Finishing a polymer 80 percent lower is another alternative to buying a stripped lower receiver for your AR build. Before polymer units came out, you’d be stuck choosing between a billet or forged aluminum lower. Drilling and cutting aluminum can be challenging (even dangerous) if you never handled power tools.
Not sure what a polymer lower is? Check our complete guide on 80% lowers.
A polymer lower is easier and safer to finish yourself, making it perfect if you want an AR build that’s more fun than challenging. These polymer units are also more affordable and they include their own 80% jig. Polymer lowers look and function the same way aluminum ones do. If you wanted to practice machining before building a billet or forged one, this is a great way to do it! Here’s how to complete a polymer 80 percent lower:
G150 Phoenix Polymer 80% Lower
For this guide, we’ll be using the G150 Phoenix lower from Polymer80. It’s built from a nylon polymer, similar to what most Glocks and other polymer firearms are made of. Many builders have used the G150 with positive reviews. The G150 is nice because it includes most of the tools and accessories you need:
- Polymer 80% jig
- Polymer 80% lower
- Trigger hole template
- Pistol grip screw
- Pistol grip washer nut
- Bolt catch dowel and set screw
- 3/8 drill bit (for safety selector lever)
- 5/16 drill bit (for bottom trigger hole)
- 5/32 drill bit (for hammer/trigger pin holes)
- 5/16 end mill bit w/ 44.5mm depth indicator
The grip screw and washer nut are unique to this receiver. Because you can’t tap threads into polymer, a square cut-out is provided in the pistol grip mounting area. The washer nut is to be inserted so you can screw your pistol grip in place. The bolt catch dowel and set screw are also proprietary, no roll pin is needed.
The nice part about the included tooling is the 5/16″ end mill bit. To make things easy, the bit includes a black line at the top of the cutting flutes (pictured above). This line is a 44.5mm depth gauge. This is the line you’ll be checking while you cut the fire control cavity. Simply plunge your bit down and cut low enough to get the black line level with the top of the red jig, and you know you’ve cut deep enough. No calipers or measurements required! Now let’s look at what tools you still need:
Tools You Need
- Drill press or mill
- Bench or cross vise
- Machining oil
- Bubble level
Since we’re working with polymer, our drill press doesn’t need to be commercial-quality. If you don’t own one, Harbor Freight offers a nice unit for around $70, called the Central Machinery 8″, 5-Speed Bench Drill Press. It’s bench-mounted and it’ll be more than adequate for our needs. We recommend using a cross vise for your drill press and jig, too.
A cross vise will allow you to precisely move the lower and jig around the end mill bit for cutting the fire control cavity.
Again, Harbor Freight has a simple vise for around $75. You can use a standard bench vise, but you’ll need to move it repeatedly and re-align your bits while you cut. Using a cross vise will make the project easier, quicker, and more accurate.
Setting up The Lower and Jig
Now we have the tools we need to get started. Let’s get familiarized with the lower and jig and see how they work together. Illustrated below, the jig snaps around and completely encloses your lower receiver. There are pre-drilled holes on the jig’s sides that indicate where to complete the trigger pin, hammer pin, and safety selector lever pin holes:
With your jig and lower seated, place your C-clamp where the yellow circle is indicated above and tighten with moderate pressure. Next, secure the assembly in your vise along the yellow line. Make sure the lower and jig are perfectly level using a bubble level before machining.
Step 1: Drill the receiver holes
We want to drill our holes before we cut the fire control cavity and trigger hole. To complete these three holes, you should drill 1/4″ into the receiver on each side.
Do not attempt to drill the holes by going through one side completely.
Doing this will make your drill bits to drift, causing mis-aligned holes. Your lower parts kits’ pins won’t line up, and you may damage the jig.
To drill these holes, it’s strongly recommended you clamp the jig and lower vertically in your vise, as pictured above. Use a hand drill to complete these holes. You should not attempt to drill these holes using your drill press. Laying the jig and receiver horizontally and clamping the jig by the top and bottom will cause the jig to flex, mis-aligning the holes.
Once you’ve drilled all three holes, it’s time to get your end mill bit in your drill press and start cutting the fire control cavity for the lower parts kit.
Step 2: Drill the bottom trigger hole
To start the fire control cavity, we recommend drilling the bottom trigger hole first, using the 5/16″ drill bit. The top of this hole can also act as your pilot hole for your end mill bit. This will be the starting point for cutting out the rest of the fire control cavity. To drill the trigger hole, vertically orient and level the lower and jig in your vise, under the drill press.
Next, lay the red trigger hole template on top of the jig’s fire control cavity template hole (circled in green). Tape the template down so it remains still while drilling. You need to drill completely through the lower receiver only for this trigger hole.
Be careful, don’t push the drill bit so far down that you accidentally drill into the trigger guard underneath. Once finished, the width and length of the trigger hole should match the dimensions of the trigger hole template. The hole should look like the illustration to the right.
Once the rest of the fire control cavity is cut, you can clean up the trigger hole using round files or sandpaper. Do this before installing the lower parts kit.
Step 3: Cut the fire control cavity
You can now remove the trigger hole template from the jig. Remove the 5/16″ drill bit from your drill press and replace it with the 5/16″ end mill bit. Ensure your lower and jig are still seated vertically and leveled using a bubble level. Align the jig and lower so the bit is directly above the trigger hole you just drilled.
To begin cutting the fire control cavity, set the starting depth of the end mill bit so the very tip of the bit is flush with the top of the jig. Adjust your drill stop to 44.5mm. You can now spin up the drill press and begin making plunge cuts using the end mill bit. Plunge the bit down into the receiver until reaching exactly 44.5mm. Repeat this step until most of the fire control cavity has been plunge-cut.
You can then begin milling the fire control cavity using your cross vise if applicable. To do this, ensure the bit is set to exactly 44.5mm, and begin using the cross vise to move the jig and lower around the end mill bit. Use the edges of the jig’s template as a guide for completing the cavity. Start in the center of the cavity, working your way around to the edges (illustrated).
Tips for finishing the cavity
On the polymer 80 lower, the rear lug pocket for your upper receiver’s takedown pin is already completed (circled in green). This pocket is stepped up from the fire control cavity. If you wish, you may leave a few millimeters of polymer material to separate the rear lug from the fire control cavity (vertical green bar). This is entirely optional.
Take care not to apply any pressure against the red-highlighted portion of the lower with your drill press or end mill bit. This area covers the safety detent and spring hole. Removing material in this area will expose the hole and spring. If this occurs, the receiver will not function properly.
Step 4: Final fitting for parts kit
With the fire control cavity finished, you now own a stripped AR-15 lower receiver! Your polymer lower is now ready for a parts kit to be installed. Any standard AR-15 lower parts kit will work. With your first attempt, you might find some parts are tight-fitting, especially the trigger components. Some simply may not fit at all. If this happens to you, take note of where to trim extra material away. We recommend using a Dremel with a mild bit on the slowest setting:
To ensure all parts fit and function smoothly, remove any burrs and rough edges using round files and sandpaper. Remember to do this for the bottom trigger hole, too. If you find the parts kit doesn’t sit in the cavity like it should, ensure your final cut depth is exactly 44.5mm. It may be necessary to go over the cavity one more time using the end mill bit. If more cutting is needed, only remove a few hundredths of an inch of material. You only want to “polish” and clean up the floor of the receiver.
Your trigger assembly may also rub or interfere with the polymer covering the safety detent and spring. If this occurs, lightly sand the area using sandpaper until the trigger kit fits appropriately. Remember, removing too much material here will expose the detent and spring hole, making the lower inoperable.
Wit final sanding and parts fitment, you’ve officially completed your own polymer 80 percent lower. The steps for completing this lower are mostly the same for billet and forged options, so you now have the knowledge to build any AR-platform rifle or pistol you want using a receiver blank!
If you have any questions about parts, installations, or completing your lower, just give us a call or email us. We’re here to help.