How to Build an 80% Pistol thumbnail image

How to Build an 80% Pistol

80 Percent Arms   |   Mar 8th 2021

A polymer 80% pistol such as the GST-9 is a fantastic firearm for home defense, concealed carry, or range shooting on the weekends. But before you purchase one, you must know how to build a polymer 80% pistol, which isn’t overly complicated once you see each step before you in detail. Let’s begin!

Introduction to Polymer 80% Pistol Builds

First and foremost, before we delve into the nitty-gritty of building a polymer pistol from scratch, there are some key factors you must understand. For instance, it is entirely legal for you to create a handgun in your home, without registering or serializing your newly constructed pistol. You need not hold an FFL or visit a local gun shop, pay fees, or file paperwork. That being said, check your local and state laws, as some states, such as California, are a little stricter about constructing firearms.

Tools You Require

tools needed to build 80% pistol

You cannot build a polymer 80% pistol without the appropriate toolset. You’ll need:

  • Hammer
  • Hand Drill
  • Bubble Level
  • Tabletop Vise
  • Roll Pin Punches
  • Files or Sandpaper
  • Drill Press, Dremel, or Snips

Polymer 80% Pistol Frame Kits

If you opted to buy a 80% polymer pistol frame (and you should), your purchase likely includes the following:

  • 80% Pistol Jig
  • 80% Pistol Frame
  • Locking Block Rail System
  • Locking Block Rail Pins
  • Trigger Housing Pins
  • Rear Rail Module
  • End Mill Bit
  • Drill Bits (M3, M4)

Step 1: Preparing Your 80% Pistol Frame

Insert your 80% pistol frame into the accompanying jig, securing the entire setup in your tabletop vise. You’ll want to ensure you do not over-tighten the vise, as it is entirely possible to crush both the jig and frame under pressure.

With that out of the way, pick up your hand drill with an attached sanding drum. Using the sanding drum, slowly and carefully remove the excess polymer exposed above the jig at the frame’s front. You can follow-up with a finishing file and sandpaper to remove any excess with greater precision.

Step 2: Drilling the Holes

how to build 80% pistol

With the frame carefully seated and flush inside the jig, it’s time to drill the trigger, locking block, and trigger pinholes. For this step, remember not to use a drill press. Neither the jig nor frame should be tightly-gripped at the top or bottom while drilling, as the jig may flex and misalign.

Your only other option is to use the tabletop as mentioned earlier vise, though you’ll want to level it using a bubble beforehand to ensure an even cut. It’s best to rotate the frame and jig inside of the table vise vertically for this stage. The vise should have a firm grip on the bottom half of the jig; snug but not too tight to avoid warping.

Every hole on the frame is marked with a drill bit. When drilling, we recommend going half-way through one side, then finish on the opposite side.

Step 3: Cut the Top Rails

With the holes thoroughly drilled and prepped, it’s time to move on to the top rails. You can leave the jig set up inside of the vise for the time being. You’ll note markings indicating where to remove polymer from the frame along the rails on the jig. You can use a drill press, Dremel, hand file, or snips to remove any excess. Typically, a Dremel or snips are used.

Your goal is to ensure the frame is flush along the top of the jig. Do not cut the jig, though. It’s common for first-timer builders to shave off a portion of the jig which jut up over the frame.

Once you’re done removing excess polymer, using your tool choice, sand down the ragged remnants until smooth.

Step 4: Cut the Barrel Block

Cutting the barrel block, for a newcomer, is said to be the most difficult, time-consuming step, so use caution and do not rush yourself. 

For step number four, you’ll require a Dremel, hand file, or a drill press. The goal here is to remove excess material inside of the frame. However, be wary of the slide buffer stop, as removing any polymer here will turn your frame into a hunk of junk.

Whether you’re using a hand file or Dremel, you’ll want to keep the frame and jig vertical and secure inside of the vise. Then, slowly sand and file down the material starting at the top until you come to the slide buffer’s lip.

If you’re using a drill press, use the provided end mill bit to remove the excess polymer. You’ll finish by sanding the rough edges of the slide buffer until it is smooth and rounded.

Here’s a little rundown:

  1. Install the locking block rail
  2. Install the magazine catch and catch spring
  3. Install the slide lock lever and spring
  4. Install the trigger housing and trigger
  5. Install the barrel, slide, and test both for functionality

That’s it! Your 80% pistol frame is complete. But now, you need to add a slide and internal parts to hold a working firearm in your hands.