How to Build an AR15 Lower
Building your AR15 is a rewarding experience and it allows you to configure your rifle exactly how you want it. Building your rifle from scratch is usually less costly and helps you avoid having to deal with any unwanted parts you might be left with when upgrading an existing rifle. Cheaper Than Dirt! wants to help you create your custom rifle by not only providing you with the tools, but also the knowledge you need to build your perfect AR15.
The first step in the building process is to purchase a stripped lower receiver. The lower receiver is considered a firearm unlike the rest of the AR15 parts you will need. If you want to know more about how to buy a firearm online you can refer to our easy-to-follow guide.
Step No. 2 is to buy all the parts you need to assemble the rifle. Complete rifle kits with a prebuilt upper will include everything you need to complete a functioning rifle (minus the stripped lower receiver). You can also buy the parts à la carte if you would like a more customized rifle.
Building the Lower Receiver
When you have all of your parts ready to install on your stripped lower receiver, make sure you have a clean and clear workspace.
Installing the Lower Receiver
Select Installing Lower Receiver Instruction for:
1.) Place tape around the magazine catch slot to protect the finish.
2.) Insert the magazine catch into the slot on the left side of the receiver, and then insert the magazine catch spring over the threaded stud that appears in the hole on the right side of the receiver.
3.) Once you have both those parts in place, you can start by threading the magazine release button two turns onto the threaded stud with the smooth side facing the receiver.
4.) When the button is threaded onto the stud, depress the magazine button with a large punch as far as it will go below the surface of the receiver and tighten the magazine catch until the threaded stud is flush with the button.
5.) Line the magazine catch up with the slot in the receiver and release the pressure on the button.
6.) Insert a magazine, and then depress the magazine release button to make sure your magazine drops free from the magazine well.
1.) Place tape around the bolt catch area to protect the finish.
2.) Start the bolt catch roll pin from the front of the receiver with a roll pin starter or tape the pin to a roll pin punch to hold it in place.
3.) Insert the bolt catch spring into the bolt catch hole on the left side of the receiver. The thin end of the bolt catch plunger should be then inserted into the spring. Now place the bolt catch into the cutout in the receiver with the pingpong paddle orientated to the top of the receiver.
4.) Insert a small roll pin punch from the front of the receiver to keep the holes inline. Use your roll pin punch to drive the roll pin into the receiver and bolt catch until it is flush.
5.) Press on the bottom of the bolt catch to make sure the catch moves freely and springs back into place.
1.) Insert the pivot pin install tool that has a hole into the pivot pin hole and line the hole with the spring channel. You can also use a large allen wrench, pliers and a small punch.
2.) Drop the spring into the spring channel through the hole in the pivot pin install tool. Now drop the detent into the hole and depress the detent with the pointed pivot pin install tool. Once the detent is depressed, turn the pivot pin install tool 90 degrees to capture the detent and spring.
If you declined to use the pivot pin install tool the procedure is a little different. Insert the large allen wrench just as you would the install tool until it is flush with the spring channel hole. Insert the spring and use a pair of needlenose pliers to insert the detent. When the detent is in as far as it will go using only the pliers, depress it fully with the small punch and slide the allen wrench over the detent and spring.
3.) Press the narrow end of the pivot pin against the left side of the pivot pin install tool. Use the end of the pivot pin to slide the tool to the left side of the receiver taking care not to let the pivot pin detent or spring escape.
4.) Once the pivot pin has captured the detent and spring, rotate the pivot pin to ensure the channel lines up with the detent.
5.) Depress the pivot pin fully then push it out as you would if removing the upper to test function.
1.) Place the trigger spring with the “U” shaped portion under the leading tail of the trigger with the two legs oriented to the bottom of the trigger and the loops around the trigger pin extensions.
2.) Insert the disconnector spring with the wider portion into the hole on the top of the trigger. Place the disconnector into the slot in the upper part of the trigger with the hook pointed forward and the spring cutout oriented over the disconnector spring. Now make sure the hole lines up with the hole in the trigger pin extension.
3.) Next, install the hammer spring with the “U” shaped portion around the rear of the hammer and the loop of the spring orientated to the rear of the hammer.
4.) Place the trigger and disconnector assembly into the lower receiver, and then press on the disconnector to line the holes up with the trigger pin holes in your receiver. Once you have them lined up, you can now insert the trigger pin into the trigger pivot hole. You may need to use a hammer to tap the pin flush lightly with the receiver.
5.) Now install the hammer with the flat part of the hammer facing forward, when placing the hammer into the receiver make sure to drape the legs of the spring over the trigger pin. Line the holes up with the corresponding holes in the receiver and push the hammer pin until it is flush. You may need to use a hammer to lightly tap the pin into place.
6.) Pull the hammer back to cock it and pull the trigger while holding the hammer to verify function. Do not let the hammer fly freely and impact the receiver or it may damage the receiver.
1.) Place the lower receiver on its side and position the holes in the trigger guard ears over a hole in the bench block.
2.) Insert the trigger guard between the trigger guard ears with the spring loaded detent nearest to the single hole to the rear of the magazine well then depress the detent and lock the trigger guard in place.
3.) Make sure the holes in the trigger guard ears line up with the hole in the trigger guard. Now position the roll pin over the hole and carefully tap the roll pin into place with the proper roll pin punch. Stop tapping when the roll pin is just past the surface of the receiver.
4.) Use a small punch to depress the spring loaded detent to ensure that the trigger guard swings clear of the trigger. This feature is handy when shooting with heavy gloves.
1.) Cock the hammer to apply pressure to the trigger group to keep it clear of the selector’s path. Insert the selector switch from the left side of the receiver.
2.) While holding the selector in place, turn the receiver over and drop the selector detent into the detent hole with the point facing the selector.
3.) Slide the selector detent spring into the spring channel in the grip and slide the grip onto the lower receiver with the spring depressing the selector detent.
4.) Place the lock washer on the grip screw then use a large flat head screwdriver or the appropriate size allen wrench to secure the grip to the lower receiver. Take care to not over tighten and strip the threads.
5.) With the hammer cocked, place the selector on safe to check function of the safety. If pulling the trigger does not allow the hammer to drop the safety selector is working as intended. Now place the hammer on fire and pull the trigger while keeping hold of the hammer to keep it from impacting the receiver. If the hammer is released when the trigger is pulled, it is functioning correctly. While holding the trigger to the rear, recock the hammer and release the trigger. The hammer should move forward when the disconnector releases the hammer and stop when the sear surfaces contact each other.
1.) Start by threading the castle nut onto the receiver extension with the four cuts in the nut facing the rear of the receiver extension. Place the end plate onto the receiver extension with the protruding side facing forward.
2.) Begin threading the receiver extension into the lower receiver and stop before the threads begin to cover the buffer retainer pin hole.
3.) Insert the buffer retainer spring into the buffer retainer hole first, then place the buffer retainer on top of the buffer retainer and depress the retainer with a large punch or your thumb. Continue to thread the receiver extension into the receiver until the end of the receiver extension captures the retaining pin’s shoulder.
4.) Insert the takedown pin from the right side of the receiver and face the slot in the pin towards the rear of the receiver. Then install the takedown pin detent rounded end forward into the spring channel in the rear or the receiver and follow it with the detent spring. It is totally normal for the detent spring to stick out of the rear of the receiver about a half an inch.
5.) Line the protrusion on the end plate up with the hole of the same size in the back of the receiver. Taking care not to bend the detent spring, slide the end plate against the receiver and tighten the castle nut by hand.
6.) Finish tightening the castle nut by torquing it to 40 inch pounds with your armorer’s wrench. Use a prick punch to stake the castle nut in place by using the punch to swage the endplate into the small cuts on the front of the castle nut.
7.) Install the butt stock onto the lower receiver by pulling the release lever down to make sure the locking pin will clear the receiver extension and slide the stock onto the receiver extension.
8.) Once the stock is in place, depress the release lever and make sure the stock slides freely and locks into each position to test function.
9.) Insert your buffer into the action spring then insert both parts into the receiver extension until it is held captive by the buffer retaining pin.
You have completed your lower receiver! If you have purchased a preassembled upper receiver you can now pin that to your lower receiver with the pivot and takedown pins. Once the upper and lower receiver aremated together you are ready to go to the range and sight in your new AR15.