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MP38 Inspired Build
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  vintagemx0

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Posted: December 17 2017 at 6:53pm | IP Logged Quote vintagemx0

Hello forumites,

I did a build a while back that I shared on this form "MP40 inspire build", and have since then decided to give it another go. There were several things about that build that, in retrospect, I was a little unhappy about. The core of this stemmed from the fact that I made the lower receiver from steel; it did not include a polymer foregrip as the original did. This alone made that build a far departure from the original. Although that project was started with the absence of the polymer foregrip, there were other things on that build that left me wanting to do another try having failed to capture the essence of the original that inspired the project in the first place.

So, I decided to do another project that more closely embodied the characteristics of the original design (which I guess I really wanted to achieve.) After reviewing the original MP design, I thought after looking at numerous photos that for attempting duplicate that was much closer to an original that the MP38 would be easier to duplicate as a one-off because for a one-off home build it would be easier to flute the receiver than it would be to press ribs and form a co-centric tube as was done on the MP40.

So, this project will chronical my attempt at duplicating an MP38 as closely as is possible given my limited tooling resources and the challenges I will be faced with in making this a semi-auto, closed bolt device.

To date:

I have purchased a few components and raw materials and have just begun the physical investigation into this build.

I purchased a plastic foregrip. It was advertised as "bakelite", but it is just a modern plastic that looks like it was molded from an original. After seeing prices for original bakelite ones, at $70 for what I received I am happy. I also obtained plastic pistol grip panels - both the foregrip and handle panels are brown.

I also purchased an original front sight block and "korn", both of which have waffenampts on them. I also acquired a barrel protector and barrel nut. The barrel nut is of the later two-sided variety. Because I should be using a six-sided barrel nut, and the fact that my receiver tube is of a slightly decreased diameter of the original, this nut will need to be turned-down slightly and I will grind more flats to make it six-sided. There are no waffenampts on this piece, so I don't feel so bad modifying it.

I started by making the rear tube end-cap I am currently working out the lower receiver sheet metal. In the coming days, I hope to photograph the pieces I have and the pieces I have made as I organize my approach to the project. Ultimately, I plan to SBR this one (if it turns-out worthy enough).

-Ken
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  Paraquat

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Posted: December 18 2017 at 6:25am | IP Logged Quote Paraquat

Good luck. I enjoyed reading about your last build.
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  vintagemx0

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Posted: December 18 2017 at 8:37pm | IP Logged Quote vintagemx0

I began by drilling and making slots to receive material that will serve as the tube-locking lugs inside the end cap.



I cut some small tabs to go into the slots. I turned-down a section of pipe that fit tightly inside the tubing that was to be the end cap to a depth that would set the depth for the tabs that will be the locking lugs.



The tabs were plug welded and the outer tube was cleaned-up after welding on a plug to the end cap.



I pressed the needed radii for the tube tray and lower receiver box and created some crude blanks.



In this last photo, the pieces are cut to approximate size.



The next steps will be figuring-out how to temporarily hold this together while other details are being worked out, such as fitting to the foregrip, the FCG, and the final riveting that will bind the assembly.

-Ken
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  backbencher

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Posted: December 18 2017 at 10:22pm | IP Logged Quote backbencher

Which FCG will you use?
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  vintagemx0

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Posted: December 19 2017 at 8:02pm | IP Logged Quote vintagemx0

Good question on the FCG. Real estate is a limiting factor within the lower. The interior of the lower receiver box will be (planned) .531 wide by 1.075 deep. I can't deviate larger because the box will no longer fit inside the foregrip. I have only looked at AR and AK types so far.

AR - It looks like there won't be enough room for the hammer to arc back far enough to engage the disconnector while allowing the bolt to travel further rearward clearing the hammer.

AK - Too wide to fit in there at all.

Both are too wide, actually, but the inside of the foregrip happens to be a little wider in the region where the FCG would go, so I could widen the box out to .688 in this region and get it to fit nicely in for foregrip. This would allow the trigger assembly for the AK to fit, but not the hammer or its spring. The AR hammer would fit though, and I have actually considered grafting the top of an AK hammer onto the bottom of an AR one. I can't see why this wouldn't work, unless it was prone to breakage. The bottom of the AK hammer just looks too bulky to work it down to fit and get a spring in there too.

I haven't looked at other options yet - primarily because I already have the AR and AK pieces.

-Ken
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  bikergunnut

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Posted: December 19 2017 at 8:55pm | IP Logged Quote bikergunnut

If you're up to it you could design a curved flat spring to fit in behind a hammer. That might thin up the whole thing.
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  backbencher

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Posted: December 20 2017 at 12:20am | IP Logged Quote backbencher

870 & 500 FCG may work as well.  I've got a 500 FCG on my bench, holler if you want some measurements.
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Posted: December 20 2017 at 8:11am | IP Logged Quote bikergunnut

Or sks or m1 carbine. I have those. Can't measure till Monday.
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  bikergunnut

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Posted: December 25 2017 at 9:28pm | IP Logged Quote bikergunnut

M1 carbine and vz52 rifle fcg's won't fit. My sks fcg is already cut apart for something but it looks too big.
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  Zuzzy

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Posted: December 26 2017 at 11:16am | IP Logged Quote Zuzzy

Great idea on making the tube locking lugs !
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  vintagemx0

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Posted: December 28 2017 at 7:43pm | IP Logged Quote vintagemx0

Thank you for looking into those different FCG options guys. I thought before I went too crazy on deciding on a route for the FCG, I'd better try to complete the lower receiver box. I made it from 2mm (0.080 in) 4130 sheet. I've never worked with any other sheet than mild CRS, so this became a learning experience from the go for me. This stuff is incredibly hard to bend/form and is rather "springy".

In the photo below are the parts I have to date for the lower receiver. From the top down we have the RHS of the box, the top (tray), the LHS of the box, the bottom of the box, the retainer/plunger assembly, the tube end cap, and the polymer foregrip.




I had a hard time telling from photos how the original tube retainer/plunger body was secured, so I decided to take a piece of .625 bar stock and face-off two sides so it would fit inside the box. This would secure it from turning. Shoulders are turned near the top and bottom ends to secure it up-and-down. Here's the assembly...



...And the next two photos show it in the box and how it will (eventually) be secured on the bottom. Note: The sides are secured temporarily with screws & nuts. When I am ready to close this box up I will peen-in some rivets.





A few photos of the temporary assembly...








For the lugs on the tube, I performed some etching experiments on short pieces of 4130 tubing using electrolysis wondering if it was possible to get the depth I needed. (It actually worked). I used electrical tape as a mask and the solution I used was 50/50 solution of saturated salt water and white distilled vinegar.. I'll post more about this when I begin the actual tube as I am sure some of you may find this a plausible method in other applications.



-Ken
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  backbencher

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Posted: December 28 2017 at 10:42pm | IP Logged Quote backbencher

Wow, those look like nice lugs.
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  Paraquat

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Posted: December 29 2017 at 7:08am | IP Logged Quote Paraquat

I am definitely interested in your etching process.
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  vintagemx0

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Posted: January 02 2018 at 8:27pm | IP Logged Quote vintagemx0

Given the tight quarters I had to work with, I decided to widen a small portion of the receiver box in an area that would not interfere with the inside of foregrip. I simply welded some small 2mm panels in.



This made the width of this area perfect for an AR hammer, hammer spring, and retaining pin. As I mentioned before, this box is simply not tall enough to accommodate an AR FCG, but an AK trigger assembly fits nicely in the narrow region after sanding down the sides. So, I cut my AR and AK hammers in half and grafted a bastard hammer that is AK on top and AR on the bottom.



I welded a small ledge inside the channel that was to serve as a rest for the hammer spring and as a forward stop for the trigger assembly. I accidentally cut my hammer spring too short, but I realized that this ledge was not the best idea regardless as it is a bitch to preload the spring on it while trying to install the hammer. I have since decided to cut some small channels in the side of the receiver sides for the spring to recess into. I'll do that after I order and receive a new spring.



So, it is essentially AK FCG, but I did need to grind the catching surfaces on the hammer to clear the hole in the receiver tray. There is still good engagement, no play in the axis of the hammer, and I love the simplicity of the AR style retaining pin. So far, this looks very promising.









Next, I'll be welding on rearward stops for the sear position and then begin working on the trigger/transfer bar. I've just started looking at a safety. There are definately options, but I don't want it to be to cosmetically intrusive.

Oh, and a bit of good news. I had thought the foregrip I purchased was bakelite but plastic... Well, I needed to shave-off a little bit of two of the reinforcement ribs on the inside for clearance and it is most definitely bakelite. I was happy to see that knowing that it will age well.

-Ken


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Posted: January 10 2018 at 8:56pm | IP Logged Quote vintagemx0

I got around to getting a new hammer spring. I cut slots into the side of the box deep enough for the spring to recess completely (and then some). Seems to work fine thus far.



I removed the trigger tang and welded a small rod to the back of the disconnector/sear assembly to provide linkage to a bar. I filed a snug fitting notch into a piece of 3/8 square bar to engage the pin and will serve as a transfer lever from the soon to be added trigger.

   


You can see where I'm going with this in the photo below. With the disconnector/sear assembly installed, it serves to ensure the hammer spring stays in the grooves.



-Ken
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Posted: January 10 2018 at 9:55pm | IP Logged Quote backbencher

No insult to your workmanship intended, but won't that end up as a fairly heavy trigger?
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  vintagemx0

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Posted: January 10 2018 at 10:25pm | IP Logged Quote vintagemx0

None perceived! I appreciate your input very much. Yes, I think you are quite right and I was premature locating that pivot point for the link-bar. Not much mechanical advantage where it is now and a very limited arc for trigger movement to boot. I will be moving the fulcrum much closer to the sear after I play with paper mock-ups of a trigger to find the right geometry. I'm not all that concerned about drag from hammer springs as they recess deeply into the grooves. Just another couple of holes to fill and grind...

-Ken
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Posted: February 07 2018 at 10:57pm | IP Logged Quote vintagemx0

I have done very little lately, but I have been whittling on it here and there and I think I have a plan for the trigger and trigger return spring. I had an extra AK trigger, so I added two 1/8" steel dowels - one to rock the transfer bar and one to be pressed by the return spring. The return spring rests against a tab (that the return spring rod goes through) that is welded to the top of the transfer bar immediately above the fulcrum of the transfer bar.

Here's the trigger (the tang will be cut-off and a much longer one welded on), the transfer bar (this will be re-made as this one is knarley-looking), and the return spring/plunger...



Here's how they orientate together ('couldn't fully engage the ends due to spring tension)...



Here's a mock-up of where they reside inside the receiver. Again, without cross-pins anchored, the spring tension just shoves the parts apart.



Same view as above, but with trigger pulled...



Here's a view from the bottom of the receiver. There will be a plate welded on covering this bottom and all parts will be serviceable from the top side...



Here's a view from the top side...



'Looks like this will work. I am going to redo the transfer bar, but what you see here is fully functional. Trigger pull with this short tang is just a little stiff (guessing 4-5 lb), but with a longer tang and polishing of contact points, I think it will run well. That remains to be seen, at the pace I am going, that may be a while.
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Posted: February 08 2018 at 6:57am | IP Logged Quote backbencher

Looks very nice.  I know nothing of trigger systems, but looks good.
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  Paraquat

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Posted: February 08 2018 at 7:38am | IP Logged Quote Paraquat

I like your idea of a plunger to engage the disco.
Very creative.
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