7.62mm L4 series........

[Joe H]

Yes, I have noticed that also. That's Joe's transferable converted/ running 54r. It was test day and on the lowest gas setting it was still way to much. It also has the MKI single return spring. A MKII spring has since been installed and we are trying to figure out how to choke the gas down more. Evidently the mag didn't run as we'll at the Marietta shoot so back to the drawing board. If I could get my hands on some factory drawings for the 303 mag it would be helpful. We want to modify the follower and having the original geometry of both mag and follower would be very helpful. Peter?

C310,

IMO if you need less gas then you have to modify the holes in the regulator. The regulator has to be annealed to drill it. If you modify the barrel inlet the gas regulator will only work for the holes that are smaller than the inlet. If the inlet hole is smaller than the #1 reg hole the regulator will have no effect on gas pressure, at least that's my opinion.

On the Bren mag issue, I doubt modifying the follower on a Bren mag will help using the 7.62 x 54R. The modification of the ZB39 mag to run 7.62 x54R , including the one you were using essentially includes a helper spring to point the bullet tips up after about 12 rds. In order to maintain the high capacity the orig. mag spring is weakened so that the at capacity the total force of the helper and orig. mag spring are close to the orig. spring or the first rds. can't be stripped. That leaves later rds. (below about 12) with a weaker than design spring. It's all a compromise because the mag case does not have the right geometry (Radius) for 7.62 x 54r round.

Below is a summary of the single stack angle (actual mag is double but the proportions should hold) and the inside radius of the mag.

Magazine/cartridge---------- Stack Angle ---------- Magazine Inside Radius

.303 British Bren ------------ 9.78 ------------------ 6.31"

7.62x 54R ---------------------- 5.38 -------------------- ?

8x56R ZB39---------------------- 3.92 ------------------- 10.0"

You can see from the above table why the ZB39 mag is useful for the 7.62x54R because the stack angle is much closer to 7.62 x 54r than the Bren. When you put 54R in the Bren they start pointing up as more rds. are loaded. On the ZB39 they point downward which is much easier to counter with a helper spring.

Using the above table and the stacking angles you can predict an inside radius for a 54R mag of about 9".

Awhile back some pics. were posted of North Korean PKM's with hi capacity 54R box mags. Scaling the pic and making some reasonable assumptions since I doubt Kim Jung Il will send us one I came up with a inside radius of about 9". The pic is attached below.

IMO to get a 54R mag that really works you will need to design the mag case geometry to fit the 7.62x54R round. Helper springs, follower mods won't do it.

Joe

Attachment 57919

[Brian Dick]

c310, He can do it. Just waiting on you.

[Peter Laidler]

Good afternoon Joe. Good technical points and really got the grey matter ticking over............ Re the gas port in the barrel situation, I see the point. That a larger hole further down the gas passageway will have no effect in a physics theory lesson but in a situation like a Bren, we have to remember that the gas being delivered is an explosive force as opposed to a sustained 'push' such as you'd get in , say, a steam engine or even a high performance car engine. Here we have an impulse force which delivers almost mind blowing accelleration to the piston and with it, the associated inertia/momentum. And it's this momentum that we really want to control. And you HAVE to control/reduce it before it gets to the piston

Re the magazine. Mmmmmmmmmm. Perhaps the most difficult part of any weapon to design and even MORE difficult to get right! An apprentices classroom discussion about just this subject could go on for weeks but here goes....... At the end of any travel DOWN the magazine, however the rounds manage this, the only point that matters is that the top round in the magazine (the round waiting to be fed) must momentarily be positioned EXACTLY in the same position of the previously fired round and all of the other rounds before that. If not, it's doomed to fail. All that we need is for that round to be presented at the correct angle downwards and inwards and it will (?) work/feed...... yes?

I've never converted a .303 mag to 762R so I am theorising that the magazine radius of/for the requirement between the two rounds is marginal and even less so because they are double stacked. Thus, with a bit of titivating or perhaps adding a slightly more pronounced downward 'wedge' effect to the front of the platform should take up the slack so to speak.

This can work as the mag platform is able to 'articulate'* to some degree within the magazine so this should be able to cater for what's needed. After all, the good quality HK .;22" rifle magazines do a more difficult job really well.

* 'articulate' is the wrong word as it can't articulate as it's solid but I can't think of another word to express what I mean. Maybe 'level itself out' or 'able to accommodate' might be a better choice.

[Joe H]

Hi Peter,

Can't argue with your theory on the gas because I haven't tried it. Here in the US were are stuck with semi-auto unless we have a very large "Toy budget". I've found that my Bren's , a Mk I & MkII both require a #3 and for total reliability a #4 gas setting. I redid the barrel inlet, regulator holes and outlet holes in a .303 barrel so that the #2 setting was equivalent to an original # 4 setting. I haven't had a chance to try it out yet.

I've have no first hand info on full auto. Unlike C310 FA 54r Bren my semi-auto 54R runs about the same as .303 British requiring a 3 o4 setting?

The info I posted about the mags. was a result of my research trying to build a 25 rd. 7.62x54R using a ZB39 mag. It does work with semi auto but I had my doubts about FA due to speed and reduced spring tension. I previously posted the details on this Board. I have tried to "articulate" the follower only to have them jam about halfway thru the mag. You are definitely right about the round being at the right position for the bolt to grab it. A fixed follower won't work.

On the other hand if you grind down the horns on ZB39 follower and remove the follower guide so that the follower can freely rotate the mag will work with a 7.62 x51 Nato . The difference being that you are dealing with a almost 0 stacking angle rimless case. The steep rimmed cases seem to be the culprit.

I've attached a pic of 17 rds. of 54R in a .303 Bren mag. First round is fine as rounds are added they rotate. If you eliminate the follower guide so that the follower can rotate they will at some point jam. The ZB39 mag is just the opposite at about 12 rds. they point down and don't clear the lip.

It would sure be great if someone could come up with an easy solution but so far I haven't heard of any.

Attached are pic of 17 rds. of 54R in a unmodified Bren mag & a unmodified ZB39 mag. Bren on right.

Joe

Attachment 57924Attachment 57925

[c310pilot]

Peter, the C130 is a great airplane. Unfortunately the Cessna 310 could sit on the C130's elevator! It's a small twin engine 6 seater. I owned a 61' 310 F model long ago. The username is a carry over from a by gone era. I now fly CRJ 200/700/900s for US Airways.

I might have to get Joe to chime in on this board as he has much more vested interest in the mag conversion. I have only been present for the successful tests and have not witnessed any of the failures.

[TactAdv]

It would sure be great if someone could come up with an easy solution but so far I haven't heard of any.

All-
The issue you are facing here isn't voodoo, it is simple mechanical design cross purposing. The entire reason you see what you call "nose diving" of the cartridge column when trying to use 54R cartridges in the ZB39 (8x56R) magazine is the issue of what is properly known as column drag. This, particularly with a prominently rimmed case like the 54R, is the problem of the total OUTWARD pressure exerted by the cartridge column as it is increased by stacking in the magazine box. This is most prominent with rimmed cases as the near total outward pressure is thus concentrated solely on the very thin rims themselves, against the inside sidewalls of the magazine box. Parallel sidewall, rimless, or rebated-rim cartridges might seem to have more contact area but this in practice is not generally so. The effect you are witnessing is that at some load value (# of 54R cartridges stuffed into the ZB39 mag), the total RIM DRAG of the 54R rims on the inside of the magazine box body causes the column to begin stalling out, and will eventually jam solid......the rims are dragging so much on the inside they cause the very tiny contact area to become a fulcrum point and the then heavier weight of the stack of the cartridges forward of that fulcrum point start to lower due to the free weight nature of the still moving forward mass of the column. THAT is your "nose diving".

The sole area of concern here, the "why" of this all lays in the magazine follower itself, namely and specifically, the SHAPE of the original 8x56R cartridge dummy shape on the top of the follower, and how that shape directly determines the internal outward stack force of the cartridge column against the inside walls of the box body. There is no issue loading the ZB39 mag full to its limit with the original 8x56R cartridges, because the follower dummy shape was properly designed to position cartridge #1 & #2 in the proper centerline-to-centerline relation, i.e., proper "height" of cartridge #2 over #1, and succeeding......as they stack up in the box body. That proper relation, IN COMBINATION with the angular measure of the radius of the box itself is what determines the internal frictional load of the cartridge column, as a total value. This is why follower dummy shapes differ with cartridge geometry and magazine capacities!

In order to use 54R successfully in the ZB39 magazine box body, you will need to relieve the much increased internal sidewall pressure, i.e., "DRAG", that stuffing 54R into it causes. And that issue is solely caused by the original 8x56R follower dummy shape positioning the 54R cartridges too far apart in the stack, cartridge centerline-to-centerline. To visualize this, you need to realize that a cartridge column that places the 54R cartridges closer together, though "higher", in relation to each other will reduce the internal frictional component as that column of cartridges tries to articulate through the ZB39 magazine box body in trail.

The way to affect this ("affect", with an 'a') is to modify the ZB39 8x56R follower dummy shape such that it will place cartridge #1 closer to the magazine centerline, versus just placing a 54R against the original shape. There is another issue at play too, in that the 8x56R follower dummy shape is angled to guide the 8x56R cartridges to the proper angular positioning inside the box body (proper for 8x56R), so that when the cartridge is stripped, the shoulder properly contacts the steering shoulder of the box body as it leaves the column stack and is thus properly positioned to enter the barrel chamber cleanly. You will need to modify, by milling, the original 56R dummy shape to position a 54R cartridge more toward the magazine box centerline, and slightly increase the "inward cant angle" of the 54R cartridges. Once the 54R cartridges can sit "closer and higher" in relation to each other, you will have (in general terms) reduced the internal drag sufficient to function properly. PROPERLY stacking 54R cartridges will reduce the available column height of the box body capacity to less than the original 25rds of 8x56R, though.

The ideal way to do this is mill OFF the original 8x56R dummy follower shape, then weld on a newly contoured replacement. Once can use a Lahti-Saloranta LS26 magazine follower as a donor shape, or as a guide. Or you can wing it and carve one out. Using plastic to first carve a shape and using tape to affix it to the milled flat ZB39 works to test the concept.

Restoring the internal cartridge sidewall drag to a level approximating that designed for the original 8x56R cartridges in a column will cause the ZB39 mag to feed 54R well. There is no need for any "helper spring" when the internal frictional values are restored, and thus no "nose diving" either.
-TomH

[Peter Laidler]

If we could all get our practical and theoretical heads together and do a few hours of brainstorming I think that a combination of both plus a bit of the bleedin' obvious would/could solve this problem quite easily. I wouldn't be starting with a blank sheet of paper but a few 'knowns' such as a) a gun that we KNOW will reliably shoot and function with 762x54R (yours'll do C-130!) and a magazine that we know will feed and function with a similar round. Yep, a standard and plentiful .303 Bren mag.

Together with a small workshop and................

Then start from there and deviate to 'unknowns' or variables if we need to. You game TacAdv? C-130? Joe? Brian?

[Joe H]

Well Peter,

You have thrown down the gauntlet! I've got a modified ZB39 mag that works with 25 rds of 7.62x54r in semi-auto, maybe or maybe not in full auto. The ZB39 mags. were available in the US for $25, but no longer. They are now a hot item because of their use with Bren caliber conversions. The best outcome, as Peter mentioned, would be a conversion using a standard .303 Bren gun mag.

If Tom is correct it should be "a piece of cake". If not, it's a much more formidable challenge. C310, the ball is in your court. You have access to the 54R FA Bren and the ability to try Tom's relatively simple conversion. I've got a Lahti-Saloranta LS26 mag I'd be happy to send to you. BRP has very inexpensive Bren .303 followers.

Joe

[Peter Laidler]

And C-130 could whip over here in his plane and fly me over with a xxxxxxg big box of magazines............

[TactAdv]

Guys....remember too that I left certain things unmentioned, as they seemed obvious, but in the interest of shining light on all let me state that.....

-Many people don't realize that most times in designing a new gun like the Bren (nee ZB Vz26/30, etc.) the magazine is designed first, once the cartridge is decided, then the gun mechanism is built to accommodate THAT. Anything AFTER that first magazine design, matched to the original gun concept, is just many forms of modifications.

- The magazine follower shape and size is only the first element of the proper positioning of the cartridge column that the magazine body is designed to contain and control. Once the follower configuration is designed, the mag box body has to be matched to hold the cartridge stack at that relative spacing, so the box sidewalls are designed to THAT spacing, laterally.

- No matter how much work you modify the ZB39 follower to better suit the lateral spacing of the 54R cartridges, you're still stuck with the box body sidewall spacing in the ZB39 magazine design.....which probably is not ideal for the 54R cartridge. So, that issue is still to be reckoned with.....

- An LS-26 magazine can be used to judge the lateral spacing thusly determined for the unique 54R dimensions. That spacing, laterally stacking of the cartridges, is what you want to try and emulate as best as possible.

-TomH