Cranky ZB-39 Mags (ideas)?

[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

[djandj]

I'll experiment with the helper spring Joe. Where did you get/make the spring guide on top? It looks like it has a square top to match the underside of the mag follower. Nice.

Just to report in so far, I have had some success in leaving the anti-tip in and simply adding a couple of pieces of sticky back felt to the top of the follower in front. This causes the first round's nose to pop up a bit more. Naturally each round follows a bit further up. With this (humble) experiment, I think I can get 15 reliably. Taking it to the range tomorrow to try. I'll report back.

[djandj]

UPDATE! By just throwing in a 3 inch spring from Home Depot into the front compartment, I managed 15 round reliably and up to 20 now and then. A great improvement.

[Peter Laidler]

They do say that the most complicated part of the weapon as a system, especially in a magazine fed machine gun is the actual mechanics of the magazine. One point that was al;ways emphasised time and time again was the fact that the FIRST round fed from the magazine was the most difficult to solve because the breech block is fired from very close to/behind the top round and it has to strip the round off the already tensioned magazine spring with ONLY the loading imparted to it by the return spring - that in itself has to overcome the friction of the tensioned magazine spring.

The next rounds should/will be easier because now (in the automatic role), the breech block starts its loading/feed run from much further back in the body. PLUS it has a greater spring load as a result, PLUS it has already gained a bit of inertia.

In a Bren, especially with the early Mk1 magazines, the top round (or the bottom.....) being fed is CONSTANTLY trying to drag the NEXT round forwards. As a result, it is constantly dinging the nose of that round into the inner face of the magazine and it didn't take long to find dimples being formed inside but protruding OUTWARDS in the soft mild steel case of the magazine, just above the front locating lug, that would make magazines difficult to insert into the guns.

That's the reason why there was a hard, spring steel rubbing strip placed into the next mark/type of magazine. Have I gone off at a tangent yet...........?

Anyway, there's another bit of useless Enfield related info

[Joe H]

Djandj,

You did exactlty what I didi the first time around. The spring OD was around .78" Seemed to work fine. Then I started getting mag jams. The coils of the two springs were jamming. I've been improving the design since.

I just changged the spring wire to .055 from .051 greased the springs and oiled the mag. Did a little polishing and now am getting 25 rds reliably.


Joe






i

[djandj]

So Joe, where do you find 5" .55 springs. Home Depot only had a few compression springs, so I got what they had. I haven't played with it enough to say for sure, but it appears that the two springs are not in conflict (so near as I can tell)

[Joe H]

I haven't played with it enough to say for sure, but it appears that the two springs are not in conflict (so near as I can tell)

Hopefuly it will stay that way. I wind my own springs on a lathe. Pretty simple if you have access to a lathe.


Joe

[Vincent]

Ace hardware has a better selection of springs than HD. You might find one that will work there.

[djandj]

Good idea, I'll see what they have in the 5" range.