Home Build A Semiautomatic Japanese 99 LMG
By Cain M. Grocox

Many NFA owners are both avid shooters as well as collectors. We are always on the hunt for one more item to add to the collection or that missing accessory that we need to fill out our collection. At least that is the story we share with our spouse or significant other. After the 1986 FOPA, which included the infamous Hughes Amendment, building a new full auto cannot be done as an individual, but other items can still be made. Of course we all know what that has done to the price of transferable machine guns.

With the ever increasing prices on many of the NFA world’s most sought after items, there is a market that continues to increase in size. This market is the semiautomatic versions of the original full auto weapons. There are the normal semi-auto versions of the AK, 1919A4, MG34, MG42, and M16 to name a few. There are also semi-auto versions of more rare transferable weapons such as the M2HB, PKM, RPD, SG43, BREN, and Vickers HMG. This article will focus on the build process for a “hole in the collection item.”

The author was able to acquire a complete all matching Japanese 99 Light Machine Gun (LMG) parts kit. Then the thought process of how to turn a full automatic only open bolt LMG into a closed bolt semi-auto version began. Since there was no semi-auto 99 design on the market, a working design would have to be developed. At this point, the author teamed up with Lonnie Ingram to study the BATFE approval letters for other designs and to incorporate these principles into the semi-auto Japanese 99 design.

The first step in the build process was to mill the sear engagement area off the bottom of the bolt carrier and the hammer on the carrier that would strike the firing pin since neither would be needed in a semi-auto design. This changes the function of the bolt to a closed bolt as opposed to an open bolt weapon.

The second step in the build process was to remove the rear sight bracket to install a denial island. A 3/16-inch stainless pin was welded into the left rear of the receiver that extends .060 into the inside of the receiver. A corresponding clearance cut is then milled on the bolt and the carrier. This prevents an unmodified bolt or carrier from being installed in the receiver and cannot be seen from the outside of the weapon. Note that similar denial islands / pins are an integral part of the semi-auto 1919, Bren, and MG42 designs on the market. The goal of steps 1 and 2 is to remove any full auto features before attempting to complete the receiver.

The third step was to weld the receiver. The best advice is to go slow and alternate from side to side on the welding process while using copper backing plates inside the receiver. This will help to minimize warping and excess material to remove after the welding process is complete.

The fourth step is to modify the bolt. A new firing pin that is over twice the length of the original was made. This would also require a new bushing to be installed in the carrier for the new firing pin. This functions as another semi-auto feature that prevents an original firing pin from being used. With the modified bolt carrier and bolt now sliding freely in the receiver it was time to figure out how to hit the firing pin.

With the firing pin being located above the drive spring for the carrier, the semi-auto design would not be able to utilize any type of hammer. A striker similar to the Bren, PKM, or RPD designs was chosen for the basis of design. This striker would have to function as the hammer while catching on the sear in the lower and allowing the original drive spring for the carrier to function. The striker was built using a single block of 4140 heat treated steel. Also note that the 99 LMG has an ejector that pivots on a pin and side ejects. This required another clearance cut on the striker that was not anticipated until the use of snap caps to check function showed this flaw. After fixing the ejector issue, the first major obstacle to overcome would arise. There has to be a spring to drive the striker and that spring has to have an area to retreat to.

On the semi-auto PKM and RPD there are inner and outer springs that drive the carrier and the striker. Lonnie Ingram was able to come up with a design that was able to use a Wolff extra power M1 carbine recoil spring in a tube that would come out the back of the buffer cap. The spring is preloaded and surrounds a rat-tail taken from an FN FAL carrier on the back of the striker that extends into the tube. In addition, the tube is able to be removed for maintenance and cleaning as required. To a trained eye this tube is definitely not found on an original 99 LMG but blends in quite well with the lines of the weapon and does not affect looking through the sights mounted on the left side of the weapon. With the entire upper section done, how do you make the trigger group work?

After several ideas that worked much better on paper than in reality, the semi-auto function of the Swedish K SMG was used as the basis of design. A new trigger, sear, and disconnector were made along with a wood block that the entire trigger group could be mounted. All the trigger group components were made from 4140 heat treated steel and AR trigger return springs are used to power the sear and the return the trigger. Being able to mount the trigger group on the outside of a wood block allows you to see how each piece functions and tweak as needed. Needless to say, this was a trial and error phase of the design development that led to at least 4 disconnector hooks being made before the final design was completed. This would lead to the second big hurdle of finding a way to fit the components into the existing lower.

An additional piece of steel was added to the front of the lower to allow more area for the semi-auto trigger group to function. Once again this blends in well with the lines of the weapon but to a trained eye is this not found on the original. You can see from the photos that there is a considerable amount of material that must be removed from the stock lower to allow the semi trigger group to fit. As a project note, be careful when working on the lower. The exact quality of steel used to fabricate the original WW2 lower allows the lower to bow and twist when welding and will cause issues when attempting to install the completed lower on the receiver.

After finishing the design and the build, there is nothing like the reward of actually seeing it bust caps and fire. The final outward appearance looks original and unless you are a RKI, most people would have no idea that the complete weapon is a semi-auto 99 and not an original LMG. There is nothing cheap or easy about the build, but there are states out there that the only option for owning this weapon would be a semi-auto unless it was a dealer sample. The author and Lonnie went through several design alterations before ending up with a design that actually works. Thinking through the design and putting it on paper pales in comparison to the actual level of craftsmanship required to bring the paper design to reality. I am fortunate to call Frank Hatten (semi-auto Uk-59 designer) and Lonnie Ingram (semi-auto Vickers) friends and I try to learn every single thing about weapons that they are willing to teach.

It is not a particularly hard build, but requires a great deal of patience and attention to detail in the trigger group and striker areas of the design. This is project was truly a labor of love that will provide a great deal of satisfaction. Additionally, it “fills that hole in the collection” and looks great in the man cave next to the semi-auto 1910 Maxim, TNW MG34, BRP MG42, and the semi-auto Vickers.

This article first appeared in SmallArmsReview.com on July 5, 2013


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