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MP40 Inspired Build
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  UKBiker

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Posted: April 03 2015 at 6:05am | IP Logged Quote UKBiker

vintagemx0 wrote:
That's sad that activity has declined. Perhaps with the economy changing boys will start making toys again..

As a newb with no mill, I know that the general over-all quality of this build will be lacking, so I think it is important to pay attention to the little details in order to make it more interesting-things like ribs and lines... It will then be at least (hopefully) interesting and a good facsimile of what inspired it to begin with. Not to mention, it's just plain fun to build and think about how you're going to go about the next step. I guess everyone here already understands that well.

I ordered a chambered barrel blank. I do have a HF mini lathe, and I plan to turn it down, along with a trunnion. The magwell fits very snugly on the tube, so I think I will just cross-drill and pin. If that is inadequate, I could always plug weld it.

Thanks,
-Ken





I think you do yourself a huge disservice on that, the quality of this build so far is magnificent, working with the limited equipment you have at your disposal you are still managing to produce excellent quality parts, you have a real skill there and I for one am watching this build with quiet excitement I think you're doing a fantastic job, and your method of ribbing the sheet sections is brilliant simple yet extremely effective. I am really looking forwards to watching this build progress, thankyou for posting your work up for us to see.
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  Zuzzy

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Posted: April 03 2015 at 11:03am | IP Logged Quote Zuzzy

vintagemx0 wrote:

As a newb with no mill, I know that the general over-all quality of this build will be lacking, so I think it is important to pay attention to the little details in order to make it more interesting-things like ribs and lines... It will then be at least (hopefully) interesting and a good facsimile of what inspired it to begin with. Not to mention, it's just plain fun to build and think about how you're going to go about the next step. I guess everyone here already understands that well.



Thats the best part, as long as you dont make some great mistake in design or manufacture you cannot repair later. Lots of ideas comes randomly and with time; for example you see something and contemplate how to adjust it for your build.

Lacking of tools and machines combined with innovative approach in making parts augments the value of any build, and shows the skill and intelligence of the builder.

To me, builds that are completely made with expensive machines, on near industrial level, have absolutely no diy value or merit.
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  vintagemx0

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Posted: April 05 2015 at 10:41am | IP Logged Quote vintagemx0

Thanks for the kind words. You can do a lot of things without a mill, but it can get a little messy. Case in point...



I'm making the receiver in two halves, and then will join them together (weld and grind). There's some pretty blobby looking welds inside, but they won't be seen even when field stripped. The two holes near the magwell are for the FCG. Since this receiver body is only 16GA mild CRS, I'm allowing enough width to laminate some hardened steel over the holes to reinforce the pins. I'm going to use some small pecees cut from an old circular saw blade.

Next up will be piecing the two haves together and and completing by adding the tube end-cap and facilitating installation of the grip.

-Ken
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  Zuzzy

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Posted: April 06 2015 at 7:59am | IP Logged Quote Zuzzy

"I'm making the receiver in two halves, and then will join them together (weld and grind). There's some pretty blobby looking welds inside, but they won't be seen even when field stripped"

Thats exactly hows yugo m56 smg lower receiver was made, including the weld bead still very visible on the inside;

https://weaponeer.net/forum/forum_posts.asp?TID=22555&ge t=last
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  backbencher

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Posted: April 06 2015 at 11:53am | IP Logged Quote backbencher

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  vintagemx0

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Posted: April 09 2015 at 12:53am | IP Logged Quote vintagemx0

That M56 is pretty cool. I studies that autopsy pretty long last night - Thanks guys. I didn't fully understand how the retaining pin worked though.

I have don a little work on my build. Actually, a lot because I had to stop and think some things through. With the FCG and trigger bar, it makes it difficult to emulate the MP assembly. I was going to go with a twist-lock like the original, but a good way to latch it from rotating is elusive. I decided not to twist-lock, but use a tang that gets pinned with a 3/8 pin running side-to-side through the receiver just above and behind the trigger.

Here's some pics...









It's just a shell with a lot of love needed in all parts. 'Pretty much just going for the over-all "high altitude" fitting of peices and thinking about the details as I go. I think I have a good plan for retaining the tube and the FCG, which will be the next two little projects I will perform. That and starting a little grinding/sanding/polishing of the parts. I hope to receive my barrel any day and I can begin starting to think a bout trunnion.

-Ken
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  Sixtysixdeuce

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Posted: April 09 2015 at 12:57pm | IP Logged Quote Sixtysixdeuce

Lookin' good!

Fabbing guns without a mill is doable, but tricky.  I did a few simple designs before I had my mill, but never attempted an autoloader.  Goes to show that where there's a will, there's a way.  You just have to get creative when your tooling is limited! 


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  SOP9

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Posted: April 09 2015 at 11:14pm | IP Logged Quote SOP9

Welcome to Weaponeer.
Interesting build. Im a fan of the MP40 along with other WWII guns.
That build of yours is awesome. Thanks for sharing.


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  vintagemx0

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Posted: April 13 2015 at 8:07pm | IP Logged Quote vintagemx0

I have a question for the experienced builders. The diameter of my barrel blank is 0.795 inches. I want to secure it to the trunnion in the same fashion as the original, that is with a barrel nut. In order to do so, I would need to weld the two flanges onto the barrel in order to employ the half-rings that the barrel nut would bear against.

Is it inadvisable to weld to the barrel? Am I asking for deformation problems, or is a 0.795 inch diameter (9mm bore) sufficient to resist such problems?

I hope to have my trigger and FCG sorted out soon and have the lower receiver completed soon.

Thank in advance..
-Ken.

edit.....OK, I've had a look around the internet on the subject and have decided that welding on the barrel = bad idea. I think I'll just press fit and cross pin. I will employ a barrel nut for looks though....
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  blurrededge

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Posted: April 13 2015 at 8:51pm | IP Logged Quote blurrededge

You can weld on the barrel, but I would use heat sinks (aluminum blocks) on either side of the area, and stress relieve it immediately afterward (400*F in a preheated oven for 1 hour if I remember correctly).

However, I would agree that you're much safer to press and pin it, and add a barrel nut for looks.

With that said, the shell looks amazing thus far! Looking forward to see how the rest turns out.

P.S. Take LOTS of pics (including during the fabrication process)
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  vintagemx0

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Posted: April 13 2015 at 9:06pm | IP Logged Quote vintagemx0

Thanks for the input blurred edge. Yeah, I really didn't give that whole topic very much thought. Press & pin for me.

Also, after looking around some other builds on here it did occur to me that I wasn't sharing many pics. I haven't closed-up the receiver yet while still sorting out some details, so when I do, I'll be sure to take plenty of good pics of the elements and recount what's been done. You helped remind me of the detail I enjoy reading and looking at when I'm looking at others builds.

-Ken
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  vintagemx0

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Posted: April 18 2015 at 12:05am | IP Logged Quote vintagemx0

So, let me start by just say'n - Weld and grind method with sheet metal is a bitch. You are basically "sculpting" with sheet steel, weld filler, and a ton of grinding. My lower receiver weighs about 3 pounds now, and I have about 5 pounds of grinding dust on the floor. Perhaps I'm exaggerating (a little), but it is definitely not an "easy" path.

Having said that, it is fun as long as you can take it slow - which I have been. I have developed the trigger/FCG to a working condition.



In the photo above, you can see that I added a fulcrum to the pistol grip for a lever bar that pivots up and down. When the trigger is pulled, the front end of the lever lifts up to rock the trigger group of an AK FCG. It seems to work very well, but the trigger pull is very heavy. I have a pull-force scale at work and will measure and report back. I cannot even estimate, but it is heavy.



Above is a photo of the entire assembly inside the lower. The tray for the receiver tube is not in place to show all the inner workings.



In the photo above, I am showing how the pistol grip assembly is installed. It simply slips in with care that the transfer bar rests beneath the AK trigger group and then is secured by securing nut in the tube end and a screw in the bottom (see following two photos).








The photo above shows the tube tray in place. This will be perimeter welded and then sanded smooth. I am not yet ready to commit to this as I am still tweaking the innards a little. The FCG will be serviceable with the aid of a hooked pick through the holes provided in the tray and the entire pistol grip assembly will remove, so sealing will not be an issue.





The two photos above are just a couple of extra exterior views of the receiver nearing completion. I hope to get the lower essentially completed this weekend....we'll see...

-Ken
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  backbencher

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Posted: April 18 2015 at 6:41pm | IP Logged Quote backbencher

That is slick. Mad skillz you have.
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  vintagemx0

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Posted: April 18 2015 at 10:45pm | IP Logged Quote vintagemx0

I mentioned in an earlier post that I was abandoning the twist-lock method of securing the receiver tube due to issues related to the FCG placement. What I came up with is a spanning tube in the lower receiver assembly with a 3/8" ID that allows you to look through the receiver for the purpose of inserting a pin. I used one of those retaining pins from the hardware store that has an integral spring-loaded ball bearing. I pre-drilled a hole on the spanner for the ball bearing to rest.

Anyway, I cut a 1/8" wide slit 1/2 the through the spanner and welded a tab to the end of the receiver tube. There is a slot in the tray above the spanner that allows the tab to pass. You simply install by inserting the tab into the slot, then push the tube all of the way rearward, then insert the pin.



The photo above shows the end of the receiver tube with the tab hovering over the slot in the tray that accepts it.



The photo above shows the spanning tube with the receiver notch above it.



In the photo above, you can see the receiver tube in the final installation position whereby the tab engages the slot in the spanner tube.



In the photo above, the pin is ready for insertion. I am able to install this pin with extreme thumb pressure, but you need to tap on it with a tool to get it back out. I think that worked out fairly nicely.



In this last photo above, the pin was trimmed so as to be flush on both sides of the receiver body. I don't think it is too imposing.

So, I was thinking that I would need to add a simple pin protruding from the front of the receiver that would go into a hole on the mating surface of the mag well in order to retain any up-and-down movement of the receiver tube, but so far I am finding that this tube is secured extremely firmly. If things wear to the point wear some wobble or movement is detected, the pin/hole idea could be added at any time.

Next, I'll work on finishing the ends of the lower and move on to a redo of magwell. There's a number of little things about the magwell I have already made that I am not happy with, so I'm going to call it a practice peice and redo one based on the things I have learned.

-Ken
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  yaybacon

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Posted: April 19 2015 at 4:01am | IP Logged Quote yaybacon

Probably already know, but you can lighten the triggerpull by re-working the leverage to be in your favor, and also removing possible flex in that rockerarm..  Could move the rocker pivot further forward and/or  lengthen the  protrusions out the back of the ak trigger.  Can't explain how jealous I am of this build t.t
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  vintagemx0

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Posted: April 19 2015 at 11:10am | IP Logged Quote vintagemx0

Thanks yaybacon. I should have mentioned in a prior post - I found the big problem with the heavy trigger was the course grinding and sharp edges where the trigger and bar made contact. I polished those points up and it feels pretty good now. I was fearing having to move the pivot point because it would have effected the travel of the trigger as well, but fortunately it sorted itself out by simply polishing the parts.

-Ken
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  vintagemx0

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Posted: April 20 2015 at 10:51pm | IP Logged Quote vintagemx0

I have a general over-view plan sketched-up for the tube assembly for this build and thought I would post it on here welcoming any comments/concerns/ "been there done that"...

Below is a two dimensional drawing of all of the major elements I had in mind. Below the photo, I'll try to explain what I am contemplating with references to the numbered parts in the diagram.




(1) is a heavy metal end cap that is designed to sit in the heavy washer that is welded into main tube end cap. This will be pinned to the tube and serves as an end cap for the small telescope tube, centers the telescope assembly in end cap, and is heavy enough (hopefully) to withstand repeated recoil blows.

(2) Teflon or nylon pressed into (1) to serve as a "soft" striking surface for the buffer plunger (5).

(3) Recoil spring

(4) points to one of three sections of the telescopic tube enclosure.

(5), (6), and (7) are the buffer assembly - Plunger, housing, and spring, respectively. I was planing to thread the forward end of the housing (6) into the bolt extension [8]

[8] Bolt extension. This will be a hollow extension of the bolt hogged-out to allow passage of the hammer. I haven't thought too much about how to secure to the bolt, but I don't see too much issue with welding the two together.

(9) Hammer, AK type (shown to help orientation of diagram)

(10) Firing Pin. This will drop-in from the front and be secured by the bolt face (14).

(11) Firing Pin Spring

(12) Bolt

(13) Bolt Face securing cross-pin.

(14) Bolt Face - AK style. This is just the front inch or so. Removal of cross-pin (13) allows for its removal and servicing as well as the firing pin.

(15) Trunnion.


I haven't studied the geometry yet to check for travel, interference, and general practicality, but this is an over-view of the path I would like to go.

I do not have any experience with either buffer assemblies or spring-loaded firing pins. I suppose any amount of buffering is a good thing, but does this design look destined for failure? The main reason I went with the spring-load firing pin was because the original AK setup was about 1/2" too short, and this just seemed like a doable alternative that provides any easy way to secure everything. See any scary things going on there?

Thanks in advance for your valuable advice.

-Ken
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  blurrededge

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Posted: April 21 2015 at 12:12am | IP Logged Quote blurrededge

My first thought is that it appears to be a little over complicated in some areas, but that doesn't mean I'll try to dissuade you from building it (as long as you take lotsa pics )
-The recoil assembly- not sure you need the buffer in 5 & 7. The tail will help keep the spring centered.
You probably don't need the telescoping spring cover.
-If you enlarged the bolt tail, and got rid of the little buffer inside it, you would probably have enough space to be able to rear load the firing pin, and retain it with a conventional cross pin, (similar to how the STEN bolt's are done for hammer fired mods) This would save you from having to mess with the front of the bolt to install and remove it.
This Thread has a bolt where an AK FCG was used, might give you alittle insight on how to minimize material removal and still use the AK hammer (Cut one side of the hammers humps off the head if they aren't engaging a trigger hook )
However, you ARE doing an MP-40 inspired build, so in the German spirit of Over engineering, I think you're on the right track

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  Paraquat

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Posted: April 21 2015 at 8:16am | IP Logged Quote Paraquat

The quality of your work is amazing given the lack of tooling.
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  vintagemx0

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Posted: April 21 2015 at 3:13pm | IP Logged Quote vintagemx0

Thanks blurrededge for the comments. Yes, typical German over-engineering. That is partly why I want to produce it this way. The telescopic recoil enclosure is classic MP40 style, and so it is one of those elements I want to emulate. I still need to be able to remove the AK bolt face from the bolt (carrier) in order to service the extractor, but I'm gonna give that some more thought. The little buffer - it will screw in so I can remove at any time. Perhaps I'll begin without one, then try one and see if it makes any difference, and then either leave or remove later...

Looking again at the diagram above, I see now that the recoil enclosure tubes will probably need to be two pieces instead of three. There needs to be tome compression on the bolt to hold it closed against the breach. In my design in the illustration above, there is none because these tubes are full extended already. In order to supply compression (preload) in the bolt closed position and have travel required, I'll probably need to change to two tubes.

-Ken
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