Upgrading HK G3/HK-91 style rifles

I ended up with this crazy rifle that was a conglomeration of parts; some surplus CETME parts, some HK G3 parts, and some US-made compliance parts. It had been assembled by Century Arms and changed and damaged and welded on and added to by subsequent owners. If Frankenstein ever assembled a roller-locked rifle, this would be the one.


Frankenstein’s Roller-Locked Rifle. Notice that the buttstock adjustment lever dangles, as it does not fit the cheap plastic tube.

Well, it was pretty ugly, and not very practical, so we set out to change that.

Modernizing the configuration

The rifle started its life as a Spanish CETME rifle, the rifle the HK G3 was based upon. It was cut up and imported as a parts kit by Century Arms.

The rifle now had two buttstocks; a surplus HK G3 stock that was cracked in two places and a cheap plastic receiver extension that had an ill-fitting M4 stock of dubious origins on it. We decided to install a FAB Defense M4-G3 FK, but substituted the GL-SHOCK stock for the GLR-16 stock, taking advantage of the recoil-reduction of the GL-SHOCK. Since the rifle is a soft-recoiling rifle already, the GL-SHOCK stock should make recoil almost unnoticeable.

The Frankenstein CETME prepped for reconstructive surgery.

The Frankenstein CETME prepped for reconstructive surgery, with quality parts ready for transplant.

The forend was a surplus G3 forend. We would prefer to have a solid attachment point for different bipods, so we chose a FAB Defense G3-RS. The G3-RS is a railed aluminum forend designed for HK G3 and HK-91 style rifles. It is unique in the way it mounts. Unlike other rail systems that mount in a similar fashion to the original HK handguard, the G3-RS actually clamps to the front of the receiver. The rail system is rock solid and does not move at all due to this design. In fact, you could leave the front screw out, and this rail system would still be totally solid and rigid, as it does not depend on the front screw to hold it in place.

The pistol grip was a surplus G3 grip, but we used a Century Arms replica instead in order to ensure 922r compliance.

The charging handle was also made by Century Arms and will be retained, and the trigger pack is a Century-made assembly as well.

The grip frame is a surplus G3 part and is marked S E F and the selector can be moved to all three positions. The Century Arms made grip frames block the selector from entering the bottom (full auto position, and the selector works awkwardly with the fire position being the top position and the safe position being the center (now lower) position, since the original bottom full-auto position is blocked. This is requires most shooters to shift their grip to manipulate, and is especially difficult for shooters with smaller hands. If a G3 grip frame is used, the bottom position is again available, and the selector now works like this:

G3-SEF-Selector-markingsS (upper position) Semi-Auto
E (middle position) Safe
F (bottom position) Semi-Auto

Now the selector can be used in the bottom two positions and will work similar to an M16 selector – center for safe and down for fire. However, anyone new to the rifle will need to be instructed that the S position is not the safe position.

This grip frame was designed to be pinned in place at the front, but the ATFE in all their wisdom decided that the front pin, whose sole function is to hold the trigger housing in place, makes the rifle a machine gun. Because of this, a shelf was added to the inside of the grip frame and hooks over a shelf added to the receiver to hold the front of the trigger housing in place.

This rifle had a G3 sight installed. It was welded on at a slight cant, but not so much that the sight cannot be adjusted to compensate. Of course, this means that any lateral adjustment will also cause a slight change in elevation as well. Because of the angle, I had to do some filing to true up the part of the sight base that interfaces with scope mounts.

We plan to add a scope eventually, and want the ability to use reflex sights as well, so we also chose a FAB G3-SM optics rail. This is a very low-profile rail, similar to their MP5-SM, but lengthened and designed to interface with the G3 sight base. This prevents the rail from creeping forward under recoil.

Restoring the Rifle

The first step was to disassemble the rifle, including the rear sight charging handle, and mag release, and strip out the oils. After checking the new parts and accessories for fit, the rifle and parts went into the blast cabinet to be blasted with 120 grit aluminum oxide to remove the previous coating. I would prefer a courser media, but that is the only aluminum oxide available in Alaska.

Distressing the finish

Distressing the finish

Once the rifle was blasted and blown clean, it was hung for coating and the first coat, a warm dark earth DuraCoat, was applied. The first coat was allowed to harden for a couple of days, and then a dark brown was applied. As soon as the second coat was fully dry to the touch, it was distressed carefully using steel wool. When this was complete, the parts were blown clean and a clear coat was applied.

Finished and ready for assembly

Finished and ready for assembly

Some assembly was done prior to the second coat and the distressing, so that the distressing would look correct, and other parts were distressed carefully so that areas that are protected when assembled were left brown.

Next, the rifle was allowed to cure for about 28 hours and then assembled.

Upgrading the Rifle


The M4-G3 Stock system is a perfect flush fit with the G3 back plate.

The FAB Defense G3 buttstock system was very easy to install. Since the G3 back plate is made from welded stampings, there tends to be some variation from rifle to rifle, and the M4-G3 kit is designed to be a tight fit to compensate. One spot needed three strokes with a file, and then the M4-G3 kit could be installed in the back plate. It installs like the original buttstock, using one large screw from the rear attaching to the back of the buffer, and using two small screws from the front. I use a bit of blue Locktite to ensure the screws don’t back out over time.


The GL-SHOCK stock will reduce the felt recoil and muzzle rise.

The G3-RS is a drop-in installation. The rear part simply slides back over the front of the receiver, and then the front swings up around the barrel. The rear screw clamps the rail system to the front of the receiver as it is tightened. On a G3 or HK-91 rifle, the front screw goes through the pin hole in the handguard hanger. On a CETME rifle, it goes between the barrel and cocking tube, where the original pin went.

The G3-RS becomes an extension of the receiver itself.

The G3-RS becomes an extension of the receiver itself.

The G3-SM clamps to the receiver lugs.

The G3-SM clamps to the receiver lugs.

Once the rifle was assembled, the G3-SM was installed. The G3-SM scope mount is a very low-profile MIL-STD-1913 rail designed to mount to the HK-type receivers. The design keeps the optic low, clears the iron sights, and prevents movement of the rail on the receiver.

GL-SHOCK-CheekpieceAn adjustable cheekpiece was added to the GL-SHOCK stock for a proper cheekweld both with iron sights and with optics, and the restoration and modernization was complete.

The finished rifle has a unified battle-worn appearance, but is completely modernized with military-grade accessories.

The finished rifle has a unified battle-worn appearance, but is completely modernized with military-grade accessories.

Before firing, we will check the headspace again, since I have found that sometimes removal of the original Century finishes inside the receiver can change the headspace on these rifles.

For aftermarket accessories like stocks and rails, I prefer the FAB Defense G3 accessories, available from The Mako Group, because they are well-designed and built for military use. If I am looking for parts for these rifles, I always  check first with RTG Parts since they have a great selection, good prices, and an expert knowledge of HK-type rifles.

The finished rifle ready for optics.

The finished rifle ready for optics.

Any G3 or HK-91 or CETME-type rifle that uses foreign-made parts must have no more than 10 parts from a list of specific parts in order to comply with U.S. Code – Title 18 – Part 1 – Chapter 44 – § 922 (r). Check this link for a list of the 922r parts that apply to the G3 and CETME rifles, as well as some other HK-style rifles.

3 thoughts on “Upgrading HK G3/HK-91 style rifles

  1. That’s not how your safety should work and that isn’t how any of mine have functioned.
    Sicher – SAFE
    Einzelfeuer – Single Fire
    Feuerstoss – Fire


    • Figured it out. You have a cetme fcg in an hk trigger housing. Invest in an hk/ptr fcg i think you will need a new short axel hk selector lever


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