Finishing an AR-15 80% Lower Receiver

The jig is a router jig that makes machining your 80% lower easier, faster, and safer by utilizing a router instead of a drill press. The jig allows you to complete an 80% lower quickly, easily and pretty fool proof. This jig is easy enough for anyone to use. You also save money on tools. You do not need to own a drill press, mill, or any measuring tools to use the Jig. You might be able to borrow the tools from a friend or relative. The jig can be reused for dozens of lowers.

The lower receiver is legally the firearm, and as such it is the controlled part. Generally, the law requires licensed manufacturers and importers to mark the designated receiver with a serial number, the manufacturer or importer, the model and caliber.

“Unfinished receivers”, also called “80 percent receivers” or “blanks”, are partially completed receivers with no serial numbers. Purchasers must perform their own finishing work in order to make the receiver usable. The finishing of receivers for sale or distribution by unlicensed persons is against US law. Because an unfinished 80% receiver is not a firearm, purchasers do not need to pass a background check and it can be shipped directly to them.

Putting together your lower receiver is pretty easy if you follow these steps:


• Install magazine catch into recess on left of receiver.
• Install spring onto threaded portion of magazine catch from the right side of receiver.
• Screw button onto threaded portion of magazine catch 3 or 4 turns.
• Use a punch (larger than the hole in magazine button) or wooden dowel to push in the magazine button so you can turn the magazine catch clockwise until the end of the catch is flush with the magazine button head. You can hold the receiver and press the punch against the table to do this, but put something between the punch and the magazine catch button to prevent marking it. Do this step first, as it will prevent the bolt catch from getting in the way as you turn the magazine catch into place.


• Attach front of trigger guard assembly to the receiver using the detent.
• Lay receiver on you bench block or a block of wood, and drive roll pin into receiver and rear of trigger guard using drive pin punch.


• When installing the bolt catch, first drive the roll pin about halfway into the rear hump from the rear of the receiver using roll pin holder. It can be very difficult to get this pin started. If you don’t have a roll pin holder, then try holding it in place with a pair of needle nose pliers while you drive using a 5/32″ punch.
• Install spring in hole on left side of receiver.
• Install bolt catch plunger on top of spring with round portion on top and small end into receiver. Make sure it moves freely in its hole.
• Install bolt catch in receiver.
• Use a 3/32″ punch to hold the assembly by placing it through the front hump.
• The pin can be driven the rest of the way from the rear as the punch will be pushed out and while holding the bolt catch in correct alignment.


• Insert spring and detent into receiver.
• Compress detent in recess using 3/32″ punch and rotate tool.
• Push out tool with pivot pin and rotate until detent is in groove of pivot pin.


• Shoulder trigger spring onto trigger with ends of spring forward and under.
• Install disconnector spring with the wider portion of spring down towards trigger and push until it locks in there.
• Position disconnector on top of trigger, where trigger pin will hold both in place.
• Insert trigger assembly into receiver.
• Insert trigger retaining pin through receiver, trigger, and disconnector. The trigger pin has 2 grooves in it; one in the middle of the pin and one off to one side. It does not matter which way it is inserted, though common practice is to insert from left to right, with the groove to the left.
• Insert hammer pin from opposite side to help align things as you push the trigger pin in and the hammer pin out. You will have to push down on trigger assembly to align the holes and get the pin in all the way.


• Install spring onto hammer, ends of spring to rear and shoulder on back of hammer.
• Install hammer in receiver with feet pointing rearward away from hammer
• Use 5/32″ punch to retain hammer in place as you insert hammer retaining pin.
• Like the trigger pin, you may have to push down and align the holes perfectly in order to push the pin in all the way.
• Ends of the hammer spring will rest on top of the trigger pin, with one end in the groove on the trigger pin.


• Install detent, pointed end towards the selector, and the spring into the receiver from the bottom.
• Carefully compress the spring with the grip and make sure spring fits into hole in grip.
• Check the function of the selector with the grip held in place; if too tight (unable to rotate) you may need to either cut the spring, or clean out the hole in the grip.
• Once feel is acceptable, secure the grip in place with the screw and lock washer.


• Insert retainer spring and retainer into recess in lower.
• Install buffer tube, backplate, and locking ring onto receiver and depress retainer when necessary to get it to rest under the extension tube. Turn tube until it is about 1/4 turn past the correct location.
• Install takedown pin with groove to the rear and install detent and spring from the rear of the receiver.
• Backplate will now hold spring in place when you turn the extension tube back 1/4 turn.
• Tighten locking ring using telestock wrench for a snug fit.
• With the hammer down, insert buffer spring and buffer into buffer tube until retainer snaps up to lock it in place.
• Function check takedown pin, stock, and buffer retainer.