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MP40 Inspired Build
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  backbencher

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Posted: September 08 2015 at 10:21am | IP Logged Quote backbencher

My initial impression is those are some DEEP firing pin strikes.

Are you using factory loads or reloads? Interesting that all the brass is still shiny around the rear web. The ruptured case appears to have a small setback ring?
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  vintagemx0

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Posted: September 08 2015 at 8:20pm | IP Logged Quote vintagemx0

Yes, they are deep. The edge of the firing pin is fairly square too, so I'm surprised none of these hits actually punched through.

The rounds were factory PERFECTA, 115 Grain FMJ, brass case.

The ruptured area corresponds with my feed ramp. I was pretty amazed at how much of these casings were still shiny. I'm now seeing some evidence that suggests that the edge of my clearance notch in the breech for the extractor is colliding with the extractor. This would explain this apparent lack of seating. I'll extend the clearance before the next testing.

I confirmed to day that the ejector is simply too short - easy to fix. The take-down tang is bent due to the lower receiver taking blows upon recoil. I'll beef the tang up substantially and look at some kind of simple buffering. I hope to re-test next weekend.

-Ken
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  sevenla

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Posted: September 09 2015 at 5:23pm | IP Logged Quote sevenla

vintagemx0 wrote:
Yeah, you guys are probably right. But, I think I can make these pieces and decide later if I want to abandoned them. This is a hobby effort , so I don't mind some trial and error (emphasis on error...)

I've been busy with other things lately and haven't worked much on this project, but I was belly-aching about what to do about a safety? The more I looked at this build, and the more I searched for examples on the internet, the more I liked the idea of a push-button style right in front, and to the top of, the trigger guard. Much like what is on a .30 cal. M1 or a Ruger .22 rifle. There is a nice big "delta" in my pistol grip where a button would fit nicely. I really like this idea over any other I have considered (AR, AK, various 180° toggles, etc...)

So, I drilled a hole in this area only to realize it was substantially hollow in there, so I filled the region with weld, ground clean, then re-drilled a 3/8" hole clean through:



Next, I purchased a 3/8" diameter pin (I don't know the proper term, but this pin is offered in various lengths and diameters, has a grasp ring at one end, and has a spring-loaded ball near the other end.) I fashioned this pin with a flat spot ground in it to limit the travel when the pushed from side-to-side when pushed. The idea is to expose a depth where material has been removed from the pin to allow a pin to recess into when in the "fire" position. When in the "safe" position, the pin will be blocked and thereby not allow the trigger to rotate on it's axis.



I had some issues on my first try. I was able to get the pin to be captive with a retaining screw, but I hollowed too much material for the interference pin. Also, when in the "fire" position, some of the area that has been removed is exposed on the exterior of the weapon. I thought it would look terrible, but the width of the lower receiver really shields this view and it looks pretty good.





Anyway, like I said - I pretty much screwed up this first pin and need to re-do. After I get it working properly, I'll post details about the whole safety selector system. I'm also going to use a bolt hold-open (J-groove in tube) as I like this feature, but I really feel a closed-bolt weapon needs a trigger safety as well.

The devil's in the details. This little pin has already taken much time, and it's a complete redo. Funny how little things can become more complicated than the larger things.

-Ken


Where did you get this pin?
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  vintagemx0

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Posted: September 12 2015 at 11:54pm | IP Logged Quote vintagemx0

I got it at Ace hardware



-Ken
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  vintagemx0

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Posted: September 13 2015 at 12:42am | IP Logged Quote vintagemx0

I made repairs on my build and am ready to take it back out for another trial.

I didn't take any photos of it, but I put a little weld on top of the ejector and ground it down to fit. I thought this would easy, but I ran into a problem with the ejector getting hung-up on the forward end of the striker pin that is behind the firing pin. I ground a flat spot on the pin, but had to add a little tab that rides inside the ejector slot in the bolt to keep the pin from rotating and causing the same problem again. Anxious to see how she spits 'em out now.

After studying the relationship between the bolt face and the breech, I discovered I had done a very stupid thing very early on. The extractor is located 45° from top of bolt face, and I had made a clearance notch on the breech face starting at the top and going 45°. The middle of this notch should have been located at 45°, not one end of it. As a result, the extractor was hitting breech face and hold the rounds slightly out of battery.



So I inserted a spent round (because it made of brass) and welded the old notch closed, ground, and re-notched.







And then I rounded and polished the edge around the recess for the round on the bolt face.



The extractor now nestles nicely in the new clearance notch. I also ground the firing pin a little. I think I'll take a file out with me and adjust more as required. I also beefed-up the take-down tab that had cracked before...



I feel much better now about the over-all fit and function, but we'll see (hopefully) tomorrow. Interestingly, I was having trouble before with failure to load when only one round was in the mag (snap caps), but now it seems to chamber a lone snap cap quite reliably. Wish me luck...

-Ken
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  OldCoot

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Posted: September 17 2015 at 1:14pm | IP Logged Quote OldCoot

Standard firing pin protrusion is usually around 0.060".  Might want to measure yours....
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  KernelKrink

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Posted: September 17 2015 at 4:56pm | IP Logged Quote KernelKrink

That feed ramp in the chamber is pretty deep, it's exposing a lot of the case where the brass starts to get thin. 
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  vintagemx0

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Posted: September 20 2015 at 11:29am | IP Logged Quote vintagemx0

Oldcoot: Thanks - I did take it down some more. I'm at about 0.061" and the strikes look pretty good, however they do look a little strange. It looks like the primers have been bulging-out slightly in the area around the firing pin where there is space between the pin and the bolt face. My clearance for the extractor runs unp to the pin and you can see pretty clearly in the photo below at about 4:00 o'clock position where this bulge is happening...



I' thinking that my headspace is 0. The bolt is closing fully and contacting the trunnion, but I think that the round is bottomed and that there is zero headspace - not negative, just absolute zero.

Kernelkrink mentioned concern about the unsupported region at the bottom of the rounds due to the feed ramp depth. I share this concern, but my last 100 rounds look really good - no bulging of any kind or any defined lines impressed in the brass. I did use a different ammo this time, so perhaps that is why, meaning maybe my luck may vary with different ammo?

Question: I was thinking that perhaps I should obtain a chamber reamer and ream the chamber a little deeper to afford some headspace and at the same time, the unsupported region on the bottom would be that much less. Does this sound like the right idea?

Thanks for the comments fellas. I really do appreciate it.

-Ken
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  vintagemx0

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Posted: September 20 2015 at 12:24pm | IP Logged Quote vintagemx0

R A N G E   R E P O R T   2

I had taken it back out, but again I had major failure to eject issues and ended that testing abruptly. I extended the ejector further and am happy to report that she digests rounds with good reliability now. It doesn't spit them out far, maybe about 3-4 feet, but the failure to fully eject now is about 5 out of 100. I'm going to experiment with protruding the tip of the ejector further forward and seeing if that helps eject the shells earlier and helps eliminate any fails.

I also added a crudely improvised buffer. In the photo below, you can see where I added a rubber bumper to by bolt spacer and a disc to the end of the receiver tube to take the blows.



I replaced the nut that secures the pistol grip to the tube with an extension nut..



Then the end plate gets screwed-in until it is seated tightly...



I liked it. The amount recoil on this 8.5 lb. weapon is light anyway, but the amount of recoil felt is no less than it was before. It does however feel very different. Before, the recoil blow came from the spring becoming fully collapsed. Now, the rubber bumper strikes the end plate before the spring is fully collapsed and the blow just feels a little different - more refined. Sorry, I can't explain it any better than that.

I also decided it would be wise to add some securing means to the front end of the receiver. I drilled and tapped a 1/4-20 hole in the front near the top and threaded in a short piece of bolt that I turned-down on one end to make a smooth little protrusion. I drilled a hole in the back of the magwell to accept the protrusion when the tube is slid back into it's installed position. This was really easy and works great...



Lastly, I fashioned a charging handle. I drilled a hole through it so any available pick or small screw driver could be used as a tool to screw it in or out.



I think I will fashion a more permanent and clean buffer based on the general design above. The feel of this gun when it fires is a real treat. The last 25 rounds, I did a mag dump for heck of it (Got to do this at least once, right?) No fails, even fired the last round. Although I like the feel of the trigger, it is heavy and therefore you can only fire so fast. No chance this gun could bump fire ever.

The feed problem identified earlier due to the mag being mounted a tad low relative to the chamber is still there, but only happens on the last round and about 80% of the time. This is a lot less trouble than I was anticipating, but I am still going to try to solve this entirely. I just want to wait a longer before I do because it may require some plug welding and I'm still working out other issue.

The sights are way off. Consistently hitting about 6 inches low and 4 inches to the left from about 25 yards. I have a chamber laser that indicated to me that they were near perfect at home, but obviously not... I paid very little for the laser - another case of you get what you pay for...

-Ken
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  blurrededge

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Posted: September 20 2015 at 1:37pm | IP Logged Quote blurrededge

Personally, I would shorten the chamber. All the blowback guns I've built have about a papers thickness between the bolt face and rear trunnion, with the chamber just a hair shallower then the trunnion. This way, the barrel itself doesn't take any contact when the bolt closes on an empty chamber, and the only time the bolt should hit the trunnion, is when it's closing empty, with no rounds. This was the way most of the full auto open bolt guns were designed, partially, I'm assuming, to ensure that the round fully chambered and was fired during the slam fire operation taking place, but also to reduce impact stress on the bolt face and rear trunnion.
That's my thoughts on your zero headspace, but others may have a different idea.
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  vintagemx0

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Posted: September 20 2015 at 5:29pm | IP Logged Quote vintagemx0

'Sounds like I could accomplish that by taking the face of the bolt down a little. The recess is deep and there is a lot of bevel on the face of the extractor, so it could be shaved down a hair as well.

Other thoughts?
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  Zuzzy

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Posted: September 25 2015 at 9:48pm | IP Logged Quote Zuzzy

blurrededge gave you a good advice, check this link also, especially the pictures;

http://www.uzitalk.com/forums/showthread.php?67921-Trunion-t o-barrel-fit-issues
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  vintagemx0

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Posted: September 26 2015 at 1:09pm | IP Logged Quote vintagemx0

Thanks Zuzzy. I know blurrededge is likely right as I respect his experience and willingness to help very much! Meanwhile, I'm just sorting out gross issues like my sights. Although I followed the original attachment of the sling to the right side, I'm not sold this is the best fit for me and will try the lefts side. She feeds well at the moment, but the last round fails to feed more often than not and that is my next priority. I'm actually very satisfied at the moment of the performance and condition of spent shells when using Winchester 115 grain target loads.

As soon as I improve last round feed, I'll likely move forward on the magnetite finish that I'll post for the benefit of others contemplating this type of finish.

As always, thanks for all of the input guys!

-Ken
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  OldCoot

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Posted: September 27 2015 at 2:37pm | IP Logged Quote OldCoot

Very neat gun, vintagemx0. Looking forward to seeing your finishing operation...
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  backbencher

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Posted: September 27 2015 at 11:52pm | IP Logged Quote backbencher

Zuzzy wrote:
blurrededge gave you a good advice, check this link also, especially the pictures;<br><br> http://www.uzitalk.com/forums/showthread.php?67921-Trunion-t o-barrel-fit-issues


http://www.uzitalk.com/forums/showthread.php?67921-Trunion-t o-barrel-fit-issues
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  vintagemx0

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Posted: September 28 2015 at 8:24pm | IP Logged Quote vintagemx0

I haven't been able to work on my project here lately - which is probably a good thing since I now have some research to do (thanks backbencher for the link).

I ran into an old friend that brings a lot of interest to this project. In an earlier posting on this this thread, I mentioned that this project will fit right-in with another project I had completed. It was a scratch-build replica of a 1943 VW Typ 82, or best known as simply "kubelwagen". My friend owns a real one and lives very close to me, so I would go over to his house and get detailed dimensions and so-on. He needs my help soon to repair abad transaxle leak, so this weekend I was sprucing up mine and test driving (it's been parked for 6 months) so we can trade and I can keep his while we're repairing it. Well, this was a good chance to take a few photos... So, (and this is for you primarily UK biker), here is the MP with kubel..









The other col thing is that my friend also owns a real operable MP40 and says I can go shoot it with him anytime. How fun will that be? Full auto real-deal. 'Looking forward to that.

I know these pictures kind of suck as they cut-off parts of the vehicle. Next time I'll take some MP40 action "shots" in the woods with the car using a real camera and not my phone.

-Ken
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  Inkfideltattoo

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Posted: September 29 2015 at 1:19pm | IP Logged Quote Inkfideltattoo

Just a quick thought on your last round feeding issue, I've had a few tube guns including a S&W 76 that were notorious for hiccups if you put certain pressure on the magazine (as in using it as a fore-grip). Any chance that magwell play could be a factor here? Also, would modifying the follower improve feeding function? If those pics were in black and white it would look like it was straight from a wartime gallery!
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  UKBiker

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Posted: September 30 2015 at 7:49am | IP Logged Quote UKBiker

Fantastic thanks Vintage, my old neighbour had a Kebel replica several years ago, we used to zip around town in it having a great laugh with Wagners Flight of the Valkiries blasting out on the stereo.
Unfortunately neither of us were lucky enough to have an MP40 though
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  vintagemx0

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Posted: September 30 2015 at 8:15pm | IP Logged Quote vintagemx0

Thanks Inkfideltattoo - Yes I have found that the attitude of the mag can be a bit sensitive. Iavoid holding the mag and hold either the magwell or the foregrip area of the lower (preferred). The foregrip area of the lower is really nice and I can only imagine that folding butt stock would make it that much nicer.

Perhaps a future "build" will be applying for the SBR and pay the tax to do it. I want to make sure this thing purrs first and appears to ave a long life ahead.

UKBiker - Nothing much more fun than tooling around town in a Kubel! I'm glad we share that memory.
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  vintagemx0

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Posted: October 05 2015 at 11:16pm | IP Logged Quote vintagemx0

I had the pleasure of meeting up with an old friend this weekend who happens to own an MP40. First time I ever aw one up close - what a treat for me!

'Took some photos of the two together...





We'll be going out before long to fire them as well. That will be fun.

I had a good look at the breech of his weapon and can report that the trunnion is about 1 mm prouder than the end of barrel. Goes along with blurredege's advice. Thank you sir! I'm going to see what I can do about it.

-Ken
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