Cranky ZB-39 Mags (ideas)?

[djandj]

I took my Bren MKII (converted to 54R) out to where I can use the mags at full capacity (as opposed to blocked to 10 rounds. I was using two different ZB-39 mags with Russian Steel cased surplus ammo. I had a real hard time getting the mags to feed with more than 13-15 rounds in them. they kept dipping nose down and thus would not feed. Any ideas on how to resolve that?

Thanks,.

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[WarPig1976]

Are you getting "rim lock"? Something to try is load each round so the rim is in front of the last.


The soldier above all others prays for peace, for it is the soldier who must suffer and bear the deepest wounds and scars of war.

Douglas MacArthur

[Joe H]

djandj,

13 - 15 rds isn't bad with the ZB39. You can get 20 rounds reliably with a helper spring. You may get 25 rds (mag capacity with 8x56r) but it won't be consistently.

Check this link for a detailed discussion of the ZB39 mags. & 7.62x54R:

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The pic shows how I set up mine.

Attachment 46793

Joe

[djandj]

djandj,

13 - 15 rds isn't bad with the ZB39. You can get 20 rounds reliably with a helper spring. You may get 25 rds (mag capacity with 8x56r) but it won't be consistently.

Check this link for a detailed discussion of the ZB39 mags. & 7.62x54R:2

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The pic shows how I set up mine.

Attachment 46793

Joe

WOW! Very nice, but you still have the anti-tipping part in the mag. The first thing I was told to do was lose that so the follower can adjust to a better angle with more rounds in the mag.

[Joe H]

djandj,

With some leaves annealed on the mainspring I had occasional problems with rd #1 &#2 (first rounds inserted). Probably caused by the weakened mainspring. Using the guide solved the problem but it has to be ground back at the top to allow the rotation. If you want to use 7.62 Nato in the ZB 39 mag just remove the guide, use the original mainspring, no helper required.

As far as I know there never was a 20-25 rd box mag made for 7.62x54R. There was a 20rd made for 7.62x53R but it is not very adaptable to the Bren.

IMO the only way to get 25-30 rds of 7.62 Russian reliably in the ZB39 mag.is to change the curvature of the magazine which really isn't feasible.
Joe

[djandj]

Thanks Joe - I'm beginning to think that 20 rounds may be as good as it gets!

[Joe H]

djandj,

I can use 23 rds without too many problems. The last two start the problems (up to 25). 20 rds good all day. If you load 25 rds of 8x56r in the mag which is what it was made for and it's capacity the force on the last round is 14-15 lbs. So the force exerted by the helper spring must be deducted from the original main spring for the last round to strip, with the total force on the follower in the 14-15 # range . If you increase the force on the helper spring to push the tips up you must decrease the main spring. At some point the mainspring will become ineffective for the first round or two. This limits how much force can be exerted by the helper spring. At 25 rds there is very little space left above the follower so the solid height (totally compressed) of the helper spring comes into the design. Also the spring is unstable so some follower or lateral suport is required so the helper spring does not foul the mainspring.

Joe

[Peter Laidler]

It might be absolutely nothing to do with what I'm going to say but worth bearing in mind........ One of the interesting things about the Bren gun and the magazine feed was that unlike some guns before it, the feed horns on the breech block are fixed and as a result of this, when the top round has been pushed from the magazine, the next round is pressed down into the feed position. All well so far...., but........

When the top round has fired and the breechj block is now in cock mode, returning to the rear, the fixed feed horns now LIFT the whole stack of rounds in the magazine. So what you have in effect within the Bren magazine AND YOURS of course, is the whole stack of rounds constantly being not only pressed DOWN, but being lifted up as well. Albeit just a bit, but they are! On a Breen, this has the effect of constantly shifting the rounds making any internal magazine stoppage (except the rim-behind-rim scenario) a rare occurrence.

This is VERY interesting to watch in a skeletonised magazine on high speed video as they jump up 3/16" or so and then drop drastically then lift and so on down the magazine.

While I'm not familiar with your conversions, I wonder in my bones whether while this might be helpful in a Bren magazine specially designed to do this, it might have the opposite effect with your complicated mag platform magazines.

Just a thought

[djandj]

Wow Peter, and I thought the math was bad. That hypothesis is even more scary. The entire stack of rounds going up and down with each shot. I of course don't know what is happening inside during cycling, but I do know that when it sticks, I remove the mag to find a round usually half way forward out of the mag, stuck. Or, when I have 13+ rounds in it the bolt sticks back on the initial charging as the nose of the first round is buried in the mag. However, I have also noted that when loaded and I try to strip a round off a round with say a screwdriver, the tension of the spring sometimes causes a round to "stick" against the feed lips making it very difficult (if not impossible) to push a round out merely by pushing from the back like the bolt would do. On these occasions, I have to push the other rounds down in order to release the tension between the top round and the fee lips.

[Joe H]

I had mentioned that the feed horns drive the next shell up into the mag. Peter's comment that it is not only the next shell but the whole stack of cartridges in the mag that move up and drop back down added a lot more insight into why it is difficult to get the full 25 rds of 7.62 r in the ZB39 mag. You can balance the mainspring force and helper spring @ 25 rds to be about 14-15 # total which is the design force on the 25th rd of 8x56R which is the cartridge that the mag is designed for. The helper spring keeps the follower rotated upward to compensate for lack of magazine curvature. As you go from 20 to 25 rounds it becomes much more difficult for the helper spring to keep the follower rotated at an angle it was not designed for and push the whole stack of cartridges quickly back into place after the feed horns have pushed the whole stack back up into the mag. If you increase the helper spring force the cartridges will not strip.

djandj,

Anything above 10-11 rounds in a ZB39 mag using 7.62x54R without a helper spring will cause the problems you mentioned. Regarding the round sticking on the feed lips. Trying to push it out by hand is not what happens when gun is firing. Push it back in and with a small wood dowel push it forward smartly like the bolt would hit it and it will come out if the bullet is pointed up and not down into the mag. If you make up some dummy rounds you will see that any small nick on the rim can dig into the adjacent case and this will cause feeding problems with steel cased rounds. Look carefully at a jammed cartridge before you reuse it.
If you run some dummy rounds through your mag and you get jams you will quickly notice that the bullet tips are getting battered from hitting the front of the mag.

Another possible problem with a semi is the gas system. The gas system was not designed for closed bolt semi auto fire. The gas system has to move more weight and cock an additional spring. If everything else is operating without binding the gas system unless opened up is just marginal for a closed bolt semi. Bad return springs could also be an issue. IMO I think your main problem is lack of a helper spring.

Joe