The GST-9 MOD100 uses a familiar Gen 3-style lower-parts layout, but there are a few important differences to keep in mind. Most notably, the GST-9 uses separate left and right rear rails rather than a one-piece rear rail module. It also uses an O-ring in the trigger-pin area that must be properly positioned before the trigger pin is fully installed.

This guide walks through the general installation order for the GST-9 MOD100 lower-frame components. Always refer to the official GST-9 instructions for exact part orientation, pin placement, and model-specific requirements.

Safety notice: Make sure the firearm is completely unloaded before beginning. Remove the magazine, visually and physically check the chamber, and keep all ammunition away from the work area. Wear eye protection and never force a component into place.

What This Guide Covers

Use these links to jump directly to the section you need.

• › Parts and tools
• › 1. Magazine catch assembly
• › 2. Trigger assembly
• › 3. Slide lock and spring
• › 4. Place the trigger assembly
• › 5. Split rear rails
• › 6. Front rail/locking block
• › 7. Slide stop lever
• › 8. GST-9 O-ring
• › 9. Trigger pin
• › 10. Final lower inspection
• › Troubleshooting
• › Final safety check
• › Frequently asked questions

Parts and Tools

Before getting started, confirm that you have all required components:

Begin with the magazine catch spring and magazine catch.

Using needle-nose pliers, insert the magazine catch spring, which looks like a small straight metal rod, into the narrow vertical hole inside the center of the magazine well. Press it down until it is fully seated. Once the spring is properly seated, install the magazine catch and make sure the spring engages it correctly.

Press the magazine catch several times to confirm that it moves freely and returns to its resting position. It should not stick, drag, or feel unusually loose.

Before beginning, gather the trigger housing, connector, trigger spring, trigger bar, and trigger shoe. Some lower parts kits include a trigger shoe that is already attached to the trigger bar, while others provide the two pieces separately.

If the trigger shoe is not already attached, insert the front end of the trigger bar into the slot at the top of the trigger shoe. Align the pin holes and secure the two pieces using the small pin supplied with the trigger. Follow the trigger manufacturer’s instructions for the exact pin direction and placement.

Next, locate the long tab on the connector. Press this tab into the slot on the side of the trigger housing until the connector is fully seated against the housing.

Attach one end of the trigger spring to the opening at the rear of the trigger housing. Hook the other end of the spring onto the trigger bar. Make sure both ends are securely attached and that the spring is not twisted.

Once the spring is connected, rotate the trigger bar over the top of the trigger housing. Guide the rear edge of the trigger bar into the groove in the housing so the cross-shaped portion rests on top of the housing.

The trigger bar should settle into position without being forced. Before placing the completed trigger group into the frame, confirm that:

• ✓The connector is fully seated.
• ✓The trigger spring is attached at both ends.
• ✓The spring is not twisted.
• ✓The trigger bar is resting in the housing groove.
• ✓The trigger bar moves without sticking or binding.

Gather the slide lock spring, slide lock, frame, and a small punch. The slide lock spring is not a traditional coiled spring. It is a thin metal bar with a bent end.

Locate the narrow groove in the frame directly in front of the trigger opening. Insert the bent end of the slide lock spring down into this groove. Press it into place until it is fully seated. The opposite end of the spring should remain slightly raised so it can apply upward pressure to the slide lock.

Next, identify the small notch along the bottom of the slide lock. This notch must face downward toward the spring. The rigid side of the slide lock should face the rear of the frame.

Begin sliding the slide lock into the frame from either side. As it approaches the spring, use a small punch to gently press the raised end of the spring downward. Continue sliding the lock across the frame until it is centered and the notch sits directly over the spring.

Release the spring and confirm that it pushes the slide lock upward. Press down evenly on both sides of the slide lock, then release it. The lock should move down smoothly and immediately return to its raised position.

If the slide lock does not spring back up, sits unevenly, or feels stuck, remove it and check that:

• ✓The bent end of the spring is fully seated in the frame.
• ✓The slide lock’s bottom notch is positioned over the spring.
• ✓The slide lock is facing the correct direction.
• ✓The spring is not bent, trapped, or sitting beside the notch.

Do not force the slide lock into place. When the spring and lock are correctly oriented, the parts should move freely and remain under spring tension.

Before installing the trigger assembly, confirm that the trigger bar is properly seated on top of the trigger housing, the connector is fully inserted, and the trigger spring is still attached at both ends.

Hold the frame upright and guide the trigger shoe down through the trigger opening from inside the frame. Keep the trigger shoe centered as it passes through the opening so it does not catch on the surrounding polymer.

At the same time, lower the trigger housing into the rectangular pocket at the rear of the frame, directly behind the magazine well. Press down evenly on the trigger housing until it sits completely inside the pocket. The housing should rest flat and should not rock, lean to one side, or sit above the surrounding frame.

As the assembly is lowered into place, make sure the trigger bar rests naturally along the inside wall of the frame. It should not be trapped underneath the trigger housing, pressed tightly against the polymer, or caught on another component.

Once the assembly is seated, check that:

• ✓The trigger housing sits flat in its pocket.
• ✓The trigger shoe is centered in the trigger opening.
• ✓The trigger bar is resting along the inside of the frame.
• ✓The trigger spring remains attached and is not twisted.
• ✓The connector has not shifted out of the trigger housing.
• ✓The assembly does not rock or lift out of position.

Do not force the trigger housing into place. If it will not sit completely inside the frame, remove the assembly and recheck the trigger bar, connector, and trigger spring before trying again.

Before installing the rear rails, confirm that the trigger housing is fully seated in the rear pocket of the frame. The trigger housing should sit straight and should not rock or lift when light pressure is applied.

The GST-9 uses two separate rear rails: one for the left side of the frame and one for the right. Compare the rails with the official GST-9 diagram before installation so each rail is placed on the correct side and in the correct direction.

Place the first rail into the space beside the trigger housing. Lower it straight into the frame and press it down until it is fully seated. The rail should rest alongside the trigger housing rather than on top of it.

Repeat the process with the second rear rail on the opposite side.

Once both rails are installed, check that:

• ✓The left and right rails are on their correct sides.
• ✓Both rails are pressed fully into the frame.
• ✓The upper rail surfaces sit at the same height.
• ✓The rails are straight and parallel with one another.
• ✓Neither rail is tilted, twisted, or resting on another component.
• ✓The trigger bar is not trapped underneath either rail.
• ✓The rear 3mm pin holes line up with the opening in the trigger housing.

Look through the rear 3mm pin hole from one side of the frame. You should be able to see a clear path through the frame, rear rails, and trigger housing.

If the holes do not line up, gently adjust the rails and trigger housing by hand. A small punch may be used as an alignment tool, but do not use it to enlarge the holes or force the components into position.

Once everything is aligned, insert the rear 3mm pin. Apply steady pressure or use very light taps with an armorer’s hammer until the pin is centered in the frame.

After installing the pin, check the rails again. They should remain level, sit securely beside the trigger housing, and should not move or rock.

Before installing the front rail/locking block assembly, confirm that the trigger assembly and rear rails are fully seated and that no internal parts are extending into the front rail opening.

Identify the front rail/locking block assembly and compare its orientation with the GST-9 diagram. The notched side should face toward the rear of the frame. Lower the component straight into the opening at the front of the frame, keeping both sides level as it moves into place.

Press down evenly until the front rail/locking block assembly sits completely inside its molded pocket. It should rest flat and should not rock, lean to one side, or sit above the surrounding frame.

Before installing the retaining pin, check that:

• ✓The notched side faces the rear of the frame.
• ✓Both sides are fully seated.
• ✓The component sits straight and level.
• ✓The pin openings form a clear path through the frame and front rail/locking block assembly.
• ✓The trigger bar or another internal part is not trapped underneath it.

If one side sits higher than the other or the pin openings do not line up, remove the component and reposition it. Do not use the retaining pin to pull a raised or misaligned front rail/locking block assembly into place.

Once the front rail/locking block assembly is seated correctly, secure it with the appropriate front retaining pin identified in the official GST-9 instructions. Apply only reasonable pressure and make sure the pin is centered when installed.

Before beginning, make sure the front rail/locking block assembly is fully seated and its smaller retaining pin is already installed. Leave the larger trigger pin out, or only partially inserted, so the slide stop lever and GST-9 O-ring can be positioned before the pin is pushed completely through.

The slide stop lever is the thin metal part with an external thumb tab and a small wire spring attached to one end. Do not confuse it with the slide lock, or takedown bar, installed earlier.

Hold the lever so that:

• ✓The external thumb tab remains outside the left side of the frame.
• ✓The spring end points toward the inside of the frame.
• ✓The hook-shaped portion of the spring faces upward.

Guide the spring end of the lever down into the frame beside the trigger assembly. Slide the lever underneath the front retaining pin and move it fully toward the left side of the frame. The hook-shaped spring should rest against the front retaining pin, while the external thumb tab sits flat against the outside of the frame.

Make sure the lever is positioned beside the trigger rather than underneath the trigger bar or trapped between the trigger assembly and frame.

Before continuing, check that:

• ✓The external thumb tab rests flat against the frame.
• ✓The spring hook faces upward and contacts the front retaining pin.
• ✓The spring is not bent, pinched, or trapped underneath the lever.
• ✓The lever can move upward slightly without binding.
• ✓The lever returns to its resting position when released.
• ✓The trigger-pin holes remain aligned.

Do not force the trigger pin through at this stage. On the GST-9 MOD100, the O-ring must also be positioned in the trigger-pin area before the trigger pin is fully installed. If the lever feels stuck, unusually loose, or does not return under spring tension, remove it and verify the spring orientation before continuing. Once the lever and O-ring are correctly aligned, the trigger pin can be installed without excessive force.

Before installing the O-ring, confirm that the front rail/locking block assembly is fully seated and secured with the front 3mm pin. The trigger assembly, left and right rear rails, and slide stop lever should also be in place. Leave the larger trigger pin out until the O-ring has been positioned.

Locate the small O-ring included with the GST-9 parts. Gently slide it into the narrow space between the slide stop lever and the trigger shoe. The O-ring should sit flat in this area without being folded, twisted, or stretched.

Look through the trigger-pin opening from the side of the frame. The center opening of the O-ring should line up with the trigger-pin holes in the frame and the other internal components.

Use a small pick or punch to carefully move the O-ring into alignment. Use the tool only to guide the O-ring. Do not puncture it, pry against it, or force the tool through the rubber.

Before installing the trigger pin, confirm that:

• ✓The O-ring sits between the slide stop lever and trigger shoe.
• ✓The O-ring is lying flat and is not twisted.
• ✓The center of the O-ring is aligned with the trigger-pin holes.
• ✓The slide stop lever remains properly positioned.
• ✓There is a clear path for the trigger pin through the assembly.

Once everything is aligned, begin installing the trigger pin. The pin must pass through the center of the O-ring as it moves through the frame.

If the trigger pin stops partway, do not strike it harder or attempt to force it through. Stop, move the pin back enough to relieve pressure, and check the position of the O-ring and slide stop lever. Use the pick or punch to realign the O-ring before continuing.

Before installing the trigger pin, confirm that the front rail/locking block assembly is fully seated and that the slide stop lever and O-ring are already in position. The trigger pin is typically the larger of the frame pins and may have grooves cut into it. Follow the official GST-9 instructions for the correct insertion direction.

Look through the trigger-pin opening and make sure there is a clear path through each component. The following parts must be aligned:

• ✓The trigger-pin holes on both sides of the frame
• ✓The front rail/locking block assembly
• ✓The trigger assembly
• ✓The slide stop lever
• ✓The center opening of the O-ring

Begin pressing the trigger pin into the frame by hand. Keep the pin straight as it enters the first side of the frame and passes through the internal components.

As the pin reaches the slide stop lever and O-ring, it may meet slight resistance. Gently move the slide stop lever up or down while applying steady pressure to the pin. This small amount of movement can help the openings in the lever and trigger assembly line up.

For the GST-9 MOD100, also watch the O-ring as the pin moves through the assembly. The pin must pass through the center of the O-ring rather than pushing the O-ring aside. A small pick or punch can be used to carefully hold the O-ring in alignment, but take care not to puncture or stretch it.

If the trigger pin stops partway:

1. 1Stop applying pressure.
2. 2Move the pin back slightly to relieve tension.
3. 3Recheck the slide stop lever and O-ring.
4. 4Confirm that the front rail/locking block assembly and trigger assembly have not shifted.
5. 5Realign the components before continuing.

Do not use heavy hammer strikes to drive the pin through. If the pin catches, gently move the slide stop lever while rechecking the alignment of the trigger assembly and O-ring. Do not force the pin.

Once the pin passes completely through the assembly, center it in the frame. The ends of the trigger pin should sit evenly on both sides and should not protrude noticeably from either side.

After installation, confirm that:

• ✓The trigger pin is centered.
• ✓The slide stop lever moves without binding.
• ✓The slide stop lever returns to its resting position.
• ✓The O-ring has not been pinched or pushed out of place.
• ✓The front rail/locking block assembly remains fully seated.
• ✓No part of the trigger assembly shifted during installation.

Before installing the slide, inspect the completed lower-frame assembly from both sides and from above. All components should sit straight, securely, and in their proper positions. Nothing should appear raised, twisted, pinched, or damaged.

Confirm that:

• ✓The magazine catch moves freely and returns under spring pressure.
• ✓The slide lock moves downward evenly and springs back up when released.
• ✓The slide stop lever moves without binding and returns to its resting position.
• ✓The trigger housing is fully seated in the rear pocket of the frame.
• ✓The trigger shoe is centered in the trigger opening.
• ✓The trigger bar is not pinched beneath a rail or pressed against the frame.
• ✓The connector remains fully seated in the trigger housing.
• ✓The trigger spring remains attached at both ends and is not twisted.
• ✓The left and right rear rails are fully seated, straight, and level with one another.
• ✓The front rail/locking block assembly is fully seated and does not rock.
• ✓All frame pins are centered and sit evenly on both sides.
• ✓The O-ring remains properly aligned around the trigger pin.
• ✓No rail, spring, pin, or internal component appears bent or damaged.
• ✓There are no cracks, stretched pin holes, deep gouges, or other visible damage to the frame.

Pay particular attention to the trigger-pin area. Confirm that the trigger pin passes through the center of the O-ring and that the O-ring is not folded, twisted, torn, stretched, or pushed out of position.

Lightly handle the frame while watching the pins. None of the pins should begin moving sideways or working their way out of the frame.

Do not install the slide or continue if:

• ✓A control binds or fails to return.
• ✓A rail is raised, loose, or uneven.
• ✓A pin is not centered.
• ✓The trigger bar is trapped or pinched.
• ✓The O-ring is damaged or displaced.
• ✓Any component appears bent, cracked, or incorrectly positioned.

Correct the affected component before continuing. Do not attempt to compensate for improper alignment by forcing a pin, bending a rail, or removing material from a part.

Common Installation Problems

Most installation issues come back to alignment. Use the checks below before applying more pressure to any part.

Final Safety Check

Keep all live ammunition in a separate area while completing the final inspection.

After the slide is installed, perform only the unloaded function and safety checks provided in the official GST-9 instructions. Continue to keep the firearm pointed in a safe direction and all ammunition away from the work area.

Stop and have the assembly inspected by a qualified armorer if:

• ✓The trigger does not move or reset correctly.
• ✓A control binds or fails to return.
• ✓A pin begins moving out of position.
• ✓A rail is loose, raised, or uneven.
• ✓The O-ring appears damaged or displaced.
• ✓An internal safety appears damaged or improperly installed.
• ✓Any component required unusual force during installation.
• ✓You are uncertain whether a part is installed correctly.

Never attempt to test-fire the firearm when a control is binding, a pin is moving, a rail is uneven, or a safety system is not operating properly. Taking the time to correct a questionable installation is better than forcing a component, damaging the frame, or creating an unsafe condition.

Frequently Asked Questions

Need the Official GST-9 Reference?

Use this article alongside the official GST-9 instructions for exact part orientation, pin placement, and model-specific requirements.