Yugoslav Reanimator
The ZB30J has been on the American market in the form of parts kits for a few years now and even longer in the form of dummy guns for display. Most builders jumped on getting a kit to only to discover a few unique quirks about the 30J preventing them from being constructed into functional semi autos. As a bit of background, the 30J is a variant of the ZB30 made exclusively for the Yugoslav armed forces. Brno was contracted to create a specialized version of the 30 that utilized an adjustable gas block similar to that used on the Romanian contract 30's. Yugoslavia had mountains of different weight 8mm ammunition in it's armories from WW1 captures along with various sources post war and wanted a LMG that was able to run it all. As part of integrating the adjustable gas block into the design Brno shortened the overall length of the gas system, including the piston, by approximately 4". Another odd feature of the 30J is the lack of cooling fins on the the last ~10" of the barrel toward the chamber end; there is no real reason known behind this design choice.
One would suspect that there are plenty of intact barrels for these 30J's in the US, imported prior to 2005, for all of the kits on the market. The truth is the opposite; there are very few barrels in the US compared to the amount of kits available on the market. With a majority of them paired to transferrable MG's they seldom come up for sale and when they do, they command eye wateringly high premiums compared to other MG barrels. Supply and demand at it's finest. For a long time the 30J was relatively unknown in the States and not until recently have they garnered much attention, therefore driving the prices through the roof.
I wanted a 30J to be a part of my Yugoslav Small Arms collection for quite a long time but kept waiting for any available barrel to turn up. I finally got tired of waiting, so I bought a kit and decided I would find a way to make my own barrel come Hell or High Water.
This is my own personal experience and not a direct guide, your mileage WILL vary.
Step 1 : Barrel
I decided to create a lined barrel using the cut barrel sections as the outer sleeve. Over the course of military history, there have been many instances of this being done including the Chinese lining ZB26 barrels to 7.62x39 and the United States using Stellite to line machine gun barrels to prevent erosion. Most practicing gunsmiths have also lined antique rifles from their obsolete rimfire cartridges to accept modern ammunition. This process goes against any and all safety precautions and can be extremely dangerous resulting in permanent injury and/or death if done improperly. This is your reminder that when firing this rifle there will be pressures of up to 30K psi within inches of your head, face, neck, and chest. DO NOT ATTEMPT THIS PROCESS UNLESS YOU ARE FAMILIAR WITH ALL DANGERS AND RISKS INVOLVED.
All of the parts were laid out and measurements were taken of the original chamber depth, most importantly rim to breech face using a go gauge. This was 0.110" and is notably the same exact measurement of the chamber erosion gauge that is on the gunner's combo tool/hammer. The barrel was stripped of all components and the bores were cleaned extremely well. Each barrel section was initially piloted to .578", drilled out completely to .593" and then reamed to .600". This is a very slow and tedious process on a manual lathe with the need for constant chip clearing and essentially flood coolant. A gun drilling machine would make very quick work of this process. A shoulder joint was turned between the finned and knurled section; when slipped together the seam was almost invisible.
With the barrel sections bored and reamed to size, the next step was turning the liner to size. Initially, I had hoped to create an interference fit between the liner and sleeve. A Mauser 24/47 barrel was turned to .603" OD along it's entire length and attempted to be press fit into the barrel sections. Unfortunately, when attempting a press fit over 6" without the help of dry ice and a furnace, its very common for things to go wrong and that's exactly what happened. The liner encountered a spot in the sleeve approximately halfway down and froze into position. With 20 tons of pressure on it, the exposed part of the liner bent and the process had to start over.
After the stuck liner was removed, a new liner was turned from a MG13 barrel. However, this time the liner was now .595" cross it's entire length until the last 2.850" on the chamber end, which was turned to .700". A corresponding .702" bore was made in the chamber end of the sleeve. The liner now slid into the sleeve hand tight and it was fixed into place using Loctite 660 retaining compound. This was chosen to due it's excellent gap fill and curing properties along with it's incredible shear strength; it is essentially made for applications like this including rebuilding critical worn cylindrical fits. With a shoulder on the liner, there is no way for it to walk forward when firing or elongate headspace. The chamber was finish reamed to the specified measurement, original gas port recut, and the last 2.5" of liner was cut off to allow the flash hider to screw on. Headspacing the barrel to the receiver was a rather convoluted process, with many points of adjustment. The shoulder was turned to seat the barrel in deeper to the receiver, the locking collar wedge was adjusted to get the correct lock up, and once the receiver was rewelded a final pass with the finish reamer was needed.
Step 2 : Reweld
The ZB30 is essentially the grandfather of the Bren Mk1 and shares a lot of characteristics with it, but the parts are not close enough to interchange. This is important when considering that all of the welding jigs on the market are marketed towards the Bren series. They'll fit very close but not quite right. I welded the receiver using the upper half of a Bren jig that I extensively modified as a backer and general alignment tool. The torch cuts on my particular kit were rather wide, starting at 3/8" and moving up to 7/8" at it's widest. Before the receiver was completed, two denial bars were added to prevent an original bolt/carrier and original lower from fitting.
Clean up on the internals was rather easy with the largest internal feature being the rails for the carrier and dust cover. The outside was also not too difficult with the biggest challenge being refitting the rear sight mechanism.
Step 3 : Semi Auto Conversion
The Bren series has had semi auto parts support for quite a few years in the United States and since it shares a lot in common with the ZB series, it can be used as a strong base for conversion. I am unaware of a hammer fired conversion of either platform and, even though Smith Machine Group has done one on the DP-28 which utilizes a similar amount of space in the lower, I decided to go with a striker to simplify the build process.
As originally designed, the ZB30 is an open bolt, tilting block style machine gun which utilizes a sear catch on the bolt carrier. When the sear drops, the carrier moves forward under spring pressure driving the bolt into battery. To achieve lock up when going into battery, the carrier has a camming horn. This horn is machined in a way to drive the tail end of the bolt upwards against the locking shoulder while simultaneously striking the spring loaded firing pin at the end of bolt carrier travel. This unique operating system presented a few challenges, namely the addition of a striker into the system and the extension of the firing pin to the rear face of the bolt. Before starting any other work, the sear catch was milled off and the carrier modified for the denial bar along with restricting the lower to semi only with welding.
In most Bren conversions, the firing pin is extended by way of a slitting the middle of the camming horn and using a custom pin that is machined flat in that area. In my experience, I know that sharp straight corners in load bearing systems lead to stress risers and eventually cracks - the camming horn is subjected to a large amount of load under recoil. Using a highly radiused endmill, I machined away the sides of the camming horn in order to fit a "wrap around" extension into the bolt body. This brought the striking surface of the firing pin back to the end of the bolt allowing a striker to hit it. This was pinned in place with a set travel distance.
The striker was fabricated from a piece of 1018 I found in the shop; the shape was derived from measuring the distance on the rear of the bolt/carrier at full recoil. In my own work around of having to "remake" the recoil and buffer system, I opted to route the striker spring into the stock itself through the socket in the lower receiver. Using a series of aircraft extension drills, a hole was made that ran at an angle to the contour of the stock and terminated at the recoil spring housing. A guide rod was welded to the rear of the striker and a hole for the bolt recoil rod was drilled as well. This allowed the bolt to move independently of the striker and kept the spring system easily accessible for adjustment during the tuning phase. A few checks were made for function and strike reliability.
Step 4 : Tuning
The tuning phase of any semi auto conversion is usually the most frustrating. This project stood out as being the most challenging to date. Starting with surplus hard primed ammunition, light strikes were an occasional problem. Initially the gun would lock up under recoil and refuse to reciprocate. This turned out to be the system binding at the denial pin and the narrow extractor cut. Once those were remedied, testing showed that the gas system just didn't have enough volume to reliably eject casings. This is common on semi auto conversions using a balance system between two springs. The gas port was opened up to .250 along with the block; the original recoil spring was trimmed by 1.750" and the gun showed signs of wanting to cycle. Using high pressure 198 grain surplus ammunition, it would eject and feed occasionally but not consistently. After swapping the original recoil spring to the outer recoil spring of a Bren Mk1 and moving over to new manufactured ammunition, the gun cycled and functioned very well. Occasionally, there would still be feed jams which I attribute to the magazine that came with this kit but overall is acceptable.
Conclusion
Overall, The ZB30J Project was a successful undertaking. Granted there were several roadblocks and associated headaches along the way, however none of them were too egregious to bring the project to a complete halt. This build has been holding up well and has approximately 300 rounds through it at the time of writing. All of the welds are still fine and the liner hasn't moved. The provided magazine seems to work best when only loaded to 10 rounds. It's an absolute beast to bring to the range and does fairly well at the 200 yard gongs.
Each build is different and faces it's own unique challenges, but I hope that this helps you in your journey in creating your own semi-auto ZB30J.
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