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Building the FGC-9


FGC-9 Pistol Build
The FGC-9, Built as a Pistol

Since the FGC-9 was introduced, the ability to build one has become ever more streamlined and 3D printing techniques have evolved to allow for tougher prints and integrated fasteners. We are going to walk through an FGC-9 build with a couple of upgrades that show how the base design can be improved upon while at the same time simplifying the build process for the average person.


This article is not designed to replace the original build guide, but is instead to be used as a supplemental resource showcasing some alternate parts or build techniques that we believe enhances the overall design. Before attempting this type of build you should already be familiar with your 3D printer and have a good understanding of the necessary tuning to ensure you get dimensionally accurate and sound prints.


The original build guide shows the orientation in which the individual parts should be printed and we are not going to deviate from that here. All parts were printed in PLA+ using a "99 perimeter" infill style and "build plate only" tree supports. Many of the original designed parts are utilized, however, the following parts have been modified and will be shown in this build guide:


  • Upper Receiver

    • Hex nut sockets have been added for the handguard and buffer tube screws. This is done with filament changes at the appropriate layers so the nuts can be added and then printed over afterwards.

    • A picatinny rail was integrated into the receiver to eliminate the extra screws and printed rail parts in the original design.


  • Lower Receiver

    • Hex nut sockets for the takedown screws have been added for a more streamlined fitment. This includes shortening the rear takedown screw as well.

    • The lower receiver has been modified for thumb clearance over the safety selector as it was found that the original design didn't leave much room to manipulate the selector from "SAFE" to "FIRE".

    • Through hole added for the magazine catch pivot pin should the pin need to be removed in the future. This was done after having the printed pivot pin fail in the past. This also allows for the use of a metal pin if the end user so chose.

    • 1/4-28 TPI threads printed into the lower for an AR-15 grip screw. This allows the builder to use more parts from a purchased AR-15 lower parts kit.


  • Magazine catch

    • Remixed design for aesthetics only.


  • Pistol Brace

    • The stock in the build guide was swapped for a FGC-9 pistol brace for legality reasons.


Getting Started


Before we start with the lower receiver and trigger group we need to ream the selector hole using a 3/8" reamer (a 3/8" drill bit could be used in a pinch).


FGC-9 Lower
Depending on print orientation, holes will be undersized at the top and will need to be reamed

3/8" reamer for the AR-15 safety selector hole
A 3/8" Reamer is used for the Safety hole.

Now we can gather the AR-15 fire control group and lay it out. First we will install the AR-15 trigger, disconnector and associated springs into the lower receiver using the original AR trigger pin.


AR-15 FCG
AR Fire Control Group Components

FGC-9 trigger install
The trigger is seated and partially held in place with the pin.

FGC-9 trigger install
The disconnector is added and pin installed fully.

We then install the AR-15 hammer and spring with a longer hammer pin as per the build guide as the original hammer pin isn't quite long enough for the FGC-9's design. The legs of the hammer spring fit into the notches on top of the trigger pin to help secure the trigger pin in the receiver.


FGC-9 FCG install
The hammer spring legs are positioned over the trigger pin.

FGC-9 FCG install
The longer hammer pin is installed.

Now we can install the safety selector, detent and spring. The selector should be lightly oiled prior to installation to allow it to rotate freely. Install the selector detent into the receiver and spring into the grip. The grip is installed using the 1/4-28 screw and external tooth washer while making sure that you don't overtighten the screw and crack the lower, snug is plenty and the washer will keep it from loosening up.


FGC-9 FCG
Oiling the selector helps it operate smoothly inside the plastic receiver.

FGC-9 Lower
Notice the 3D printed threads for the grip screw.

AR-15 grip screw
We used an AR-15 grip screw for this build.

FGC-9 Build
Capturing the safety selector spring in the grip.

FGC-9 Build
Grip screw & washer tightened down in the grip.

Once the grip is installed the trigger group is function checked to ensure the safety and disconnector work. Make sure to capture the hammer with your thumb to keep the hammer from hitting the front edge of the fire control pocket of the lower when doing this check. You also want to check to ensure the disconnector will catch the hammer during reset as well. Things work as expected when using a high quality fire control group and this is no area to save money.


FGC-9 Build
Testing the FCG on "SAFE"

FGC-9 Build
Testing the FCG on "FIRE"

Now we can install the magazine catch assembly and feed ramp. We use the magazine catch spring from an AR-15 along with a 3D printed retaining pin (optionally you can use the same material as the hammer pin to hold in the mag catch). The magazine release also uses an ambidextrous release button that is 3D printed as well. The feed ramp is held in with a 12mm long M3 Socket Head Cap Screw (SHCS).


FGC-9 mag catch
The feed ramp and mag catch components laid out

FGC-9 feed ramp
The feed ramp utilizes a 12mm long M3 screw

FGC-9 feed ramp
The feed ramp in its fully seated position.

FGC-9 feed ramp
The feed ramp bolt installs from underneath

FGC-9 mag catch
The magazine catch uses a "button" to allow for ambidextrous use.

FGC-9 mag catch
We decided to use an AR-15 mag catch spring as well.

FGC-9 mag catch
The mag catch is held in position and the 3Dprinted pivot pin is installed. This pin is probably the weakest part in the whole design and can be replaced with a metal pin if needed.

FGC-9 mag catch
The pivot pin fully seated.

FGC-9 mag catch
You can see one of our improvements was a partial "through-hole" to allow for removing the pivot pin should it break.

After the feed ramp and magazine catch are installed we can test the magazine's fit and clearance with the feed ramp and ensure the mag falls freely when the release is pressed.


FGC-9 Build
The magazine should sit next to the feed ramp but still have clearance.

Next we will prep the buffer tube and brace. The buffer tube screw hole will need to be tapped using an M4 tap in preparation for the 30mm long M4 screw. The buffer system for the FGC-9 utilizes a dual spring setup of a primary spring (AR-15 Carbine buffer spring) and a smaller stiffer secondary spring seated into the bottom of the buffer tube. The support from the inside of the buffer tube can be used as a "pusher" tool to help seat the secondary spring into the recess in the back of the buffer tube.


FGC-9 Build
The FGC-9 buffer tube and pistol brace components

FGC-9 Build
The brace is held to the tube with a single 4mm screw.

FGC-9 Build
A standard hardware store 4mm tap is used to cut the threads.


FGC-9 Build
The screw is slipped into the brace and will sit in this position when over the hole in the buffer tube.

FGC-9 Build
Your threads should allow the screw to snug down this far.

FGC-9 Build
The screw should sit flush when fully installed.

FGC-9 Build
The secondary and primary buffer springs

FGC-9 Build
Notice the recess for the secondary spring in the back of the buffer tube.

FGC-9 Build
The tree support that was used to install the secondary spring.

FGC-9 Brace
The buffer and brace system assembled and ready to go onto the gun.

The ejector is held into the upper receiver with a single 16mm M3 screw. Make sure the isn't any support material left in the screw hole and ensure you don't use any screw longer than 16mm or you will damage the upper receiver.


FGC-9 Build
Make sure to use the 16mm long screw for the ejector!

FGC-9 Build
This is what the correct length will look like.

FGC-9 Ejector
Once installed, the screw head will sit flush. Do not overtighten!

Now we will work on the bolt shroud and charging handle. The charging handle is made up of 3 pieces (2x 3D printed parts and a 5mm allen key) that are epoxied together. I used Marine-tex brand of epoxy as I find it easy to mix and work with and it hardens up very well. We will also install the firing pin retaining screw (20mm long M3 screw) at this point and epoxy over the nut to ensure it doesn't come loose during firing (if it does it will chew up the inside of the upper receiver.


FGC-9 Build
We have found Marine-tex works well.

FGC-9 Build
Coat the upper half of the allen key with the epoxy

FGC-9 Build
Slide on the Charging Handle Bushing

FGC-9 Build
Push epoxy into the Charging Handle and install.

FGC-9 Build
Remove any excess epoxy and set aside to cure.

FGC-9 Build
The firing pin retaining screw is 20mm long

FGC-9 Build
The nut is tightened on the retaining screw.

FGC-9 Build
We don't want our screw coming loose so we cover it in epoxy.

FGC-9 Build
Remove any excess epoxy and set aside to cure.

Two of the items that make the build go much easier and quickly are a prefabricated bolt kit and barrel. Our CNC bolt kit drops into the factory bolt shroud and eliminates the need for any drilling or welding to produce the FGC-9's bolt. The bolt kit also does NOT need to be epoxied in place and allows for the bolt to be removed and used in another build. The firing pin protrusion is also set in this new design so no grinding of the return spring is needed.


FGC-9 CNC bolt & firing pin kit
Our CNC Bolt and Firing pin kit make assembling an FGC-9 a straightforward affair.


FGC-9 Barrel
A premade barrel also make the build go much more smoothly by having the headspace already set.

The bolt kit slides directly into the bolt shroud until the feel lip on the bolt stops against the cutout on the bottom of the shroud.


FGC-9 CNC Bolt
The bolt kit slides into the front of the bolt shroud.

FGC-9 CNC bolt kit
Once fully seated the bolt face will sit slightly "proud" of the bolt shroud

The barrel is set into the upper receiver until the barrel face meets the shelf in the upper receiver. The charging handle is then installed in the upper receiver and the handguard/barrel retainer is installed with 4x 40mm M3 screws. The 3mm hex nuts shown below are what are used inside the upper receiver and for the lower receiver's takedown bolts.


FGC-9 Build
We still use 40mm long screws with our modified upper receiver.

FGC-9 Build
Each screw gets an appropriate washer as well

The M3 nuts that are used throughout the build.

FGC-9 Barrel
The barrel is inserted into the upper receiver.

FGC-9 Barrel
The barrel should bottom out in the receiver.

FGC-9 Charging Handle
The charging handle is installed in the upper.

FGC-9 Barrel retainer
Handguard/Barrel retainer ready for assembly.

This shows the nut pockets that are set into the upper receiver during printing.


FGC-9 Nuts
Nuts installed during a filament change.

FGC-9 Barreled upper receiver
The Barreled upper

FGC-9 Build
View of the Charging handle, Barrel, and chamber.

FGC-9 Build
You will need a long reach allen key to install the barrel retainer screws.

The ejector needs to be in the "ejected" position before the bolt is installed into the upper receiver.


FGC-9 Ejector
Ejector in the proper position to install the bolt.

FGC-9 CNC bolt kit
Bolt installed showing the feed lip fitment in the bolt shroud.

The rear takedown screw in our build will need to be shortened down to ~33.6mm in length to allow for a flush fit on the lower receiver while the front takedown screw can be left at the 40mm length.


FGC-9 Build
Our build requires the rear takedown screw be shortened.

FGC-9 Build
The recessed "nut pocket" on the lower receiver and cutaway shelf for thumb clearance.

FGC-9 Build
The forward takedown pin "nut pocket".

The lower can now be installed into the upper receiver keeping in mind that the feed ramp's notch needs to slip into the upper receiver. The lower is slid forward until the takedown holes line up.


FGC-9 feed ramp
The feed ramp can be broken if the receivers are installed incorrectly.

FGC-9 Build
The receivers should be brought together and then the lower slid forward afterwards.

FGC-9 Build
slide the lower forward until the takedown screw holes are aligned. this can require some force from a soft mallet.

Once the lower is assembled into the upper, the buttstock can now be installed. The main buffer spring is slid over the bolt tail and the buttstock secured with 3x 30mm M3 screws. The original design utilized a 4th screw into the lower receiver but I have found this additional screw to be unnecessary with the embedded nuts and omitting it means we don't have to mess with any heat-press brass inserts further simplifying the build.


FGC-9 Buffer tube
Now the buffer system is installed

Now we have our very own FGC-9 ready to hit the range and enjoy.


FGC-9 Build cnc bolt kit firing pin
Our slightly modified/enhanced FGC-9 pistol build is ready for the range!

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