View Full Version : Runaway MP-40

July 14, 2010, 18:56
So back in June I got a chance to fire a real honest to gosh,
straight from a nazi's hands,( appox. 60 years ago.) MP-40.
Needless to say I was a happy camper.
The owner showed me the basics and did an impressive
job of controlled burts with the stock folded.
Not being all that well versed on handling ANY kind of
select fire firearm,I went with the stock unfolded.
As soon as I touched the trigger, the weapon took off on me.
I clamped down and held on and the mag finally ran dry.
This got the attention of the owner.He reloaded and
fired the MP and the same thing happened.
Just as a test,he folded the stock and the firearm went back to normal.
Whenever we unfolded the stock,and fired the piece,it would run away.
My guess is it has something to do with the stock,but I cant imagine how.
Anybody out there got any ideas?

July 15, 2010, 07:09
I'm willing to bet that you were using ammo without enough pop to push the bolt back far enough to catch the sear. Ask me how I know.

Not sure just what part the stock plays in it but I bet its understrenth ammo.

July 15, 2010, 07:29
I agree with the hippie. I'm guessing with the stock extended and braced against your shoulder, it cycles fine (but barely...) When you fold the stock, your wrist acts as a shock absorber, kinda like "limp wristing" an auto pistol and it's juuuuuuust enough to keep it from cycling fine.

johnny d
July 15, 2010, 08:22
Well that's just plain weird. The stock mechanism isn't anywhere near the FCG. I can't see how it would have any effect on sear function. I'm thinking it an ammo problem. I have noticed changes in the cyclic rate with different brands, at least with mine.

July 15, 2010, 08:29
Originally posted by GOVT1911
I agree with the hippie. I'm guessing with the stock extended and braced against your shoulder, it cycles fine (but barely...) When you fold the stock, your wrist acts as a shock absorber, kinda like "limp wristing" an auto pistol and it's juuuuuuust enough to keep it from cycling fine.

Sorry,maybe I got ya confused.It was the other way around.With the stock extended it took off on me.
When my buddy was showing off,he did it with the stock collapsed, and it worked fine.
I could see the limp wristing,like in a Glock,being a cause,but it shouldnt have with the stock in postion.
Is it possible that as loose as that old stock was, that its wobble actually made things worse?
We were using Federal 124 grain hardball.Seemed hot enough.
The whole thing has got us scratching our heads.So I figured I'd ask the
experts over here on the files.
Thanks guys.:beer:

July 16, 2010, 01:30
Well, that shoots my theory in the foot! LOL

July 16, 2010, 06:43
Originally posted by GOVT1911
Well, that shoots my theory in the foot! LOL

Thanks for throwing in anyway.Its appreciated. :beer:

July 16, 2010, 08:30
Stock folded or unfolded I still bet its ammo . You can get 3 shot burst from a semi MAC 10 by loading a hot round followed by two weak ones. I believe that the stock position is a red herring. Or a yellow sardine. Or something.
Try as I may I cant think of anything else that might cause it

July 17, 2010, 19:21
It's either the ammo or the springs - euro ammo was loaded to hotter spec's than what you get down at walmart and the springs were set up for the hotter ammo (harder to compress so need the hotter load to make bolt go all the way back to catch the sear).

With the stock extended the spring does all the work, with the stock folded you are inputing some "push" to hold the gun on target so you have moved the receiver forward enough to catch the sear evidently - must be very close to the sear to exhibit this problem.

Try to find ammo that duplicates the original NAZI ammo as close a possible and you'll probably see the problem go away.


Depends on which WWII German 9mm you are looking at and what book you are referencing.

Pistolen Patrone 08 - 8 gms (123.45 grains) bullet / 1050 fps

Pistolen Patrone 08 S.E. (S.E. standing for Sintereisenkern or pressed steel core) - 5.9 gms (91 grains) bullet / 1350 fps

Pistolenpatronen 08 m.E. (m.e. = mit Eisenkern or with iron core) - 6.3-6.5 gms (97-100 grains) bullet.

There is a very good chart from a 1944 dated Army manual on page 227 of Frank Iannamico's Blitzkrieg.

Just to note also that the German iron or steel core ammunition is considered armor piercing.

Best to stick with brass cased commerical 115-124 gr 9mm or if you reload use factory specs.

I do not shoot Wolf steel cased ammunition in any firearm I care a lot about. I will shoot it in my Sten because parts for those are cheap.

I think the 115 gr is marginal on recoil impulse, but the 124 or 147 gr Winchester White box usually works fine in my UZI. I have noticed that the nickel plated 145 HP wolf works ok in my pistols (P38, 92FS).


RichUrich wrote:

115gr. vs. 124gr.?
Some MP40's like one much better than the other for feeding and ejecting.
At the last shoot, the MP40 we had ate all the 115gr Wolf ammo, but wouldn't reliably function with 124gr. Wolf.
I have had MP40's that would not relaibly function with British SMG 9mm ammo that worked wonderfully in a Sterling or STEN.
Just make sure the 9mm ammo is HOT HOT HOT.

Rich Urich

Breaking MP40's
Guys, I have been a Class 2 and collected MP40's since 1974. I have seen and built and repaired MANY MP40's. MP40's are my specialty. I have fired thousands and thousands of rounds through MP40's. I like MP40's.
What I have seen as regards to repairs:
1. Do not assume that parts are inter-changeable. I have had MP40 bolts that worked perfectly in one weapon, that simply would not operate in another. RARE, yes, but I have seen it.
2. I have had plenty of extractors and firing pins break, or have had to replace them for customers whose did break.
3. Magazines are usually the problem if the MP40 doesn't feed / eject. Those magazine feed lips WILL wear out!
Also, clean the inside of the magazines from dirt and apply a light coat of oil inside the mag.
4. When the barrel wears out, and they WILL wear out eventually, there will not be enough back pressure to operate the bolt recoil, and the MP40 will fire sluggishly. I doubt the original "life expectancy" of the MP40 barrel was 10,000 rounds. (Compare with similar documented expectancy of other WWII German MG's)
5. When the sear and/or sear contact area on the bolt base wear, and they WILL wear, the bolt will "run-away."
6. Not all MP40 WWII German magazines are interchangeable!! Occasionally you will find mags that will just be impossible to fit in smaller dimensioned mag wells. Who knows why?

Spare parts needed:
Spare bolt
Spare extractors
Spare magazines
Spare barrel

If you have a chance, try to watch some original WWII videos of MP40's in use. Note the rate of fire and how high the empty shell casing eject from the ejection port. Seldom do I see MP40's are the ranges with the same forceful ejection.

Finally, consider VERY VERY carefully replacing the recoil springs with WOLF Springs. When removing the firing pin from the recoil assembly, remember it is under a lot of pressure!



July 17, 2010, 19:50
Wow! Thanks Artful,I appreciate all the work you did here.
Gave me some stuff to tell my buddy.