New to the Forum, Problems with my MG34

[mggunner34]

Hello all, I'm a new forum member, and I have a couple of problems with my transferable MG34 I was hoping you guys can advise me on... When I bought this gun it was actually just a receiver, I had to buy a complete parts kit to put it together, which I think is part of my problem.

First thing- the trigger seems to stick, especially when the top "semi" trigger is fired the weapons fires and ejects fine but the trigger doesn't reset I have to physically pull it back out with my fingers to fire another shot. I'm figuring that a spring may be missing or way to worn?

Second issue:
In full auto it ran Romanian 8mm great except for one round that it literally popped the primer out of the casing which caused a nasty jam because the spent casing wouldn't eject. With Yugo 8mm the problem got way worse with every other round getting its primer punched right out, and these aren't pierced primers either, the primer is actually comming out of the casing almost whole. this leads me to believe that the firing pin protrusion is out of spec, can anyone tell me what the proper FP protrusion should be? the Romanian casings that fired fine showed quite a deep primer hit. but funny enough none of the yugo ammo failed to go boom as I have read will happen.

[JBaum]

how does it run when you don't run junk ammo in it?

[mggunner34]

Isnt the Romanian surplus good ammo? I ran 3 belts through it and it still popped one of the primers right out of the casing, then it got caught in the threads in the barrel collar and jammed up (I know one out of 150 isnt bad but I've never shot anything else that had this issue with popping out primers). Should I get some new prvi to put through it?

[Matt]

there is nothing wrong with romanian or yugo 8mm. the yugo has harder primers but this is easily fixed. i would try different bolts. they are the cause of about 90% of the mg34s problems. get a hold of a new one if possible, about 275 or 300. - matt

[JBaum]

Not to give you a hard time here, but you have a $30,000 MG, and you're trying to run 40 year old ammo in it because it's cheap...... It's like buying a BMW and going to the junk yard to siphon some gas to fill the tank. A 1/20th of a second hang fire and you can have brass shrapnel in your arm and face. Blown up MGs aren't pretty, and it's a very embarrassing thing to have happen. Do the gun and yourself a favor: Try some decent modern ammo in it, and let us know how it runs.

You can't judge the performance of the gun any other way.

[mggunner34]

yes yes I know the whole cheap ammo speech but lets get off that for a moment here, its POPPING out the primers on both yugo and Romanian, how could using more expensive ammo change that? And dont tell me everyone at the everyone at the mg shoots are only using brand new prvi in their mg34s I know that's crap, most of them use Romanian and I have seen yugo run too. And this isn't my first MG, I've got a 1919 and an M16 so I am completely aware of the irony of buying a 10k++ MG then putting cheap ammo though it, and BTW the yugo I tried runs fine in my 1919 the primers aren't just falling out when fired.

Any chance someone can measure their FP protrusion so I can compare it? I'd like to salvage this bolt if possible as it seems to be in great shape, if the firing pin is out of spec Ill just get a new one. or if you know if a k98 would have the same protrusion I'll just measure that myself

[mggunner34]

I just reread my last post and it seemed a little short, I assure you I didn't mean it that way.

[TactAdv]

yes yes I know the whole cheap ammo speech but lets get off that for a moment here, its POPPING out the primers on both yugo and Romanian, how could using more expensive ammo change that?

You're considering the wrong failure mode, IMHO. For starters, for the primers to "literally pop out" as you say, they have to physically have ROOM to do so. In a normally functioning MG34 bolt + cartridge face relationship, the external primer annulus is fully supported under the entire high-pressure event cycle, i.e., there is a full circumference physical bolt face contact on the exposed primer annulus, which is maintained well after the high pressures have subsided. This full support is achieved by tight contact between the cartridge rear area, and the bolt face itself, broken only by the area opened up where the FP protrudes through, IOW's under normal circumstances of operation there is simply no possible room for the primer to be PUSHED rearward out if its pocket under HIGH PRESSURE, the only thing which will do that.

While certainly some suspect ammunitions may exhibit loosened primer retentions, the fact that BOTH diverse types of ammunition here are exhibiting the same failure mode speaks volumes- you have a serious pressure-related event occurring where the required lock-up between the bolt-face and cartridge face is not being sustained well into the extraction phase. The bolt is partially opening under high-pressures, allowing the primers to de-seat and be PUSHED OUT of their pockets, crimped or not.

Okay, so what causes this? Fortunately, it is a rather easy situation to analyze and fix. You'll recall that the design of the "Solothurn lock (Stange lock)" employed in the gun utilizes NON-POSITIVE threads, i.e, they tend to NOT stay locked tight because the mathematical pitch of the threads leads to induced pressures causing the threads to begin anti-rotational unlocking.....stated simply, under high pressures they tend to open up against the inclined plane of locking rotation. This allows the bolt(and bolt face) to begin rotational unlocking before residual chamber pressures have dropped correctly(safely) and as the bolt increases its space rearward relative to the cartridge face (now no longer fully supported), the high residual pressures press directly against the primer annulus forcing it out of the primer pocket. That is why you are seeing the same symptoms across the differing ammunitions.

This was a known issue when the gun was designed and they designed in a very simplistic and very effective cure to combat the premature unlocking tendency in the form of a "bolt lock latch" which is/was merely a heavily spring biased arm that presses against the right bolt lug, effectively positively preventing undesired rotation until it is moved out of engagement by a cam in the recoil phase, by clearing a lug on the barrel jacket interior as the barrel extension + bolt move rearward in the initial recoil phase. These springs DO wear and lose their original rated biasing force, so replacement of the spring is called for here. If not the spring being at fault, even simpler is that the actual locking latch ARM itself may have broken, be damaged, or even missing.

These are all very simple things to either repair or replace, and having access to one of John Baum's excellent manual translations will easily allow you to do the work quickly. Spare parts here are easy to find.

-TomH

[42rocker]

TomH
That was a heck of a statement and I think that I almost understood it. I think that I'll read it again to make sure. At an rate your statement about it happening with both ammo's then going from there was nice.
Interesting how the best answer to so many problems ends with the statement "get one of John's books".... Maybe it should be get several of John's books......

Later 42rocker

[junglewalk]

TomH, what an indepth explaination. My 2-brain cells are having trouble grasping it all!
..
I do know that in Daniel W. Kent's book, 'German 7.9mm Ammunition', he talks about the primer popping out problem with the MG-34 in the 30s.....Their solution was to change the primer, and to punch the primer in the case as an additional step; These rounds were placed in boxes marked, 'FUR MG'. Not all 7.92m ammo had punched primers at the beginning of the war. MG crews would grab & link such marked ammo for their MGs, and avoid using un-punched ammo, unless they ran out of "FUR MG" ammo.( I bet at Stalingrad they used punched & un-punched primed rounds)
.... By mid war, almost all of the factories producing 7.9m ammo were punching the primers, so MG crews did not have to go searching in their supply chain for the specially mark ammo.
.... I have had an occasional primer pop out and lodge itself in the craziest places in the receiver of my MG-34.....
......
.....I also have fired Germ/WWII "FUR MG" ammo (oh no!!), and still, the occasional primer pops out and makes its presence known in the receiver....I did go to a full length/longer recoil spring, and it seems to help.(don't know why)
......You could switch around bolts & barrels, it is worth trying....bh

[TactAdv]

..I did go to a full length/longer recoil spring, and it seems to help.(don't know why)

Well, (gently speaking) that is the point I was making and explained, actually. ;-)

The point is that the bolt is not STAYING locked to the barrel extension threads throughout the full high-pressure phase allowing the bolt face to incrementally back off its normal full contact support with the cartridge face allowing the primers to back out. In your case, I feel strongly as I suggested that this evidenced failure mode is due to the premature unlocking caused by the lack of proper functioning of the bolt-latch-lock in your gun. That was "why" you were able to slightly mitigate this by installing a stronger RECOIL spring which, in effect also, caused a slight added delay in the pre-mature unlocking problem that your gun currently has.

The "extra strength" recoil spring was just a happy/lucky band-aid to the actual extent MECHANICAL PROBLEM in your gun- a weak, broken or missing bolt-latch-lock spring, or a damaged, broken, or missing LATCH arm itself.

The "extra strength" recoil spring you installed temporarily added just a bit more counter-recoil force, which effectively delayed the bolt trying to unlock by itself too, the same as the normal bolt-latch-lock device does completely. (Careless use of "Extra-strength" recoil springs CAN be the cause of severe problems in an otherwise normal gun, be aware, as it quickly upsets the normal dynamic balance of the recoil-counter-recoil.)

That the Germans STILL experienced the problem of primer movement and chose to implement additional ammunition modifications to assist in prevention, even when the gun was thus equipped with a mechanical device (bolt-latch-lock) to HELP prevent premature unlocking, AND STILL SUFFERED ISSUES, should be fair warning that the basic locking design used in the MG34 is normally operating on the very far edge of safely containing the high-pressure event and as such these guns are ALWAYS a potential for issues in that regard. (Interestingly, this version of the modified-Solothurn-lock (MG34, MG81) was particularly prone to this, while earlier versions that employed a true-Stange-style locking COLLAR (MG30, MG15, MG17, MG131, etc.) did NOT suffer this phenomenon overtly.)

Your problem is NOT in "the bolt and barrels", per se, so switching them is likely to no avail. Fix the bolt-latch-lock problem. ;-)

-TomH

[junglewalk]

So Tom, you're saying the bolt latch lock may be worn and /or the spring tension for the latch may not be what it should be?........bh

[TactAdv]

So Tom, you're saying the bolt latch lock may be worn and /or the spring tension for the latch may not be what it should be?........bh

Yes. There you go...... ;-)

Hmmmmm, nice how you distilled that down so succinctly, I guess I should try it that way next time and save a few thousand words. Thanks! ;-/

-TomH

[42rocker]

But the thousand words taught so much, learned a few things in this tread.

Then to conclude with a simple replace this or that, I like that

Later 42rocker

[Bil]

Tom-I love it when you talk technical like that!
Actually,it is good to know we have folks here that have this kind of knowledge! ---bil

[TactAdv]

BTW, this phenomenon being discussed here, primer movement under high-pressures (set-back), is actually a well-known and understood event and has on a very few occasions actually been used as the sole operating principle for automatic cycling of a firearm action, being called "primer-piston actuation".

One of the original contestants in the competition to find what ultimately became the US Cal .30Carbine (Williams design) used primer-piston actuation to cycle the action. Also, in the mid-60's one of the unsuccessful entrants into the totally-failed SPIW competition also used this type of action.....point being, if INTENTIONAL primer set-back can be used to cycle the action of an automatic design, UN-intentional set-back is DEFINITELY "not good" to be happening in a design that was never intended to accommodate the movement of the primer........ ;-)

-TomH

[mggunner34]

Wow thats definately some great insight into how this weapons works, and sure enough I took the bolt latch out of the barrel jacket and its in bad shape, the surfaces are pretty worn and it actually has missing material on the inside part of it that contacts with the lug on the bolt. this is the best picture I could muster at the moment, sorry its not too great but you can clearly see the wear.

[mggunner34]

Anyone have any advise on where to get a new one? I've checked a few places online and I'm not sure what they are calling this specific part. too bad sarco doesnt have pictures because they have a bolt latch early and late (whats the difference?) is that the same thing we are talking about here?

[Matt]

they are talking about the nut on the back of the bolt that holds it all together. there is only one style of bolt catch. check on gunbroker too. there is a polish guy who sells a lot of mg parts. - matt

[mggunner34]

how about this

http://e-sarcoinc.com/mg34charginghandlekit.aspx

it lists a charging handle spring and charging handle stud, as I know the charging handle has no spring I think this may be it.