure you have the correct front sight post installed in the hood before attachment.


Replacement front sight hood installed.

The replacement front sight hood was a perfect fit and adds to a more realistic look. The barrel extension screwed on nicely and made it easy for my gunsmith to pin & weld it in place. The faux suppressor slides over the extension and is held in place by 4 setscrews.


Welded barrel extension (circled).


Faux suppressor (setscrews arrow).

Having the barrel extension welded & pinned, the last step is installing the “ATI GSG MP40 folding stock kit”. I will update this post with a range report after I get the stock installed, and get to the range.
If anyone is thinking of SBRing your GSG MP40, I recommend going this route instead. I would much rather give my money to a company like MFI, for their kit, than give the ATF anything.

UPDATE – ATI stock kit install









The installation of the ATI stock kit was not as difficult as expected. It should be noted that some prerequisites should be taken to make this process easier.
It is easier to perform this operation while the MP40 is separated into upper and lower parts. All work will be done on lower assembly, set the upper aside and BECAREFUL not to misplace the “E-clip” on the take down pin.
The assembly instructions show and reference ALL the parts individually. My kit came with most ALL parts pre-assembled. Although it took extra time to un-assemble them, it allowed me to discover some problems BEFORE they could become problems. I will annotate the issues I found as I go over the instructions.

STEPS 1 – 3
Whether your kit was assembled or not, some items NEED to be preassembled before you start. The “stock springs and stock detents”, used in (step 8), can be lubed and placed in the “pivot arms”. Wrap some masking tape around the ends to hold them ready for (step 8) use.
When grinding away the “welded spots” on the plug in (step 2), NOTE : the right side is MUCH THINNER than the left. If you just concentrate on right side, you will save a lot of time and work.


Notice how much thicker the “LEFT” side is than the “RIGHT” side

This is where having my kit preassembled provided much needed insight to what was instore. The “pivot lug spring” and “locking lug” (step 4 & 5) were seized onto the “right pivot post”. Now is a good time to brush, clean, and lube the “right pivot post” and “locking lug” making sure they slide together without binding.


STEPS 4 – 6

The first obstacle to avoid is making sure the tabs on the “locking lug” pass through the keyed opening (step 5). Mine required a touch or two from a Dremel sanding wheel before they would cleanly slide through.
Make sure the “locking lug” also slides cleanly through the hole in the “left pivot arm”. Before continuing with (step 6), check the fit of the “retaining pin” (step 7) and the “left pivot arm”. The hole in my pivot arm was MUCH SMALLER than the retaining pin. I could place a roll punch that would almost pass through the pin but would NOT come close to going int the pivot arm hole. I drilled out the hole to the proper size then started the “retaining pin” and continued to step 7.


STEPS 7 – 9

The last three steps were uneventful. After assembling everything was VERY TIGHT. Sitting down watching TV and continuously working the stock had it working as expected.

Nice documentation and great comments supporting!