With standardization of the AR-70/90, earlier AR-70 rifles have been sold as surplus and have been occasionally offered for sale in the United States with the machinegun receiver cut to comply with BATFE import requirements.
PLF of Saint Louis, Missouri, manufactures a semiauto AR-70 receiver, which is sold exclusively through JM Tech (Jeff, 847-334-2194, 11:00 – 9:00 EST). Combined with six other US made parts from the list below (total of 7) to comply with the 1989 import ban, you can build a semiauto version of this interesting rifle.
US parts (Previously Available)
There is also an adaptor that may be available to use a US AR-15 magazine (3 US parts). The adaptor attaches to the magazine. JM Tech also makes a “lockout” kit for the trigger group, which includes a hammer spacer, trigger spacer, and selector catch ($20). These parts fill the gap made by removing the full-auto parts, but do not count toward the 7 US parts required.
The AR-70 parts kit requires more skills to build into a rifle than many other parts kits. In my opinion, it is more complicated than either the AR-15, FAL, AK, Uzi, Galil, or HK. It requires TIG welding. MIG welding is simply not sufficient. It requires riveting and silver soldering is a plus. It also is made much easier by the fabrication of some simple fixtures, which may only be cost-effective if you are planning on building several. Magazines are also relatively expensive, costing from $60-$75 each depending on condition.
I found this, my first AR-70 build, to be very exciting and satisfying and learned many lessons that will make the next one go much smoother.
At this step I had already installed the new JM Tech barrel (1-9 twist, six groove, made by Wilson). You will still have the original barrel in place. Do not remove the old barrel until you have welded the trunion. Follow the trunion prep and welding steps as if the old barrel were still in place. The JM Tech barrel comes with a shoulder deliberately oversize by approximately 0.020″. This allows you to set the headspace without purchasing an expensive ($100+) chamber reamer.
I lathe-turned the shoulder until the bolt would just barely close on a Clymer .223 Remington NOGO gauge with the barrel hand tight. I figured with the relatively coarse thread pitch on the barrel, when I torqued the barrel on, it would close the headspace an additional .002″-.003″ which would result in a tight close on a .223 Rem GO gauge. Jeff at JM Tech recommended a tight headspace and I’ve also found a tight close on my .223 Rem gauge will be fine with a 5.56mm NATO cartridge. What I didn’t expect was for the barrel shoulder to be a bit softer than a FAL barrel have a greater crush factor than the 15 degrees I had planned on. When I had torqued the barrel to approximately 60-65 foot pounds, my headspace had closed to the point where the bolt would not lock on my .223 Rem GO gauge. I used a punch to indicate how far I had turned the barrel, then removed the barrel and cut the chamber .001″-.002″ deeper. When I reinstalled it, it closed nicely on my GO gauge.
Reassemble the rear sight and align the front sight with the rear sight. I aligned the front sight/gas block and held it in place while a helper scribed bottom dead center off the key on the underside of the gas block. Cut the notch to index the front sight base. Mount the front sight/gas block and using either a mill table (or lathe) index a scribe off the center of the front the front sight, then remove the gas block and scribe the second axis to precisely locate the gas port. There are other ways, but this worked for me. Using the drill bit supplied with the barrel, drill the gas port. After I made the cut, there was just the slightest movement in the front sight block. While this would be clamped by the flash hider assembly, I consider removing the gasblock and front sight for cleaning to be detrimental to accuracy and a weakness of the design. I took a wide, flat punch and tapped both sides of the keyway on the back half, which made the keyway slightly tapered. I then used a small plastic mallet to lightly tap the gas block on, making for the tightest fit – much tighter than the original.