Semi-Auto Trigger For Open Bolt Gun

If it is about the MG42, it goes here
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joe6167
Posts: 9
Joined: Fri Nov 15, 2019 5:40 pm

Semi-Auto Trigger For Open Bolt Gun

Post by joe6167 » Sat Nov 23, 2019 1:34 pm

In Poland, it is no problem at all to own Semi-auto open bolt conversions of various automatic/select-fire guns, and as a result, a whole pile of M53s were imported in the country and sold like hot cakes.

Since this is a unicorn topic (I'm sure as I type this, an ATF agent is having some kind of catatonic fit, without even knowing why), I would like to present a description of this trigger:
trigger group parts.jpg
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In the original MG-42 trigger, when you pull back on the trigger (which pivots on hole #1) a pin in hole #2 pushes up on the forward section of the sear (B), causing the back section of the sear, which catches the bolt, to drop, allowing the bolt to move forward.

In the semi-auto trigger, the pin in hole #2 is removed, and instead replaced with part D, which now acts on the sear. As Part D pivots on Hole 1 and thus the same axis as the trigger, the original MG-42 disconnector is removed and replaced with part C, which is sprung. In situ, Part C is actually rotated 90 degrees from the position shown, and actually rests under hole 1, where it presses up against part D. An additional piece of metal has also been welded onto the trigger, but I'm not entirely sure what that does. It contacts the sear and at first glance appears to stop the trigger from moving too far forward. You can also see where a bit of material was ground off on the sear to make room for Part E.
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Part C locks against Part D and thus the two move together in order to push up on the sear. However after a certain point Part D (I think) contacts the large angled surface on Part E causing the lock between Part C and Part D to break. Part D then returns to its starting position where it can no longer act on the sear, and the sear, under the pressure of mainspring pressing up on the rear, is now free to return to its starting position.

https://youtu.be/msxCKX-5ET8 (demonstration of the trigger)

Part E functions as a trip when the bolt recoils. Partially depressing the trigger causes Part E to raise into the path of the bolt lugs, and when the rear bolt lug contacts Part E, the lug forces it down, where that angled section pushes on Part D, breaking the lock between Part C and Part D, and thus resetting the sear and further helping to prevent full-auto fire. As you can see in the photos a large chamfer has been ground onto the rear lug on the bolt as part of the semi-auto conversion, presumably to facilitate engaging this trip mechanism.
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IMG_2956.jpg (50.29 KiB) Viewed 81 times

joe6167
Posts: 9
Joined: Fri Nov 15, 2019 5:40 pm

Re: Semi-Auto Trigger For Open Bolt Gun

Post by joe6167 » Sat Nov 23, 2019 1:37 pm

As I discovered when I initially went to reassemble the trigger group, the layout shown in an earlier photo I had taken, where I tried to layout the parts, is not quite correct.

Part C should be rotated 90 degrees counter-clockwise, under Hole 1, while the short end of the main spring has to be rotated counter-clockwise 180 degrees, to rest under the tail of the sear.

In the original MG42 trigger group this end of sear spring actually sits inside the trigger, so I was thrown off at first when I was trying to put it back together. But eventually I figured it out!

As well, when reassembling this, you have to hold Part C down, against that spring pressure, in order to install Part D, the trigger, and the corresponding pin back into the pistol grip! Paracord came in handy for that as well.
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The long tail on the Sear (3) is what engages the crossbolt safety, which is also the one part on the gun that the Yugoslavs seem to have modified from the original German design. They added a small detent block under the actual safety button, thus necessitating the addition of the square cutout on the grips. So MG-42 grips won't fit on an M53.

Y = Safe
O = Fire

I should also note that it appears that this modification was not carried over to the MG3.
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IMG_2603.jpg
IMG_2603.jpg (137.77 KiB) Viewed 79 times

joe6167
Posts: 9
Joined: Fri Nov 15, 2019 5:40 pm

Re: Semi-Auto Trigger For Open Bolt Gun

Post by joe6167 » Sun Nov 24, 2019 8:59 am

I should also add that these new parts look like they were cobbled together from random junk lying around the shop. Let's just say that I have my doubts as to the quality of the steel, and whether or not any kind of heat treating was done. In short, I have no idea how durable these parts may be.

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