Finished cutting and drilling your 80% lower receiver? Congratulations! You're now the proud owner of an AR-15 (well, legally). It doesn't look like much yet, and that's because you still need to assemble the thing. We're going to show you how with easy-to-follow written and picture instructions! No experience is required.
Time required: You'll need about an hour (60 minutes) to complete this install. There's also an important set of tools that are required or recommended:
AR-15 Lower Assembly: Parts & Tools
- AR-15 Lower Parts Kit. Obvious, right? We're using the super-affordable but very high-quality Anderson Stainless Kit for $39.99. It includes everything you need for the grip, trigger, safety and hammer assembly.
- Latch Plate and Castle Nut. These two little parts are pictured above, just underneath the pistol brace's buffer tube.
- Buffer Tube and Stock/Brace. Self-explanatory, take your pick based on whether you're assembling a rifle or pistol. We're using the SBA3 Tactical Pistol Brace.
Tools Required & Recommended
- Castle Nut Wrench/Armorer's Tool. Required for tightening the castle nut, securing the buffer tube to the lower receiver's threads. We've got a forged multi-wrench for $29.99.
- Roll Pin Punches and Gunsmithing Hammer. The punches and hammer are required for properly installing the roll pins for the bolt catch button and (if yours isn't integrated) the trigger guard. You'll need a 3/32" punch at minimum. This kit from AccuPunch includes both the punch and hammer, with some other helpful gunsmithing tools.
- Needle-nose Pliers. Not required, but helpful for seating small pins and springs.
- Masking Tape. Recommended. You'll want some tape to cover the finish on your receiver, lest you risk scratching it with your punches or hammer.
- Lubricant. Recommended. Useful for getting all those pins squeezed into place, lubricant will help to protect your anodized finish.
- 3/16" Allen Key. Required. You'll need this size Allen key (or Allen-head socket with a ratchet wrench) to properly tighten the bolt for the pistol grip.
Before Starting, Verify Your Parts
You don't want to be half-way through your install only to find you're missing a spring, detent, or roll pin. Check the provided parts kit diagram below to verify that you have the required parts for the AR-15 lower assembly. This diagram comes directly from the Anderson Stainless kit:
Step 1:Install Trigger Spring Onto Trigger
Install the trigger spring onto the trigger, so the loop connecting both sides of the trigger rests underneath the sear (front shelf above the trigger lever), with the tabs facing forward.
Step 2: Install Disconnector Spring
Insert the fatter end of the disconnector spring into the small hole on top of the trigger. It helps to use the 3/32" to gently press the spring in place, as it can be a tight fit. Lubricant also helps.
Step 3: Install Hammer Spring
Grab your hammer spring and install it onto the hammer. Ensure the loop connecting both sides of the spring is positioned underneath the hammer hook. The ends of the spring should point in the direction of the hammer's strike face.
step 4: Install Disconnector on Trigger
Grab the disconnector and one of the hammer/trigger pins (they're the same). Drop the disconnector into the top of the trigger.
Drop the trigger and disconnector assembly into the fire control cavity of your receiver. Apply light pressure to the top of the disconnector, so the holes in the disconnector, trigger, and receiver align. Insert the pin's non-grooved end from the left side of the receiver. Then tap the pin into place while keeping pressure on the assembly so the holes remain aligned. Lubricant helps.
The pin should be flush with both sides of the receiver once properly installed. Check the trigger and its springs inside. The ends of the trigger spring should rest on the floor of the receiver, facing the magazine well.
Step 5: Install the Hammer
Next, grab the hammer and remaining pin. Drop the hammer into the receiver so the ends of the spring rest on top of the trigger's shoulders. The ends should be facing the rear of the receiver. The strike face of the hammer should be facing the magazine well.
The hammer spring is much stiffer than the trigger spring, so additional pressure is required to align the holes. With the holes aligned, repeat the pin installation process by lubricating the hole and tapping the pin into place until it's flush on both sides of the receiver. Again, place the grooved end of the pin into the hole from the left side.
You can now perform a function check of the trigger and hammer.
Cock the hammer by pressing it down into the receiver. The sear on the trigger should catch the bottom of the hammer. Next place a finger on the hammer's strike face to prevent it from slamming into the magazine well when release.
Pull the trigger and allow the hammer to release. Keep the trigger held down. Press the hammer back down, and check that the disconnector catches the hammer hook. Once captured, release the trigger. You should hear a small "pop" or click and observe the hammer hook slide off the disconnector. When this happens, the sear on the trigger should again capture the hammer, keeping it cocked.
Step 6: Install Trigger Guard Latch
Next up, the trigger guard latch. If you bought a billet 80% lower or your receiver has the trigger guard integrated into the receiver as a single piece, you can obviously skip this step. Otherwise, simply depress the spring-loaded detent that's already installed, and press the latch into the holed ears behind the magazine well. Lubricating the ears and latch helps reduce friction between the anodized surfaces.
Next is the first roll pin install. For this, you'll need the 3/32" punch and gunsmithing hammer. The ears that hold the pin and latch in place can break here, so it's helpful to support the bottom ear with a small piece of wood or rubber. The rubber handle on our pliers provided the perfect height. Grab the longer roll pin in your parts kit and lubricate the holes on the latch and ears. Align the pin, seat the punch atop it, and gently tap the pin into place. If you meet excess resistance, stop and double-check that all the holes are aligned. Once hammered into place, the pin should be just slightly recessed on both sides.
Step 7: Install Magazine Catch
The magazine catch should be inserted into the hole and channel on the left side of the receiver. You'll need to install the button and spring on the right side. To stop the catch from falling out, you can rest a small wood pencil (or the 3/32" punch handle) against it on the table.
With the catch supported, insert the mag catch spring over the threaded end of the catch. You'll need to compress the spring as you thread the button on. Be sure not to cross-thread the button, as the spring can contribute to this. Thread the button on until it's flush with the receiver.
Now, grab the receiver firmly and use your index finger to press the button in as far as possible. This will push the mag catch back out. Rotate the mag catch clockwise three to four times, align it with the channel, and release the button. The button should be just barely recessed into the receiver. The magazine catch is now fully installed.
You can perform a function check of the magazine catch/release with an empty magazine.
Step 8: Install the Bolt Catch
The bolt catch is the hardest part of the install, thanks to the roll pin. Be sure to tape up the receiver to prevent scratching it. Next, grab the bolt catch spring and plunger and insert the spring, followed by the plunger ontop of it, into the recess for the bolt catch.
The plunger should just protrude from the receiver. Now comes the fun (hard) part: The second roll pin in the parts kit will hold the bolt catch in place between the two ears. Tapping the pin into place is difficult while trying to keep the bolt catch is aligned. It's much easier to partially seat the pin in the first ear, then press the catch into alignment, before finishing tapping the pin into place.
Once partially seated, insert the bolt catch, ensure it's aligned with the holes on the ears, and continue tapping the roll pin until it's flush, just barely recessed on both sides.
Part 9: Install Front Pivot Pin
First, insert the takedown pin detent spring into the small hole on the outside of the right pivot pin hole.
Next, carefully slide the detent into the hole atop the spring. The detent has a very high chance of shooting back out, never to be found again. To prevent this, grab a thin blade to help hold the detent in the hole.
With the detent compressed, slide the pivot pin fully into its mounting holes. Remove the blade once the pivot pin is over top the detent.
Step 10: Install The Safety Lever and Pistol Grip
Next up comes a two-for-one: You'll be installing the safety lever and pistol grip together. First, lubricate the receiver's safety lever hole and insert the safety lever fully from the left side. There may be some resistance; simply wiggle the lever to help get it seated.
With the lever seated fully, flip the receiver upside down to reveal the small hole for the safety lever's detent. Insert the detent into the hole, and insert the detent spring into the small hole drilled into the top of the pistol grip.
Next, press the pistol grip onto its mounting point and ensure the detent spring aligns with the safety lever detent. Check inside the pistol grip to verify its hole is aligned with the threaded hole in the bottom of the receiver.
Grab the pistol grip's Allen-head bolt and washer, press the bolt onto your Allen key or socket, and thread the bolt into the receiver. Be careful not to over-tighten, or you could crack the receiver. Not much force is required, as the washer will keep the pistol grip bolt from loosening when firing your rifle or pistol.
Step 11: Insert Take-Down Spring, Detent & Thread Buffer Tube
Grab the rear take-down pin's spring and detent. First insert the detent into the small hole at the rear of the receiver, below the buffer tube's threaded housing.
Then insert the detent spring over top the detent in the same hole. Next, grab the buffer tube, castle nut, and latch plate. Thread the castle nut onto the buffer tube so the "crown" of grooved side of the nut is facing the rear of the tube. Then throw the latch plate onto the tube.
Step 13: Install the Buffer Retainer
Thread the buffer tube into its housing partially, until it's just touching the takedown pin spring. It should be just about flush with the buffer retainer hole inside the buffer tube housing. Grab the buffer retainer spring and rest it in the hole.
Next, throw the buffer retainer atop its spring inside the tube's housing and compress the retainer with a large punch.
Thread the buffer tube further into the housing until it captures and holds the buffer retainer in place. The nipple on the buffer retainer should protrude next to the lip of the buffer tube. Then press the latch plate up against the back of the receiver so it captures and compresses the takedown pin spring. While holding the latch plate flush, hand-tighten the castle nut up against the latch plate.
Step 14: Tighten Buffer Tube Castle Nut with Wrench
To ensure the castle nut does not come loose during firing, it needs to be tightened with an Armorer's Wrench. You may need to remove your buttstock or pistol brace to slide the wrench onto the nut. Most buttstocks and braces are removed by pulling down on the adjustment lever, unseating the locking pin inside. Tightening the castle nut past hand-tight by one-quarter turn is sufficient. The castle nut can also be staked in similar fashion to the bolts on your bolt carrier group's gas key if you're worried about the nut loosening, though this is not typically required.
Your Lower Receiver Assembly is Complete
With the castle nut tightened, reinstall your buttstock or brace if necessary. Your lower receiver assembly is complete. If you've discovered issues with your trigger assembly failing to reset during initial use, consult the troubleshooting guide below.
Troubleshooting Trigger Reset & Issues
You may experience various issues with your trigger or hammer with a newly-assembled AR-15 lower receiver. This is not uncommon, so don't worry too much. Below are some of the common issues:
- Hammer fails to release from disconnector when resetting trigger.
- Hammer is not captured by disconnector after trigger is pulled/round is fured.
- Trigger feels gritty, or hammer resists releasing from the sear when trigger is pulled.
Fault #1 is most common, though all of the above issues can typically be fixed by addressing all the possible issues and fixes below.
1. Confirm hammer and trigger springs are oriented correctly
It is possible to install the springs onto the hammer and trigger incorrectly. If the hammer spring is installed incorrectly, it will cause various issues considering it is under a high amount of tension. Verify your hammer (and trigger) spring is installed correctly, like shown above.
2. Confirm disconnector spring is installed correctly.
If the disconnector spring is installed upside down, the disconnector will not function properly and your hammer and trigger will likely fail to reset properly. Double-check the "fat" end of the disconnector spring is seated inside the trigger assembly, and the "skinny" end of the spring is facing the notch in the disconnector.
3. Polish hook and disconnector contact points.
If either the disconnector or hammer hook are poorly machined, gritty, or not sufficiently polished, the hammer will likely fail to disconnect from the disconnector and be captured by the sear, thus resetting the trigger after you release it. To remedy this, take some steel wool and very lightly de-burr/polish both the disconnector and hammer hook.
WARNING: Over-polishing or de-burring either contact point too much can cause the hammer hook and disconnector to fail to connect at all, ruining your trigger assembly. Only lightly polish both units and continue testing your rifle or pistol until reliability is achieved.
4. Verify hammer hook and disconnector tolerance.
Inversely, your hammer hook-to-disconnector tolerance may be incorrect. With the hammer cocked by the trigger sear, the distance between the hammer hook and ddisconnector should be 0.003". You can verify the tolerance without measuring by simply releasing the hammer and pressing it down to check if the disconnector captures the hammer. If it does not, your tolerance is likely too great.
5. De-bur and polish the trigger and hammer pins.
If any of the above issues remain, or if your trigger or hammer feel gritty or resist moving freely when not captured, you may need to de-bur and polish the hammer and trigger pins. You can remove them, lubricate them, and gently polish them with steel wool.
WARNING: Over-polishing or de-burring the pins too much can cause them to easily "walk out", or fall out of the receiver assembly during firing. Only use fine steel wool and only gently polish/de-bur the pins. Reinsert the pins and test-fire your rifle or pistol to verify reliability and only continue polishing if either the hammer or trigger feel obviously gritty or get stuck during movement.