Marushin MP40 Falling Apart...

Post by Claymore

(Refer to Video:- https://mp40modelguns.forumotion.net/modelgun-videos-f2/marushin-mp40-falling-to-bits-posted-by-claymore-t59.htm)

OK after saying how reliable the mp40 was suddenly i have developed a very serious problem. When firing the gun and it only takes a couple of rounds the upper separates from the lower, now the only thing that holds the upper and lower together (to my knowledge) is the the spring assisted pin in the lower receiver which locks into the hole of the upper, i have checked the pin in the lower and this seems perfect the upper also has no cracks around the hole and does not seem worn.
I have taken the model apart and checked everything and all seems fine when the gun is put back together everything locks fine you cannot pull the lower from the upper and neither is loose or shaky but the moment you fire it the upper shoots forward from the lower.
So guys any ideas?

Post by Doc

Claymore , does your takedown pin have a flat top to it or is in slightly curved ?

If its a flat topped one then its a little too close to the moving bolt and what is happening is that the bolt if driving forward and clipping this pin sending it downwards thus releasing the upper and lower .

you should also be able to see where the bottom of the bolt has a little wear from doing this.

if this is the case then Mr dremmel is your friend , just take a little from the middle section ...will post a pic if required .

DOC
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We need guns...lots of guns....

.... well anything mechanical that can kill people is interesting ...

Post by Claymore

cheers Doc but the pin is not flat it is slightly curved however with what you say about the bolt makes me think that might be a problem as when the gun first came apart the recoil tube had opened up and the spring came out and had been bent in the middle so from what you say its possible i suppose that the recoil tube came aprt and the bolt pushe on the pin, i have a new tube to try so will try to see if that cures the problem.

Hi Claymore

I'd be keen to learn how you get around this problem please mate.
My MP41/MP40 hybrid is doing more or less the same thing as your '40.

The MP40 upper receiver assembly won't lock tightly onto the MP41 stock and lower receiver assembly.

The MP41 has a threaded screw instead of the MP40's spring loaded pin and even though the hole in the receiver has been opened up slightly to accept it, it only just about holds it. Pulling the trigger and letting the bolt spring forwards pops it off the pin.

I haven't been able, as yet, to look down the front of the receiver tube to see how far (or not) the screw protrudes into the tube. It may well be poking in too far letting the bolt strike it.

If you need a spare upper receiver tube to try out on your '40 I've got one here you can borrow if you like mate

Cerwyn
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Hobby collector of mainly WW2 era uniforms, headgear, field equipment and replica weapons.

Post by romagnus

OK. I was able to duplicate what your video shows. After playing around and inspecting my gun this is what I found. I took my gun apart and slowly worked the bolt back and forth in the tube. Clearance is real close to the take down pin. I then inspected clearance of the take down pin in the tube with no bolt and found the pin does not actually come flush with the inside of the tube. This means the bolt cannot touch the take down pin, if it did the bottom of the bolt would be destroyed and would most likely stop the bolt.

Now on my gun the take down pin spring is very very strong. I need to use some good effort in pulling it down to field strip the gun. Now if you put your gun back together your take down pin should only go back in one way(mine does) and I found that it pops back in but does not actually seat completey flush all the time! It looks good and sounds good when you here it click in place but take a closer look and you may need to wiggle the tube and push the pin in to get it to go the rest of the way.

I hope this helps. now to figure out why my rounds don't eject all the time, some getting stuck in the tube.
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DPMS AP4 carbine, CZ75, Romanian AK-47, CETME 308, Russian AK-103, XD40SC, 16guage shotgun w/short barrel, Marlin 22LR.....and some others I can't remember right now.....all real. And now a Marushin MP-40

Post by Doc

romaguns : In the instruction manual of the MP40 it shows that before you fit the ejector part you have to file away a small angle at its front edge , this should correct the angle of the rounds as they strike it and eject up and out properly .

DOC
_________________
We need guns...lots of guns....

.... well anything mechanical that can kill people is interesting ...

Post by romagnus

I'll look at that, thank you.
_________________
DPMS AP4 carbine, CZ75, Romanian AK-47, CETME 308, Russian AK-103, XD40SC, 16guage shotgun w/short barrel, Marlin 22LR.....and some others I can't remember right now.....all real. And now a Marushin MP-40

Hi romagnus,

If the tube you refer to when you say "your rounds are getting stuck in the tube", is the Detonator Chamber, I'd reckon the chamber and Det Pin are clogged up with soot and dirt from previous firing sessions.

Depending on the grease I use to lube the cartridges, I find that by the time I'm shooting the 3rd mag full rounds staret sticking in the chamber.

Try pulling the chamber and pin out, see how dirty it is.

I found your observations about the take down spring and pin very useful myself too, thanks for that!
One tip I was given is that if the pin takes a lot of effort to pull out, it's difficult to grip sometimes, take the side-plates off.
That guives much better access to the pin and I'd guess it'd be easier to confirm it's returned all the way in too.

Cerwyn
_________________
Hobby collector of mainly WW2 era uniforms, headgear, field equipment and replica weapons.

Post by Claymore

Thanks for that, yes i know about the pin mine is very strong spring and i alwasy make sure it is full home so i dont think it is the pin itself. i have been busy of late so not had time to really play around and try out the suggestions that Doc gave i hope to strat giving this my full attention in the next week.
thanks for all the suggestions so far and i will see if i can come up with other reasons for bad ejection as well.

Post by Claymore

OK, went round to kickbacks and we both had a look it seems that the problem is the rear of the upper and lower receiver, ther is a small bit of plastic on both and you twist the upper into the lower which locks it out, this has worn (god knows why) and although when you put it together it sems fine on firing this part gives out and the gun comes apart so i think on mine its the lower that has worn and needs replacing, so all those wioth the marushin beware and get some spares.

guess what! my mp40 just fell apart twice while firing it! lookin for my fix.

Hi Romagnus,

It's coming apart already
You'rs is almost new isn't it If the upper and lower receivers seperate during firing it'll either be the take-down pin's loosened and dropped a little perhaps, or the lugs that twist to lock during assembly have worn.

Once assembled, pull the bolt back into the Safety slot and look inside the tube through the slot. You should see the take-down pin sitting flush with the bottom of the receiver tube.
If not, you can adjust it by loosening and repositioning the the thumb wheel.

If that's ok, strip it again, push the upper receiver into the lower and twist to lock it as normal. The lower should also fit snugly into the slot behind the mag' well.
The locking lugs should hold both receiver tubes together quite firmly even without the take-down pin. If they pull apart, suspect the lugs.

Have a look at this... https://mp40modelguns.forumotion.net/marushin-mp40-f16/marushin-mp40-basic-reassembly-guide-t1158.htm ... for details of those lugs.

Yep mine has fallen apart again and i have already replaced the lower receiver, maybe i will have to replace the top now as well. Also i was trying hudson sten rounds in it which seem to work although i have only loaded 1 in the breach so far to see if it would fire and eject, which it does but when i tried some in the mag it would stipo and fire the first round but the 2nd just slows the bolt down to much and does not feed, so will have to work on this when i have time.

Francky sells the Upper and Lower receivers as a set and I'd guess that if one was worn the other probably would be too.
Best to replace both to be on the safe side

Was there any particular reason for trying Hudson Sten rounds Claymore I tried MGC P220 in one of mine and all I got for my efforts was a cracked bolt
I'll be sticking to Marushin's own 9mm cartridges in this one for certain

Yeah i normally stick to the marushin ones as well cos they are cheap to buy and they work, i have just been trying to see if certain rounds will fit many models and the hudsen sten round does my sten my madsen so far + i think the sten rounds would work with the hudsen inners inthe 40 so power should drop off and not destry the gun. I find that once you have fired the marushin rounds about twice they become difficult to open up and load again, the screw base i mean and iget a lot of misfires if i fire them 3 times or at least try to wheras cp rounds should fire a good 4/5 times before they start to give up.

claymore wrote:

Yeah i normally stick to the marushin ones as well cos they are cheap to buy and they work, i have just been trying to see if certain rounds will fit many models and the hudsen sten round does my sten my madsen so far + i think the sten rounds would work with the hudsen inners inthe 40 so power should drop off and not destry the gun. I find that once you have fired the marushin rounds about twice they become difficult to open up and load again, the screw base i mean and iget a lot of misfires if i fire them 3 times or at least try to wheras cp rounds should fire a good 4/5 times before they start to give up.

When you say they get difficult to open up after 2 firings, is that without a thorough clean before popping another cap into the cartridge
I've had my Marushin cartridges for 3-4 years now (about 100 of 'em). If I'm firing the MP I'll fire most of the cartridges once then strip down and clean everything completely.
to date, I've only had ONE cartridge fail and that one blew the threads off...

My MP40 needs it's detonator pin cleaning too after 60 rds otherwise I get extraction
problems.

I think my MGC Thompson rounds are the ones I've fired repeatedly most often. I have 100+ of these too and use them in my Hudson Grease Gun and M1A1 Thompson too.
Despite the rims being a bit battered they fire well. I've had no drop off in power problems due to gas seal failures.
I will sometimes re-load them with a cap without cleaning 2-3 times and they'll perform well.

Whether that's due to a thorough clean in the Ultrasonic and Polisher once I've finished because the threads and the cartridge bores are in good nick even after all this time.

Its the base thats the problem, the threads get very dirty and of course you get expansion problems, but its the dirty threads mostly that mean you cannot screw the base on well or at all sometimes also with some rounds after the 2nd fireing the base sticks and it takes plyers to remove it. i use silicon grease around the thread when i can be bothered but of course that adds time to loading although it does help and i would think it helps with the seal as well. I have also wire brushed the threads after firing to keep them clean but again bloody time consuming and boring. Of course with cp rounds i would also expect a better sound on full auto as well as being able to fire them more than twice.

Couldn't agree more with you, cleaning and preparing cartridges prior to loading can be really boring and time consuming.
Hours of work for a few brief seconds of firing fun

Although it IS an expensive way around it, having plenty of cartridges available to fire keeps me reasonably happy.
If I can put 100 rounds through any gun before the gun itself needs cleaning (Det. Pin & Chamber) I'd consider myself lucky.
All the cartridges get fired once then stripped, Ultrasonic cleaned then Tumble Polished before re-capping.
The tumble polisher will take 200-300 dismantled cartridges easily and just needs switching on. Leave it a couple of hours then sieve the polished cartridges out.
The Ultrasonic needs a little more input but is Heaven sent compared to cleaning each one indvidually.

The only rounds I've had to take pliers to are ones I found under the bed or behind wardrobes There's always one or two that vanish after a good shooting session only to reappear weeks later.
You realise then just how corrosive the powder's burnt gases can be