Full-Auto to Semi-Auto Conversions

In the American domestic market, many hurdles lie between the consumer and NFA items, especially automatic weapons. The legal barriers, the expense, and the rarity of certain models can make it difficult to gain access to them. For some, these hurdles can be impassible.

To fill the demand, non-NFA copies of popular NFA weapons can be found, which permit a much wider audience to experience the next best thing. For short barreled rifles and carbines, the workaround is as easy as dropping in a new barrel, possibly with a shroud or a faux silencer to preserve the aesthetic. With fully-automatic weapons things get more intricate, as steps need to be taken to ensure that a weapon sold as a semi-auto will stay that way to keep things on the legal up-and-up.

Below are three examples of full-autos that made the jump to the purely civilian realm, their technical underpinnings, and in one case how it can go a bit wrong.

Doing it “Wrong”: FN's G-Series FAL

Dubbed the “right arm of the free world,” the FN-designed FAL (Fusil Automatique Leger, Light Automatic Rifle) became one of the signature weapons of the Cold War, being widely embraced by the European members of the North Atlantic Treaty Organization and other users. The FAL had a lot to bring to the table in terms of features, and is by all accounts a solid rifle design for anyone in the market for a magazine-fed, full power battle rifle.

This scarce Fabrique Nationale "G Series" FAL semi-automatic rifle with scope sold for $10,925 during Rock Island Auction Company's December Premier Auction in 2020.

One of these features was an interesting answer to one of the big question that dogged the battle rifle during its tenure as a main-line infantry weapon; “it can fire full-auto, but should it?” The recoil of cartridges like the 7.62 NATO aren't as dire as something like a dangerous game or anti-material round, but is still notable, and the needs of infantry rifle design call for lightweight, maneuverable weapons that don't have extra mass to absorb recoil, field mounts to improve control, swappable barrels to manage heat, or any of the other features that really let an operator rock and roll.

A closer look at the fire/safe positions located on this Fabrique Nationale "G Series" that sold at Rock Island Auction Company in 2020.

Other solutions were out there, but sometimes involved semi-permanent modifications to the weapon that could not be undone at the unit level, such as the U.S. Arsenal solution of setting M14s to semi-auto and then tacking a plain cap in place of the selector switch. The FAL solution was in the selector switch itself, which could rapidly be switched between a semi-only switch that could not engage the selector hardware, and a full-function switch that could reach all positions. Executing this swap is as easy as popping open the receiver, pulling out the current switch, inserting the new one, and closing it back up, and you can reverse it just as fast.

This scarce Fabrique Nationale "G Series" FAL semi-automatic rifle sold for $10,925 in June of 2020 at Rock Island Auction Company.

Of course, this sort of rapid transformation is not everyone's cup of tea. Among others, the BATFE isn't terribly fond of it, as part of their general posture against items that can be “readily convertible” into a machine gun. For perspective, the ATF generally considers 6 hours of bench work to be a reasonable measurement of “readily convertible." The original offering of semi-automatic FALs, known as the “G-Series” for their serial number prefix, was essentially the same weapon they were making for their military clients, only with the semi-auto switch installed. As sold, it was not capable of full-auto fire... but if you have a full-auto switch and about 30 seconds of spare time, this was an easy problem to solve.

A close examination of the fire/safe positions on this scarce Fabrique Nationale "G Series" FAL semi-automatic rifle that sold at Rock Island Auction Company for $10,925 in 2020.

While this arrangement originally passed muster with the ATF, they reconsidered, shut the door on further G-Series imports, and told FN to hit the drawing board and come up with something a bit harder to convert. FN's response was a revised receiver design, which eliminated the pocket for the automatic safety sear. With this revision, installing a full-auto selector switch just causes the hammer to ride down with the bolt carrier if the weapon is cycled with the trigger depressed. This has become the standard for commercial FALs in the United States, both from FN itself and domestic manufacturers building up their own emulations using new-made receivers.

Up on the Shelf: HK's Roller Locked Lineup

Part of a full lineup of weapons (including the FAL's German opposite the G3 rifle), Heckler & Koch built an entire line of Cold War era weapons around a single core action, a recoil-driven roller lock that was scalable across a broad spectrum of sizes and calibers. Originally developed for the joint Spanish/German CETME rifle due to post-WWII arms development restrictions on Germany, the roller lock assembly was found to be very flexible, allowing the core CETME/G3 design to be readily sized down for 5.56mm NATO assault rifles, 9mm submachine guns, and effective at both full-automatic and semi-automatic settings for military weapons and civilian sporting arms.

This rare, early pre-ban Heckler & Koch/Golden State Arms HK41 semi-automatic rifle sold for $23,000 at the December Premier Auction in 2020 at Rock Island Auction Company.

In turn, this permitted a then-novel amount of interchangeability of parts, features, and manual of arms; a shooter trained on a G3 rifle could pick up a MP5 SMG for the first time and be holding a very familiar feeling and operating weapon, and vice versa. This parts interchangeability extended to the core trigger group, which was designed as a unified pack that could be pulled out of the weapon as a complete unit, allowing for easy cleaning, the ready replacement of an out of order unit with a fresh one, or the swapping of trigger packs between weapons, even of different calibers.

When it came time to bring these weapons Stateside, steps needed to be taken to prevent someone from shoving a full-auto trigger pack into a semi-auto carbine or rifle, so as to keep the weapons from being considered “readily convertible." HK's answer was the introduction of a new pattern of receiver. The first, dubbed the “hinge pin” or just “pin” receiver, was the base HK receiver everyone already knew, which used a hinge pin just behind the magazine well to secure the trigger group to the receiver. This mounting method was a handy way to keep the major parts together while opening up the weapon for maintenance, similar to the front hinge connecting the upper and lower receivers on an AR-15. The second pattern is known as the “shelf” receiver, which replaces the hinge pin with a solid steel lug, which engages a slot on the trigger housing to hold everything together on assembly.

This fully-automatic Heckler & Koch HK51 machine gun sold for $31,625 in December 2020 at Rock Island Auction Company.

This shelf extends far enough into the trigger group that it is physically impossible to assemble the weapon with an unmodified “pin” trigger pack installed, and as all the factory-made full-auto packs are “pin” pattern there's no way to drop an original full-auto pack into a shelf gun. To get a full-auto “pin” pack to fit requires significant, obvious modification, as you physically need to cut pieces of metal away to make a slot for the shelf, and any pack not properly marked and registered with the ATF is a federal offense.

Gut It and Start Again: Open Bolt Weapons

A de-facto standard for submachine gun design, the open bolt action has many virtues for a small caliber military weapon. Of course, from a military perspective only two of these really matter; it's cheap and it works. As a design for a combat weapon, it was a staple from the post-WWI period clear to the modern day, with sophistication ranging from the classic 1920s machined steel and polished wood Thompson, the refined simplicity of the Uzi, to the rough-made STEN and M3 of World War II fame.

For civilian use? Not so great. Many of the virtues of the open bolt design chiefly apply to full-auto operation, and do not translate well to a purely semi-automatic weapon, while its vices are still relevant. From an ATF perspective, an open bolt semi-auto is the gold standard of “readily convertible,” since all you need to do is disable a few disconnector parts to make it rock and roll. In one somewhat infamous case, an inventor tried to get around the phrasing of the NFA (which called a machine gun a weapon that fires more than once per trigger pull) by making the “sputter gun,” a Sten receiver with no trigger group that does an uncontrolled full mag dump when you release the bolt; the ATF did not agree with their interpretation of the NFA.

This IMI/Action Arms Uzi model B semi-automatic carbine sold for $2,070 at the September 2020 Premier Auction.

In contrast to other examples, which required some mild design changes to be on the square with the law, making a semi-automatic version of an open bolt gun requires drastic measures. In essence, the designers have to take the shell of a famous weapon and create a new closed-bolt action to fit inside its confines. Further, steps needed to be taken to keep someone from getting a set of open bolt parts and sticking them in the new firearm to keep it from falling into the “readily convertible” trap.

As a demonstration of what it takes to make this change and do it right, the semi-auto Uzi makes a very good example. Designed with the direct participation of the original designer, the semi-auto Uzi line replaced the original solid bolt with a new design, retaining the weight of the original bolt while integrating a mobile spring-loaded firing pin that interfaced with the sear at nearly the same point.

This Action Arms/IMI Uzi model B semi-automatic carbine sold for $2,588 at the September 2020 Premier Auction at Rock Island Auction Company.

Additional modifications were made to the receiver to prevent installation of original SMG parts, particularly a solid metal tab in the right rear (which the semi-auto bolts are slotted to pass, but flat-sided full-auto bolts slam into, obstructing the firing cycle) and a barrel support ring ahead of the bolt (which accommodates the reduced diameter of the 16” commercial barrels, but prevents proper seating of the original diameter, shorter SMG barrels). With these modifications, the Uzi Carbine and Uzi Pistol became very popular sporting arms in the 1980s, giving the look and feel of a signature weapon without the paperwork hassle.

Semi-Auto Conversions

Firearms are certainly complicated. Sometimes a fully-automatic weapon can enter the civilian market with little difficulties while other times it can result in a complete disaster. While there are many examples that were not fully explored here, hopefully this article was able to grant some insight into what happens when high-powered weapons become publicly available. If you're interested in learning more about function checking other firearms you own, try reading some of our other blogs on the topic.

As always, if there are any questions regarding consignment, future auctions, or any of the items listed here, please contact Rock Island Auction Company today.