In recent years, it would seem as if almost all of the massive Cold War stocks of submachine guns have been hitting the US market in the form of parts kits all at once. There's been waves of PM-63's and oodles of PPS-43's out of Poland, M56's and M49/57's out of the former Yugoslavian Empire, and the list goes on with even more extremes of variety on smaller scale imports like the PM12's and LF-57's out of Italy. Spain has been no exception to this sudden surplusage - take a quick look online and you can find CETME C parts kits for under $200 all day long. One of the more interesting things to come out of the Iberian Peninsula has been the Star series of submachine guns. First the Star 45's, a clunky looking amalgamation of a MP40 and a fever dream chambered in 9mm Largo. Then, more recently, the Z-70 series chambered in 9mm Luger. This model was a much needed redesign of the Z-62 and it's progressive trigger system, promising increased production at a cheaper cost. Spain is known for a rich history of cottage industry level gun makers, but everyone always like the "more for less" approach when presented. 

      When introduced to the parts kit market in the US, this oddly futuristic looking design left some builders scratching their heads wondering how to make it a reliable conversion. I've seen a few different attempts at striker style builds, and each with their own pitfalls and successes. Outside of this, there's been a large void of resurrected kits due to the lack of new made barrels on the market. When the latest batch of these kits was imported, Indianapolis Ordnance began producing new made barrels to original spec. I always like a good challenge so I got a kit and barrel with a few requirements for this conversion :

1. Must be hammer fired
2. Must retain original military configuration appearance (apart from extended barrels)
3. Must be retain the ability to move or remove the lower receiver for cleaning

With those requirements I've developed a build and conversion that is surprisingly effective and reliable.

Welding

      The Z-62 and Z-70 series are the epitome of "Close is Close Enough". Their construction leaves a lot to be desired in terms of interchangeability and overall matching appearance. I believe that a significant portion of the manufacturing process is essentially "eyeballed" and precision fits are machined later. Rather unsurprisingly, the entire affair is brazed together rather than being welded. This means that some portions of the receiver will need to be rebrazed and then other portions will need to be welded. Silicon bronze is preferred for this due to it's availability and ease of use; its also a fluxless braze alloy when used with TIG which is even better. 

      Due to the internal features of the receiver, two jigs are required to get it back together - one for the rear 2/3 of the tube and one for the shroud/front trunnion. The charging rod anchor which is brazed into the interior of the tube just forward of the ejection port negates the possibility of using one singular solid jig. 

    The first jig is simply a bar of aluminum turned to diameter with a groove milled into it to accommodate the original bolt locating features of the receiver. I cut this groove significantly deeper to allow the denial bar to be located and welded in at the same time. A piece of pipe was cut and bored to act as a very precise fitting sleeve around the bar - once the receiver was welded and allowed to cool, the sleeve is used as an "anvil" to allow the jig to be pressed out of the receiver with a hydraulic press. This prevents having to drill the jig out due to weld contraction and creates a much larger bearing surface for the pressure on the end of the receiver. To set the length of the rear 2/3's of the receiver, the lower and folding stock can be used to set distances. The second jig is multipart consisting of the rear, the main body, the nose spacer, the nose cap, washers and bolt. The main body and nose spacer can pass by the charging rod anchor, allowing the nose cap to be tightened down allowing the shroud to be held in place and distanced correctly. To adjust the length of the shroud, washers are simply placed in between the nose spacer and the interior of the trunnion. 

Bolt

      Even though the bolt may look very complex at first glance, its really quite simple. There's two push rods at the face, one for the bounce safety and one for the firing pin which needs to be welded shut. The bolt itself is already through drilled and equipped with a spring loaded firing pin cutting the work needed in half essentially. The rear cavity of the bolt where the rate reducer puck would originally sit has a tight fitting slug of aluminum pinned in place. This prevents the recoil spring from interacting with the hammer in any way while also serving as the new reducer, although a little bit lighter. A firing pin extension and block is machined and pinned into the place of the prior bounce safety and firing pin actuator. The entire bolt is then milled to allow the hammer to strike the firing pin extension and remove the full auto sear catch. At this time, the existing geometry of the bolt's "feed surface" needs to be swept back slightly to accommodate the new hammer reset distance - failing to reshape this surface will result in the bolt locking into place on the recoil stroke and possibly breaking the hammer. The original locating cut on the side of the bolt is deepened and widened to accommodate the new blocking bar inside the receiver's diameter.

Fire Control Group

      The FCG sits in the lower receiver which is originally held into the upper receiver with two split pins. To create a secondary blocking bar, the rear split pin is welded into the upper receiver with a threaded hole in the middle of it. A corresponding hole is made in the lower to allow the removal and installation of a screw underneath the grip. This style of denial feature ensures that even if the stock is deactivated to create a pistol, the lower can still be swung down from the upper for cleaning.

      To achieve the most important goal of this build I had to get rather creative with the lower receiver. The original mounting points for the FCG components is completely milled off and brought down to the sheet metal's bend. At the front of the trigger pocket, a shelf is welded into place and drilled/tapped. A commercial PPS-43 FCG pack is shortened similarly to my PM-63 conversions, then drilled under the hammer to allow the whole assembly to be screwed into the shelf. It is important to keep this screw location under the hammer in the event that is loosens - the hammer will not allow the screw to back out far enough for the entire FCG to come off of the shelf. The original trigger is cut in two places with the original mounting point being welded back onto it, about 1/4" of material is removed in this process. A corresponding hole is drilled into the lower as a pivot pin location. 

      To make the hammer fired system work, a series of linkages is required to bring the trigger location from the rear to directly underneath the sear. A "pivot" is required to link the trigger and the sear together. The most important part of this system is that all of the points move freely and have zero extra "slack" in their range of motion. If there is "slack" the trigger pull will not reliably release the hammer and conversely have the disconnector engaged far enough to catch it on rebound. The style, diameter, and material of the linkages that I found to be the best are .125" coated steel wire - it is malleable enough to be bent into the required shapes but stiff enough to resist fatigue from shooting. The original fire selector switch can be modified to be used as a safety at this point. 

Conclusion

      With a few coats of VHT Wrinkle plus, this build looks like a factory new Z-70B. At the range, these conversions perform exceptionally well in all of the configurations - pistol, carbine, and SBR. The only hang up I've discovered with this process is that the Z-70 mags themselves are rather junky with more than a few of them having to be modified to simply feed and some of them being relegated to the scrap pile. I've taken to marking the functional magazines with the serial of the build they work with. This prevents chasing phantom culprits and ensures the customer always has a functional magazine with their build.

      I hope that my experience with the Star Z-70B platform outlined here has helped demystify it's swap to semi auto, I hope more than ever that more of these Spanish TEC-9 look-a-likes get built and back out on the range!