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

[djandj]

This is beginning to involve "math" here Joe. I was told there'd be NO math involved in the operation of my Bren. (lol) I'll be happy with 20 rds. but actually part of my problem seems to be the main spring is too stiff as even at 15 rounds, the first few rounds often will not strip or get stuck on the mag feed lips. (or dip down of course). I have removed the anti- tipping device and polished the front lip of the mags. I will try that out at the range and see where I get and report back.

[Joe H]

djandj,

The spring is not too stiff at 15 rds. I assume you have the normal mag spring and have removed the guide. From about 11 rds the problem is the bolt return. On the top of the bolt there is a claw that grabs the cartridge and drives it into the the chamber. There is a reverse slope on the claw so that when bolt is driven back the reverse slope of the claw pushes the exposed cartridge back up into the magazine about 1/4". Without the correct mag curvature or a helper spring the cartridge does not pop back down properly so that when the bolt returns the nose of the bullet hits the magazine rather than going into the chamber. Look at the tip of the bullet. You can see it is hitting something. The cartridge position may look good when you insert the mag but the last shell may not be in the correct postion after the bolt has pushed it up.

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