How to Build a Glock on a GST-9 80% Lower

With the media hype over “Ghost Guns,” most people are at least semi-aware of the existence of 80% lowers and 80% pistol frames. In this article we’ll look at how to build a Glock on an 80% lower, specifically a GST-9.

I had never attempted an 80% build, so when 80% Arms contacted us and offered one of their GST-9 80% Glock pistol frames, I couldn’t jump on board quickly enough.

To be absolutely up front: this article isn’t here to debate any topics about the legality of 80% lowers; as of this writing it is 100% legal for anyone who can own a firearm to build and personally use a firearm with an 80% grip frame or lower receiver.

Let’s be clear: If you can legally own a gun in the United States, you can build and run the 80% Arms GST-9 Pistol Frame.

Now, without further ado, onto the incredibly cool subject matter!

What is an 80% Lower or Grip Frame?

An “80 Percent” lower is simply a lower receiver that has only 80 percent of the machining processes completed by a manufacturer. The lower is, naturally, a major firearms component as it houses the trigger components and serves as the frame for most of the gun. It almost always includes the serial-number.

Without the final 20 percent completed, a company can offer these non-serialized parts to the public to perform the final steps – usually to include minor drilling, fitting, and machining – in the comfort of their own home or shop.

The user can then use this self-completed component to assemble a firearm. The caveat is that the person who will be owning, assembling, and storing the firearm MUST BE THE PERSON COMPLETING THE FINAL PROCESSES. You can’t legally bring your GST-9 pistol frame or 80 percent AR lower to a machine shop and have them complete the work. It’s all you, buddy. 

But you know what? There’s no reason to have to bring your GST-9 Glock pistol frame to the machine shop, because with the 80% Arms jig and a couple simple hand tools, finishing the GST-9 is just ridiculously easy. I know because I did it.

80% Arms generously supplied me with a GST-9 80% Pistol frame, a parts kit, and a jig with drill bits and frame rails. The whole process took me about 45 minutes – maybe an hour. I’ve got gunsmithing experience and some mechanical aptitude, so I it might take others a bit longer to learn how to build a Glock from such a lower.

If this is something you’re interested in, here’s how I turned a hunk of injection-molded polymer into a sexy, badass, great running hybrid Glock.

80% Arms GST-9: What’s in the Box?

The GST-9 Pistol Frame itself is based on the Glock 19 frame size. However, the GST-9 is modular and for an extra $19.99 (as of this writing) you can purchase a longer grip frame that transforms the grip length into Glock 17 dimensions – but is completely removable in case you wanted to go back to the shorter size. 

The GST-9 is also replete with dust cover accessory rails – four of them in lieu of Glock 19’s paltry one. The additional rails help the GST-9 be more flexible in its accessory accommodation and placement. The 80% Arms GST-9 Pistol Frame is compatible with all Glock holsters.

The GST-9 has some thought put into it, too. Though it will only accept Gen 1/2/3 Glock parts (a bummer for me since I wanted to swap the goodies over from my Gen 4 G19), the GST-9 has as standard many modifications that people incorporate into their Glock grips via the aftermarket: A beavertail and undercut trigger guard to allow a higher grip, thus helping to get the bore axis lower in your hand for improved controllability.

80% Arms also scalloped the area around the magazine release to ensure you can reliably dump a mag while wearing gloves. The finger grooves have been eliminated, and “gas pedal” shelves have been built into the GST-9 for index points and forward thumb placement. 

The jig kit consists of a simple but sturdy box jig that encapsulates the grip frame, as well as the necessary bolts, nuts, and washers required to clamp the jig onto the GST-9 frame you’re working on.

It also includes the necessary non-Glock specific parts required to make the GST-9 play nice with your Gen 3 Glock slide and internals: nickel-boron coated stainless steel slide rails, and the pins to hold the rails and guts together inside the 80% Arms GST-9 Pistol Frame.

Rounding out the package are instructions, a sticker for your toolbox, an allen wrench for the jig fasteners, two drill bits (3mm and 4mm, specifically), and a burr-type cutting tool that will plunk right into your Dremel tool or hand drill.

Note: the jig kit does NOT come with the 80% grip frame; the GST-9 must be purchased separately. Also, due to certain regulations, the frame rails must be sold with the jig kit and NOT the grip frame, so one jig kit purchase is required for each GST-9 Pistol Frame you require. It sucks, but that’s the way of the world.

If you want to feel saucy, you can rob all the parts out of your Gen 1/2/3 Glock and pop them into your GST-9 – the mag release, trigger bar, disconnector, slide stop, slide lock – they’ll all drop into the GST-9 80% lower once you correctly finish the processes.

However, I recommend just buying the G19 OEM parts kit from 80% Arms – it includes everything you need for a 5 lb trigger pull, and ensures that you’ll have everything you need to complete your GST-9 without robbing parts from a functional gun; the only parts you’ll need from your donor Glockster are the slide, barrel, and recoil spring assembly – an easy, five-second swap.

How to Finish a GST-9 80% Glock Lower

Without hyperbole – I was incredibly surprised by how easy it was to complete the process required to transform the as-mailed GST-9 frame into a fully functional Glock.

1 – Assemble Your Parts and Tools

To begin, you’ll need a few tools to accompany the parts supplied with the jig kit. If you have a rotary tool like a Dremel, the job is far easier and faster – though, of course, easier and faster to remove material you might not need to remove. If you don’t have a Dremel already, rest assured that you will end up using it for many other unrelated projects as well.

Beyond a Dremel a steady hand is also important. Probably best to skip those double-espresso pumpkin spice lattes before you start working. You also need eye protection, a hand drill, utility knife, and sandpaper. These are minimums. A large bench vice is a godsend as well.

2 – Read the Instructions

It is important to READ THE INSTRUCTIONS. Then read them again. And again.

Understand the processes and what you’re going to be accomplishing before you even pick up a tool. Instructions are located online so you can check out the processes before you even pull the credit card out of your wallet.

The job isn’t complicated or even large, but it is detail-oriented. The first time you work on one of these 80% lowers, you want to go slow, take your time, and double-check references. If you do, you’ll be rewarded with a slick, non-serialized piece of polymer freedom. If you rush, you can throw it all in the trash and get back on the waiting list for another jig and GST-9. Your call.

3 – Get Jiggy with It

Now is the time to actually get started in gunsmithing. To begin, I locked the grip frame (without the grip frame extension) into the jig.

This is a simple job. All you do is snap the grip frame into the jig the only way it fits. Use the supplied nuts and bolts to clamp the two halves of the jig together, completely encapsulating the GST-9. 

The jig’s exterior is very clearly labelled what you need to do, and in what order. To start turning the hunk of plastic into a finished Glock pistol frame, you start drilling in order – A, B1, and B2.

80% Arms recommends using a drill press, which I did have available; however, for the purposes of this article I decided to use the much more common hand tools that I assumed most people would be using. So, I locked the larger 4mm drill bit into my Dewalt cordless drill chuck for the “A” hole.

I did snug the jig into my big bench vise to make sure I had both hands available for the best control and to ensure I had the drill as perpendicular to the jig as possible.

4 – Drill It

The instructions say to drill the holes one side at a time – that is, don’t run the drill bit all the way through the grip frame and out the other side of the jig! The possibility of drilling a hole in the wrong location on the back side of the jig is nigh near certain – so resist the temptation to punch all the way through.

I placed the drill bit in the jig’s hole marked “DRILL A (4MM).” I made sure the drill was as straight as I could humanly make it, took a deep breath, and lightly squeezed the trigger. 

The grill bit grabbed the plastic aggressively at first try. I would heartily recommend keeping the drill bit in the jig hole, but not making contact with the GST-9 when you start the drill. The drill bit went through the glass-reinforced polymer like, well, a fast drill bit through plastic.

Don’t push hard, just let the weight of the drill do the work and your end results will be much better; my first hole was a bit less clean than I wanted. But this is a learning curve; I went slow on the next 3mm B1 and B2 holes and my results were perfect.

Once the three holes were drilled on one side, I flipped the jig, and repeated the process on the other, always being careful not to run the drill bit out through the other side.

Once the six holes are drilled – three per side – you can put the drill bits away.

5 – Remove the Cross Blocking Member and Top Tabs

Up next is the process that kinda daunted me – removal of the cross blocking member and the top tabs. If one was running full minimalist, you could use a utility knife (and a lot of time) and scrape these parts away, smoothing with sandpaper afterward.

However, I live in the 21st century and threw the provided burr bit in my Dremel tool. Keep in mind this bit is fairly aggressive and will absolutely chow through plastic faster than you can blink. If you sneeze while using this bit in a Dremel at 20,000 rpm, consider using your GST-9 as a polymer paperweight. Prudence and a patient, steady hand is required.

There are four tabs at the top of the frame that need to be carefully removed, as well as the front cross blocking member that runs across the front of the GST-9’s dust cover. These protrusions are just thin polymer and will get eaten away quickly by the burr bit, so again, go slow. You can remove the top tabs while the GST-9 frame is in the jig, but be forewarned: you can dig into the jig if you’re rotary-tool happy so heads up.

You’ll need to remove the GST-9 frame from the jig to buzz away the front cross blocking member. For me, these all were eradicated quickly; I went with the dremel till just a small bit of the offending plastic remained; then I used my freshly-sharpened Benchmade to gently scrape away the rest. Some 220 grit sandpaper followed by 320 grit made everything smooth and happy.

6 – Clean It Up

Once you’ve removed the top tabs and the material in the dust cover opening, you’re pretty much completed with the “machining” part of the process – congrats! However, some simple-but-critical actions remain. First, you need to use compressed air, Q-tips, or a healthy jet of water to remove all your drilling, buzzing, and sanding debris.

Pay special attention to the pockets on the interior of the frame by the “B2” hole you drilled earlier. This area needs to be completely devoid of debris, as your stainless steel front rail/locking block part (labelled as part “D” in the instructions) will need to sit inside this channel cleanly with no obstructions.

If debris is still inside this pocket, the locking block won’t sit down as far as it needs to and the pin holes won’t align and you’ll be practicing combining your curse words into fun new forms. Cleanliness is next to Godliness on this step. Don’t shirk on ensuring your snazzy new GST-9 frame is free of debris and burrs.

Next, I took a small piece of 320 grit sandpaper and rolled it into a cone shape, to just VERY lightly touch up the sharp edges of all the holes I drilled.

The instructions don’t say to do this, but I know from many years of working on guns that if your holes have any burrs or sharp edges, the plastic pins may have trouble going in. While I was with sandpaper in hand, I touched up the molding seams and any sharp edges on the frame as well. 

7 – Assemble the Assembly

I won’t give you a detailed course on how to assemble a Glock grip frame; there are hundreds of viable YouTube channels that can help you out here. – but, luckily, 80% Arms has their own dedicated video! Watch it here:

I can go over the 80 Percent Arms’ specific parts however. There are a couple – namely, the Front Locking Block and Rear Slide Rails are proprietary to this system and drop right in, very simply. These two parts are impressive; they offer a much longer frame rail surface for the slide to interface with over a stock Glock system.

These parts are also nickel-boron coated stainless steel parts, so you can be assured of long life and self-lubricating properties. Nice touch by 80 Percent Arms here!

The Glock lower parts kit (trigger, connector, slide stop, pins, takedown release, mag release, etc.) seemed very OEM Glock-ish – namely, high quality and no-nonsense.

When assembling my GST-9, I did notice that the lower parts kit they sent me was missing the trigger spring; I contacted 80 Percent Arms directly – not telling them I was reviewing – and they offered to send me a spring free of charge.

I had one in my parts stash so I declined the offer, but it’s nice to know that 80% Arms stands behind the products they offer and make sure their customer base is 100% squared away.

I’m happy to report that assembly was straightforward and in line with building a standard Glock lower up. The only issue I had with the finished product was with the front frame locking block/rail; they seemed to sit too high and when I installed a slide on the completed GST-9 it sat high up front and dragged when I operated the handgun.

I pulled the assembly out, ensured my channels were clean (they were) and scratched my head a bit. My solution was to assemble the lower completely without the front pin that retains the front frame locking block. I then took the appropriate drill bit that 80% Arms provided for the machining process, then pushed it through the frame and the front frame rail block in lieu of the pin.

Then, I took a set of padded welding locking pliers, and clamped them on the top of the frame rails and to the dust cover, securing them snugly – but not so tight as to deform anything. I then put the drill bit – still sitting in the frame – into the chuck of my Dewalt cordless drill, and then gently spun up the drill and worked the bit back and forth slowly and carefully a couple times with the drill running, taking extreme care not to elongate the holes in the polymer frame

I then carefully retracted the drill bit and inserted the appropriate pin; everything aligned and assembled perfectly. There were no other issues to report once I got this malady squared away.

Running the GST-9 with a .22 Conversion Kit

Elated that I now know how to build a Glock from my first 80% lower, I decided to first try the GST-9 with my Advantage Arms Glock 19 .22 Conversion kit. This kit has been sitting around with no real home, as it doesn’t play nice with my Gen 4 Glock 19.

To be honest, I was thinking that the .22 conversion kit would make a nice permanent companion to the 80% lower, but alas, it was not to be. The Advantage Arms kit – usually ammo-finicky and somewhat unreliable when perched on other Glocks – proved consistently bad when installed on the GST-9 as well. <Insert sad trombone sound here.>

The Advantage Arms/GST-9 combo had merit as a bug out bag gun or a nice training pistol, but its lack of reliability will relegate the AA .22 kit to the parts bin. However, as a proof of concept to ensure the GST-9 worked, it was a perfect combo.

It must be said that the Advantage Arms kit was the source of unreliability, not the GST-9 frame. To prove this, I moved on to bigger and badder.

Older Model Glocks

As I am not the proud owner of any Gen 1, 2, or 3 Glock 19s or 23s, I borrowed one of each – a Gen 3 Glock 23 and a Gen 2 Glock 19 – from family members to try with the GST-9. 

First was the G23 in .40 S&W. I robbed the magazines, slide, barrel, and recoil spring assembly and moved these parts to the GST-9 frame. Some light handloads with 180 grain plated round nose bullets were stuffed into the mags, and I let fly at the steel plate rack at my local gun club.

Results were most gratifying; six bangs, six plates down. Happily, it seemed this was a repeatable process, as the .40 caliber G23/GST-9 setup gobbled up several magazines of this training handload without a hint of malfunction; it ran and ran, and was very easy to get hits with. The 80% Arms lower parts kit’s 5-lb trigger assembly was crisp for a Glock, and helped me look pretty good at clearing steel plates.

However, when I switched to some hot Remington 155 grain JHP defense ammo, I encountered problems. The ammo would regularly – usually 3 or four times per magazine – nosedive into the feeding ramp instead of coursing into the chamber. Accuracy was excellent with the defense load, and once the round found its way into its home in the chamber, the gun always went off – but I was having failures to feed and that wouldn’t do.

The GST-9 was later vindicated as I was able to try the same ammo in the G23 slide on its original Glock grip frame, and the same results occurred. Guess that particular Glock didn’t like hot big-ass hollow points. C’est la vie.

The GST-9 functioned at 100% after I assembled it with the Gen 2 Glock 19 slide, barrel, and recoil spring. It’s pictured above with the Olight Baldr Mini weaponlight.

The 9mm proved to be a much happier gun in my hands, gobbling up my standard training FMJ handload, 124-grain FMJ Blazer Brass, 147 grain American Eagle FMJ, some old Cor-Bon 115-grain JHP ammo I had, and my usual 9mm defense ammo, SIG Sauer 124-grain V-Crown Elite JHPs.

Accuracy seemed equivalent between the stock Glock grip frame and GST-9; using one grip frame over the other with a given upper assembly didn’t seem to degrade or improve accuracy markedly in either direction.

One Criticism

However, I must report that my shooting experience was quite improved when I was using the GST-9 frame. The lack of finger grooves in the front of the grip, the slight beavertail and undercut trigger guard, and the “gas pedal” ramps all combined to make the gun faster to obtain a proper firing grip that was higher on the frame, allowing the gun to slit lower in the hand for a lower bore axis and better recoil control. 

My only real beef with the GST-9 is probably due to my personal mechanics and you may not suffer from this malady. With the higher grip taken, my right hand middle finger found its way jammed over the top of the magazine release.

While I never had any issues with the magazine releasing with a strong grip, the sharp edges of the mag release dug into my digit rather uncomfortably under recoil. I’ll probably try sanding the stock mag release down to make my life easier, or perhaps search the aftermarket for a more accommodating piece.

Other than this foible, I have had zero issues with the GST-9, and am positively thrilled with the ergonomic benefits it offers over a Gen 1-3 Glock.

Accessories

The GST-9 was designed to be incredibly accommodating for your Glock goodies. The aforementioned rails worked perfectly with multiple lights I had kicking around – Olight’s Baldr Mini and Baldr IR snapped right on the rails, and Streamlight’s TLR-1 and TLR-7 were mounted easily by the multiple sets of rails offered on the GST-9. 

The biggie for me, though, was the fact that the GST-9 slid into every Glock 19 holster I had; light-bearing or not. A leather Bianchi IWB holster, a Raven Concealment Perun, and a Bravo Concealment OWB all swallowed up the Glock/GST-9 combos with excellent retention and security. Kudos to 80 Percent Arms for offering a product that actually works with existing Glock accessories; I’ve heard horror stories of other 80% Glock manufacturers not working so well with holsters.

How to Build a Glock Summary

With the obvious arguments for and/or against a legal, non-serialized handgun set aside, the GST-9 offers excellent benefits for the handgunner. One can go to 80% Arms, Brownells, or any other number of manufacturers to buy the rest of the parts needed to complete your own truly custom, modular handgun; you don’t need to cannibalize an existing Glock.

The benefit of afterthought and a critical eye applied to an existing design allows an aftermarket manufacturer like 80% Arms to offer a product that fixes many of the existing “flaws” that some may not enjoy with an OEM platform. 80% Arms offers modularity, drastically increased ergonomics, and platform compatibility, turning a time-tested, stalwart design into a superb pistol, top to bottom – all without sacrificing or compromising everything that makes a Glock great.

Definitely check out 80% Arms’ GST-9 if you’re looking for a slick, DIY upgrade for your Gen 1-3 Glock 19. The process of finishing an 80% lower on a Glock is easier than you might think – and the results are better than you ever imagined.