Ready to become an at-home gunsmith? Build your own black rifle (or pistol) from scratch? This visual and instructional guide's going to teach you how to complete an 80% lower using an 80 lower jig, a router or drill press, and some basic hand tools. We'll walk you through how to complete the four primary steps of this project with detailed instructions and photos:
The Four Major Steps
- Cut out the fire control cavity
- Drill the hammer pin hole
- Drill the trigger pin hole
- Drill the safety selector lever hole
The jig being used in this guide is a router jig, and we're using a standard forged 80% lower. Lucky for you, this is one of the most basic jigs with a nearly universal design. Even if you're using a different jig and you're completing a .308 or AR-9 lower receiver, you can still follow these general steps for reference.
First, let's take a look at all the tools you should have ready before getting started:
Getting Started: Tool Checklist
- 80% lower jig
- Handheld drill
- Drill press or router
- Drill press bench vise
- Canned air
- Cutting fluid
- Painter's tape
- Ear protection
- Eye protection
The router we're using for this project is the Porter Cable 450 Laminate Router. This is one of the most popular routers for completing a receiver blank, and most jig manufacturers make sure their top plates and router plates work with this particular model.
We're also using a heavy, cast-iron 4.25" drill press vise to keep things stable. Cutting fluid is a must. It'll keep your bits cool and prevent nasty cuts and burrs. A simple brush and canned air will help to keep your work area free of debris.
Step 1: Set up your lower and side plates.
We're going to cut the fire control cavity before we drill our hammer, trigger, and safety selector holes. To prevent damage to our receiver's anodized finish, we've taped it off with painter's tape. Be sure no tape rolls up or touches the top of the receiver. The top of the receiver must be flat and free of obstructions.Once your receiver's been masked off, you need to grab your side plates. Most side plates are labelled "LEFT SIDE" and "RIGHT SIDE". If you're using a multi-platform jig, just follow the markings on the side plates. They'll tell you how to orient the receiver before securing everything together. Each side plate is uniquely shaped to fit the external dimensions of the 80% receiver.
Step 2: Secure the side plates with Allen key and bolts.
Once you've confirmed your receiver is oriented correctly, secure it using the other side plate and provided Allen head bolts. Take care tightening the bolts, and don't over-tighten. While tightening, go in a star pattern (like the lug nuts on a wheel) to ensure each bolt is evenly torqued.
Step 3: Secure the first top plate template.
Next, it's time to secure our first top plate. Each top plate acts as a template. The templates show you where to drill with your drill bits, then where to cut with your end mill bit.
There are threaded holes atop each side plate. These holes correspond with the smaller holes on the sides of the top plate. Secure the top plate to the side plates using the appropriately sized bolts and Allen key. Again, tighten the bolts in a star pattern and take care not to over-tighten (you don't need to gorilla-arm this thing to keep it in place).
WARNING: The rear shelf on an AR-15 receiver sits higher than the main cavity. This top plate separates the two areas, labeled "REAR" and "MAIN". Usually, one of the four bolts securing your jig's side plates goes right through the rear shelf area (and rear takedown pin holes). To prevent accidentally drilling into the rear shelf with the top plate (damaging the lower and jig), it's a good idea to mask the area off.
Step 4: Set your drill bit's "MAIN" depth.
Most jigs come with pre-measured drill depth guides on the top plates. These depth guides tell you how far down into the fire control cavity (the "MAIN" area and the rear shelf or "REAR" area) you need to drill your pilot holes. This particular jig also comes with a drill stop that gets secured to the bit shank with set screws and an Allen key.
For this jig, you'll need to grab the included 3/8" drill bit and the template you'll need is the Rear Shelf Pocket Template. If you're using a different jig, simply refer to the written instructions to make sure you're using the correct depth gauge and you're setting the appropriate depth. With this jig, we must going down to the very bottom of the "MAIN" depth guide. Once the depth is set, secure the drill stop with the set screws so it's flush against the bit and the template plate (pictured).
Step 5: Secure your jig and receiver, drill the pilot holes.
Now the fun part begins! You're ready to begin your first machining step as a real gunsmith. Drilling the pilot holes for the fire control cavity can be completed with a handheld drill or with a drill press. But first, you need to secure the receiver and jig in your bench vise, as pictured below. Don't over-tighten the vise jaws. You could warp the side plates and cause your entire setup to become mis-aligned.
A drill press is recommended (pictured) since it offers better control and guarantees a perfect vertical alignment. A handheld drill can be used just as well, you simply need to take your time and ensure the drill bit is aligned vertically in each hole. Before drilling, ensure you've lubricated the top plate, the holes you're drilling through, and the receiver with plenty of cutting fluid.
Now you can drill!
Once ready, drill out each hole on the top plate. Go down until the drill stop touches the top plate (pictured below). This is how you know you've drilled far down enough. Most drill presses also have a stop or measuring gauge. Use it to double-check your depth, if applicable.
It's a good idea to periodically apply more cutting fluid and to occasionally clean out aluminum debris (pictured above) from the holes with canned air.
WORKING TIP: Check your bit depth with the depth gauge after each hole has been drilled. This will ensure the drill stop hasn't moved, and that you're not drilling too deep or shallow.
Once you've finished drilling the "MAIN" pilot holes, your receiver should look like this:
Step 6: Set the "REAR" drill depth. Drill the rear shelf.
Only perform this step if your 80% lower's rear shelf IS NOT pre-milled (pictured below). Our receiver's rear shelf was already milled out (pictured above). We're simply exampling this step just in case yours is not. If your rear shelf is already milled for you, skip this step and go directly to step 7.
If your 80% lower doesn't have a pre-milled rear shelf, then you'll need to remove the masking tape from the "REAR" holes on the top plate template.
WARNING: LOOSEN THE SIDE PLATE SCREW THAT GOES THROUGH THE REAR TAKEDOWN PIN HOLE FIRST.
DO NOT REMOVE THE SCREW COMPLETELY.
You must loosen the side plate screw that secures near or through the rear shelf area (circled in green) before drilling the rear shelf area. Removing the screw completely may allow the lower to slip. This must not occur, or your alignment will be off. Loosen the screw enough that it no longer protrudes into the rear shelf area. Leave the screw in the jig just enough that it partially holes the receiver, pictured below.
Fail to loosen the screw appropriately, and your drill bit will contact the screw, damaging it and the receiver. Once the screw has been loosened and the rear shelf area is free, it's time to set your drill bit's new depth.
Using the Trigger Pocket Template (if using this jig), repeat step 4: Set the bit depth so it bottoms out in the "REAR" depth gauge.
Tighten the drill stop with the set screws, and repeat step 5, drilling the pilot holes for the rear shelf.
Step 7: Set up the next top plates for finishing the fire control cavity.
With all the pilot holes for the fire control cavity drilled out, it's time to swap out the first top plate for the next set of plates. These next plates will allow us to complete the fire control cavity with our end mill bit, removing the remaining aluminum that the drill bit didn't remove initially. You'll need the router base support plate (center-left), the template spacer (top-right), and the trigger pocket template plate (bottom-right):
First, place the spacer template atop the side plates like so. Other jigs may not have a spacer, and may only use one plate:
Then, lay the trigger pocket template atop the spacer like so:
Lastly, rest the router base support plate atop the trigger pocket template like so. Secure all three plates to the jig side plates using the appropriate screws and Allen key. Tighten in a star pattern and be careful not to over-tighten:
Step 8: Set up the end mill bit for cutting (drill press or router).
Now that the jig and receiver are ready for cutting, it's time to set up your power tools. It's recommended to use a router for this step, though a drill press can be safely used with the end mill bit by performing plunge cuts and making shallow cutting passes. Drill presses are not designed to take side-loads like a milling machine, so do not try to perform these steps as described with a drill press. These steps are written with a focus on using a router.
First, secure the included end mill bit in your router or drill press. Grab the rear shelf pocket template and use the depth gauge on the template to set the initial depth of the end mill bit:
Step 9: Cut the fire control cavity to completion.
Once your router (or drill press) is set up with the end mill bit and the initial depth has been set, ensure your tools are turned OFF. Apply plenty of cutting fluid to the bit, the receiver, and inside and atop the plates. Finally, seat the end mill bit down into the fire control cavity so it's resting inside one of the pilot holes.
Once set, turn on your router or drill press. If using a router, gently move the router and bit between pilot holes in a zig-zag pattern. This will begin the cutting process, shaving off layers of aluminum. If using a drill press, do not attempt to make this same zig-zag pattern unless you own a quality press and you're cutting with a very shallow depth with each pass. Otherwise, make plunge cuts with your drill press, moving the receiver and jig around while you move the end mill bit up and down to accomplish the same cuts.
With the first cutting pass complete, your receiver's cavity should look like this:
Take a moment to clean out the cavity using canned air. Apply plenty of cutting fluid and then increase the depth of the end mill bit using the depth guide. For billet (6061 T6) lowers, increase the depth by 1/4th to 1/3rd of a tick mark. For forged (7075 T6) lowers, only increase bit depth by 1/6th to 1/4th of a tick mark:
Continue increasing bit depth until the end mill bit bottoms out in the depth gauge. When this happens, make one final pass to finish the "MAIN" part of the fire control cavity:
Continue cutting with the final depth until the bottom of the cavity is smooth and flat. At this point, the main fire control cavity is complete! If your rear shelf was not pre-milled, you should have two pilot holes remaining in the location.
If your rear shelf was pre-milled, skip to step 11.
Step 10: Cut the rear shelf.
Now, remove the router base plate and the trigger pocket template plate. Leave the spacer plate in place. Locate the rear shelf template and plate it atop the spacer plate. Orient this new template so the larger hole is near the buffer tube housing. This is the hole that will be the template for the rear shelf.
Be sure to loosen (but not remove) the screw that goes through the rear shelf area, as described in step 6 (pictured below). Re-secure the jig assembly with the Allen-head screws, sandwiching the rear shelf template between the spacer and the router base plate, as pictured:
Once secured, grab the Trigger Pocket Template and use the "REAR" depth guide on the plate to set the depth for the end mill bit and rear shelf:
Follow the same steps as described in steps 8 and 9 to finish cutting the rear shelf. Continue increasing bit depth until you bottom out the gauge on the trigger pocket template. The finish rear shelf should look like this:
Step 11: Drill the trigger slot pilot hole.
We're finally almost finished with the fire control cavity. The last thing to do is complete the hole for the trigger in the bottom of the receiver.
First, we need to drill a pilot hole for the slot. Remove all plates from the jig, as pictured above. Grab the trigger pocket template once again, and secure it to the jig side plates as pictured below. You can only use only two of the four screws because of how the plate must sit atop the jig and receiver:
Next, secure the included 19/64" drill bit in your drill press or handheld drill. Drill the pilot hole for the trigger slot. There is no depth gauge here, you're just drilling through the floor of the receiver. Be careful not to go too far down. You could accidentally drill into the trigger guard or bench vise underneath.
Step 12: Set up the jig to cut the trigger slot.
Just like the "MAIN" fire control cavity, we can now mill out the trigger slot with the pilot hole we just completed acting as a starting point. Remove the trigger pocket template and locate the rear shelf template once again. Install it atop the jig side walls as pictured (do not use the spacer plate underneath), so the smaller cut-out is atop the pilot hole you just drilled. Then, install the router base plate using two of the four Allen-head screws to secure everything together:
Next, set up your router or drill press with your end mill bit once again. No depth gauge is required - again, we're just cutting out the trigger slot until we go through the floor of the receiver. No drill stop or "final depth" is necessary. Make cutting passes with your router (or plunge cuts or very shallow cutting passes with your press), increasing the depth incrementally until the entire pocket is milled out.
The finished trigger slot should look like this:
Step 13: Drill the hammer, trigger, and safety pin holes.
You're almost done! Completing the fire control cavity (and potentially the rear shelf) is the most complex part of this project. Now you simply need to drill the hammer, trigger, and safety pin holes. We're going to re-orient our 80% lower and jig so they're resting horizontally on our vise.
WARNING: We strongly recommend against trying to drill each hole by going completely through one side of the jig and receiver. Your drill bits will likely flex and "walk", mis-aligning the holes and ruining all your hard work. Instead, drill each hole on one side of the receiver. Then flip the receiver over and do the other side. Each jig has each hole marked with the appropriate drill bit:
Step 14: Clean up the receiver.
Pretty self-explanatory, right? Remove any aluminum shavings left over, and wipe the receiver down with some oil.
Congratulations! With all the holes drilled, your 80% lower is no more. Now, you're the proud owner of a real AR-15 stripped lower receiver.
You're ready to install a lower parts kit and trigger, slap on a buffer, buttstock and upper, and hit the range! Your fire control cavity may not look perfect - that's okay. As long as you followed instructions and set your bit depths correctly, your lower parts kit and trigger will install just fine. Your jig is reusable, too, so grab another receiver blank and build another one when you're ready to master your skills!