80 Percent Arms GST-9 Mod 1 Completed (image courtesy JWT for thetruthaboutguns.com)
80 Percent Arms GST-9 Mod 1 Completed (image courtesy JWT for thetruthaboutguns.com)
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Last year, I had a good time building an 80 Percent Arms GST-9, not quite GLOCK 19 pistol. That was a fun gun to build, and it has run without a hiccup. Since then, I’ve ordered and built just about everything 80 Percent Arms makes, and a few of those builds have been detailed on TTAG.

When 80 Percent Arms’ announcement of a new version of their pistol frame kit, the GST-9 MOD1, I had zero hesitation in ordering one.

80 Percent Arms GST-9: MOD1 Pistol Frame
Image courtesy JWT for thetruthaboutguns.com.

I’m glad I did. The GST-9 MOD1 is just as good as the original, and may be even easier to build. On top of that, the 80 Percent Arms GST-9 MOD1 frame kit is educational, enjoyable, and the end result is a reliable EDC pistol.

80 Percent Arms GST-9: MOD1 Pistol Frame
Image courtesy JWT for thetruthaboutguns.com.

Beyond, the frame, the 80 Percent Arms GST-9 MOD1 build requires that you also purchase the Jig Kit separately. It’s not possible to complete the the frame without the Jig Kit, which also includes the metal rails for the gun.

The rails are specific to the MOD1, and they aren’t sold separately. You will need to purchase a Jig Kit for every frame kit you purchase in order to finish the frame. Even if you were to purchase the entire pistol build kit, which includes a slide, (currently out of stock), you will still also need to purchase a Jig Kit.

The fact that you must purchase at least two separate products in order to finish a frame is important for BATFE legal compliance as it currently stands. Look carefully, and you’ll see that 80 Percent Arms doesn’t even list the Jig Kit with the other parts for their builds. It’s always listed separately, and sold separately.

The BATFE is rarely consistent in its decisions, but as it’s sold, to consider the 80 Percent Arms GST-9 MOD1 Frame Kit a firearm would be a stretch well beyond any reasonable definition of the law. Than again, a bump stock is currently considered a machine gun, so….

80 Percent Arms GST-9: MOD1 Pistol Frame
Image courtesy JWT for thetruthaboutguns.com.

When comparing the internals of the GST-9 MOD1’s frame (above right) to a GLOCK brand GLOCK frame (above left), the differences are fairly obvious. The rear rails of the GLOCK are molded into the frame, whereas the 80 Percent Arms frame has the rails separate. The metal forward rail sections of the GST-9 MOD1 are also almost twice as long as the GLOCK, and aren’t split by polymer, as they are on the GLOCK.

This is part of the intelligent design that’s gone into the 80 Percent Arms GST-9 and MOD1. The long, all metal rail allows for easy installation, sitting right over the frame, once you’ve removed the frame tabs fore and aft. Beyond simple installation, there may be an added benefit, if only theoretical.

80 Percent Arms GST-9: MOD1 Pistol Frame
GLOCK left, GST-9 MOD1 right. Image courtesy JWT for thetruthaboutguns.com.

In general, I’m more confident in the durability of an all-metal rail. That said, I’ve heard of exactly one GLOCK rail section breaking in my life, and I’ve never actually seen it, so I’m not convinced that durability is really a concern, with either the GLOCK or the 80 Percent Arms rail system.

I am also completely unconvinced that either system adds any real functional improvement over the other. Still, give me a choice between metal or polymer and I’ll pick metal.

There are just a few differences between the original 80 Percent Arms GST-9 version I reviewed this time last year. Inside, the biggest difference is that, for the new MOD1, the rear rails are in two separate pieces. The older version was joined by a crossbar at the bottom.

Some users reported trouble getting the horizontal bottom of the original rear rail set to fit into the frame. The two-piece rail set, each side of which is held in place by molded slots in the frame, solves this potential issue. I had no issue with either version, and both slipped right in and then held fast once assembled.

80 Percent Arms GST-9: MOD1 Pistol Frame
Image courtesy JWT for thetruthaboutguns.com.

Also, take a look at the front portion of the frame of both the old frame (bottom) and the new version (top) in the photo above. You’ll find the newer version has a shorter metal marking bar, presumably for a serial number if required by your state. This corresponds to more and thicker material just forward of the trigger guard.

It also means there’s one fewer slot for the Picatinny rail section of the frame. Three slots was plenty for me to mount a variety of devices to the finished gun.

80 Percent Arms GST-9: MOD1 Pistol Frame
Image courtesy JWT for thetruthaboutguns.com.

Externally, the MOD1 (above left) removes the stair-step thumb rest above and forward of the trigger guard, replacing it with a simpler ridged border. I liked the original version (above right) better, as it was obvious and grippy, even with gloves on.

For folks with larger hands and longer thumbs — like me — the old version may get a little uncomfortable during long strings of fire when bare-handed, so I can understand the change.

80 Percent Arms GST-9: MOD1 Pistol Frame
MOD1 (left), original frame (right) Image courtesy JWT for thetruthaboutguns.com.

The grip texture on the front strap is also changed on the MOD1, eschewing the stippling of the original (right) for broken horizontal cuts (left).

Otherwise, the feel of the MOD1 is the same as it was on the original GST-9. You’ll find stippling throughout the frame, a high cut on the trigger guard, and a generous beaver tail at the rear of the pistol. Just like the original, it feels good in the hand and gives the shooter plenty of real estate for solid control of the gun.

80 Percent Arms GST-9: MOD1 Pistol Frame
Image courtesy JWT for thetruthaboutguns.com.

For my first GST-9 build, I did everything as carefully as I could. My tools included:

  • Drill bits and cutting bit (included with the kit)
  • Drill press
  • Dremel tool
  • Clamps
  • Wire snips
  • X-Acto knife
  • Compressed air
  • Metal rulerFile1200 grit sand paper
  • Punch
  • Mallet

I’ve built a few more 80 Percent Arms guns since then, and those include other pistol builds. So for this GST-9 review, I decided to keep it as inexpensive as possible, and go down-market a bit on the tool kit.

For the MOD1 build, my tools included:

  • Drill bits and cutting bit (included with the kit)
  • Handheld drill
  • Pocket knife
  • Clamp
  • 1200 grit sandpaper
  • Punch
  • Mallet

If the build quality suffered, I certainly can’t tell.

Like I said, I’ve done a few of these now, and this time the entire process didn’t take me 15 minutes. The longest part of the entire process was screwing the two halves of the jig together.

80 Percent Arms GST-9: MOD1 Pistol Frame
Image courtesy JWT for thetruthaboutguns.com.

After you put the frame in the jig, you’ll need to remove the four plastic tabs sticking out of the top of the frame as well as remove the polymer shelf at the front of the frame.

If I can provide any advice on these builds, it’s to leave the rotary tool on the shelf, and simply cut this material out with a thin bladed knife or scalpel. It’s harder to accidentally cut out too much material this way, and at the same time, it’s proven to actually be faster…at least for me. Smooth these cuts up with sand paper or a file.

80 Percent Arms GST-9: MOD1 Pistol Frame
Image courtesy JWT for thetruthaboutguns.com.

After that, the only thing you need to do is drill some holes and install the rails. It’s almost impossible to screw that up. The instructions are color-coded, simple, and comprehensive at the same time. Really, there is no doubt as to your ability to complete this frame using the kit and instructions provided.

After the frame was complete, I installed the G19 OEM Lower Parts kit. It goes right in just like any other OEM kit, and there are many videos online to walk you through the process if you are unfamiliar. It will take you more time to watch the video than to actually install the parts.

80 Percent Arms GST-9: MOD1 Pistol Frame
Image courtesy JWT for thetruthaboutguns.com.

My kit included a Lone Wolf Distributors trigger, trigger bar, and trigger housing. Note the trigger housing had a set screw to control trigger overtravel. When mine came, it showed up like in the photo above. The tiny set screw is too far forward, preventing the eventual release of the striker. This means the gun will install fine, assemble fine, look fine, cock fine, and the trigger will pull just fine…but will simply never release the striker.

If you don’t recognize this prior to installing the trigger housing, it will lead to an extremely frustrating experience, and make you may seriously reconsider your purchase. Ask me how I know.

Just like the original, the MOD1 allows for two different grip lengths. It comes stock with a grip that will fit GLOCK G19-length magazines. For a penny shy of a twenty dollar bill, you can purchase the full-size grip extension. Two pins pushed out and reinstall, allowing you to swap back and forth as you please, essentially between a G19 or G17 grip and magazine length.

Since the GST-9 MOD1 frame accepts GLOCK 19, GLOCK 19L, and GLOCK 19XL slides, the modular grip length allows you to complete a fairly wide variety of combinations. The 19X style, with a G19 length slide and a G17 length grip is my favorite set-up, and is another combination easily doable with the $20 full-size grip module.

Staying with the inexpensive theme, I put a Tactical Kinetics G19 Gen 3 Slide on top of the GST-9 MOD1 build. I’d never heard of Tactical Kinetics before, but they had a flash sale for 50% off their slides. They delivered a fine product for under $200, and it was at my door in less than a week. This means my total build cost for a complete pistol was a couple bucks under $500.

80 Percent Arms GST-9: MOD1 Pistol Frame
Image courtesy JWT for thetruthaboutguns.com.

The combination worked great with zero issues regarding reliability. Just to make sure the frame worked with other slides, I tried a few other slides on the MOD1 frame, including the 80 Percent Arms slide I originally had on the first build (above), a Wilson Combat modified GLOCK 19 slide, as well as a stock GLOCK 19 slide.

I ran 300 assorted 9×19 rounds through the frame, with all four slides. The gun ran perfectly no matter what slide I used. I shot the pistol with OEM GLOCK magazines as well as Magpul polymer magazines. I had no issues with any magazine loading or ejecting, and no problems with any round loading, firing, ejecting or cycling, no matter what slide I put on top of the MOD1 frame.

The MOD1 isn’t much different than the original 80 Percent Arms GST-9 build, and that’s a good thing. They all fit in the standard holsters that take their GLOCK brand GLOCK counterparts, and accept all of the same internal aftermarket parts as well.

These kits are fast, easy, and fun builds that result in great guns.

80 Percent Arms GST-9: MOD1 Pistol Frame
Image courtesy JWT for thetruthaboutguns.com.

80 Percent Arms GST-9 Pistol Frame
Price: $149.99 (currently in-stock)

GST-9: MOD1, Jig w/ Tool Kit & Slide Rails
Price: $29.99 (currently in-stock)

GST-9: MOD1, Full Size Grip
Price: $19.99 (currently in-stock)

G19 Glock Lower Part Kits (OEM)
Price: $99.99 (currently out-of-stock)

Rating (out of five stars):

Overall * * * * *
I’ve tried quite a few different builds over the last year, and the 80 Percent Arms kits are my favorites. The new MOD1 lower is no different, and I look forward to building more of them in the future.

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  1. Putting together a kit is not actually building a gunm.
    Like my son bragging to friends on a couple rifles I put together, ” Dad built this.”
    You built that?
    ” Well not really, I just put a bunch of parts together, it is nice though, I did stain and varnish the wood. “

  2. Providing the 80 rails fit the slide with little wiggle is a very good reason to buy for those who want to squeeze accuracy out of every round. A loose fitting slide lets a striker fired trigger shift the slide slightly to the left especially with OEM striker springs.

    Using an Unloaded striker fired handgun the slide may shift a bit when bouncing the trigger off its first point of resistance. Some striker handguns shift more, some shift less and some do not move even with rail play. Slide shift is visible from the back of the gun between the halves. The main reason we use the Sar9 is its removable chassis with integrated rails can be widened to .002″ clearance between the rails and slide. We have an in house made jig to hold and properly expand the chassis rails, for now the tooling and process is a trade secret.

    I agree possum…we are all lowly assemblers…However some of us can assemble things better than the factory. Hurrah.

    • Lone Wolf Arms. Their frame accepts both Gen3 and Gen4 triggers and uppers. Polymer80 only accepts Gen3.

      • Good looking out! I also found more people using the gen 4 slide on p80 frames with the gen 3 guide rod, a little adapter button, and a little extra dremel love on the dust cover of the frame. When I looked before it was like one guy that shaved a p80 down so far it was transparent. I guess the gen 3 guide rod really helps the process.

  3. “Still, give me a choice between meal or polymer and I’ll pick metal”.

    Meal would be easiest to digest.

  4. What an exceedingly clear, concise, and intelligible review and reflection on a DIY handgun kit. As always- your thoroughness and insight are appreciated, JWT.

  5. Several years ago I built a G19-type 80% and a G43-type 80%. The G19-type works like a dream. The G43-type is a piece of junk. Unless you have no choice but to build your own 80%, you should think of these kits the way you think of a trip to the casino. Don’t put up any more money than you can afford to lose.

    • It fits most holsters designed for a Glock 19; however, if the holster has extremely tight tolerances, there could be figment issues. I’ve not encountered any personally, but I don’t own that many different holsters.

    • From the article:
      “They all fit in the standard holsters that take their GLOCK brand GLOCK counterparts,.. “

      • Thanks, JWT. Looking at the side by side with a Glock, it looks like that may be the case. I got a P80 PFC9 and was sorely disappointed it didn’t fit in my Glock holsters.

  6. It’s interesting that you claim the jig must be sold separately for it to be ATF compliant, because I just purchased a Lone Wolf Freedom 80% frame and it came with the jig. I’m fairly sure Polymer80 also ships their frames with the jig included as well.

    • Read what I wrote again. I I didn’t say that it must be sold separately for ATF compliance, I said that it was important for ATF compliance. That’s why I go into detail about how, even with the ever-changing and capricious nature of ATF definitions, this particular method is well and obviously within the limits of the law.

  7. The gst line fixes all issues I had with how uncomfortable glocks frames are in the hand. They feel like an original 320 frame, very rounded and quite comfortable.

    I have to agree with the fellow above re “assembling parts” rather than building a firearm. I think the first time I felt like I actually “built a firearm” was an Ak build. Even then it’s just putting parts together in a much more difficult manner.

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  10. 80% arms sells two different kits a v1
    and a mod1…they are not the same…and are not interchangeable…and if you make the mistake of thinking they are…you are on your own…P80 doesnt play version games…and if you have an issue they are good about helping you…wouldnt suggest 80% arms…

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