The easiest way to get all the parts for a lower assembly, or at least most of the parts, is to buy a Lower Parts Kit.
First off a list tools to use when putting together an ar15 lower.
Now to assemble, step by step…
1. First lets put in the magazine catch. This is done by first taking the magazine catch like this and placing it into the lower receiver:
Flip over the lower and put the magazine catch spring and magazine catch button:
Now flip over the lower, while making sure the button stays in the hole, you will have to push it in and hold it there with your finger. You will want to push the button as much as you can so that you can spin the other end of the magazine catch to twist it in. When you twist on the catch you want to keep twisting until either the threads of the magazine catch reach the end of the button, or until you can no longer rotate the magazine catch even when pushing in the button as much as it will go.
2. Now to assemble the bolt catch. Start by taping off around the area, this protects the lower from scratches and marks. There are two ways to install the roll pin for the bolt catch, you can use punch tools, or you can use vice grips, this time we are going to use vice grips so you want to tape them up too.
Now take the roll pin that will be used to hold the bolt catch in place and just start it so it is only in one side and still allows for the bolt catch to be put into place. This is done with the vice grips, and just lightly squeezing them so the roll pin starts to go in, enough to hold itself in place.
Now you want to take the bolt catch detent spring, in a lower parts kit there are two small springs that are about the same size, one has a bigger end on one side, this is not the correct spring. The bolt catch detent spring is straight. You will take this spring and connect it with the bolt catch detent, the spring slips over the detent.
Now insert them into the lower.
Now you will be putting the bolt catch in place as you also have the vice grips ready to push the roll pin into place.
3. Now for the pivot pin, sometimes called the front take down pin. For this I really recommend using a pivot pin detent installation tool because it helps prevent loosing the detent and spring, which are very small and difficult to find when shot across the room – trust me… You first slide the tool in and align the hole with the hole for the spring at the front of the lower receiver, like in the picture.
Now you slide the pivot pin spring into the aligned hole of the tool and the lower receiver, then place in the detent. You will want to use something like a drill bit or allen wrench to push the detent far enough in that you can twist the pivot pin detent installation tool enough to keep the detent and spring in place.
Now you will take the pivot pin, align it up with the pivot pin installation tool and slowly and carefully push the pin in and the tool out. You will want to do this with pressure on the pivot pin because when the spring comes loose of the tool and before the pin is sliding in the hole there will be enough pressure that if you are not ready you could loose the spring and detent. I also recommend doing this step with a hand or towel or something over the pivot pin area to catch the spring and detent if they get loose and fly out. You will know the pivot pin is in enough when there is a click, you may want to rotate the pin to make sure the detent seats itself in the groove of the pivot pin.
4. The trigger assembly and selector switch assembly will be next. Start with installing the springs on the trigger. The trigger disconnector spring is the small spring that has one end bigger than the other, the bigger end is what goes in the hole in the trigger. The trigger spring is installed like in the picture below.
Before moving on, it is time to put in the selector switch, this is easiest before the trigger pins are in place. When installing it you may have to twist it around a little to get it through both holes and aligned for the trigger. The detent and spring to hold this in place and allow for the switch is installed when you install pistol grip, which will be done on another step.
Now take the disconnector, it goes in the groove of the trigger. Then take a trigger pin and install it through the lower receiver, through the trigger, and through the disconnector. In this case it doesn’t matter what side you start on and what end of the pin goes in first – this is from a standard trigger from a lower parts kit.
When installing the trigger pin you must push down on the disconnector to make sure it’s hole aligns with the receiver and trigger holes, then push the pin in until it clicks into place. You may have to use a punch tool, I find a flat head screw driver with electrical tape wrapped on it can be used to push this into place.
Now get the hammer and hammer spring together, put them together like in the picture. make sure they are the correct way, I once helped fix a friends AR15 after he payed a professional Gunsmith to assemble it only to find they installed the trigger springs backwards so his hammer wasn’t falling hard enough to light the primers. I have played around with the spring both ways though and on match ammo in my 6.5 Grendel the reversed spring does work quite nicely.
Install the hammer assembly into the lower receiver, be sure the spring ends go over the trigger pin, there is a notch for one of the two to rest in to keep that pin from sliding out. You will want to push the hammer down in place to align the holes for the hammer pin to go in, then install the hammer pin until it clicks into place, again a electrical tape wrapped flat head screwdriver can be of use with this.
5. Installing the Ar15 pistol grip, first you put in the selector switch detent.
You will want to carefully put the spring into the pistol grip hole and be careful as you slide on the pistol grip and allow the spring to slide into the hole of the receiver. Do this carefully as you put it all the way in place so that it does not bend the selector switch detent spring.
Now take the pistol grip screw and washer and tighten down the grip all the way. These parts were from a DPMS Lower Parts Kit.
6. Installing the standard trigger guard. One end of the standard trigger guard is held in place by a spring and detent that comes setup on a standard guard, just snap it into place, this end goes in place on the side that only has a hole on one side of the receiver. Since the other end is held on with a roll pin we start by aligning the hole on the trigger guard and push it in all the way using our taped up vice grips.
7. Installing the standard A2 stock. You start with the buffer tube, and screw it in part way onto the lower receiver.
Now take the buffer detent and spring, the detent slides over the spring and is installed in the lower receiver, the buffer tube is then screwed on enough to hold the detent and spring in.
Now you will hold down the buffer detent and screw on the buffer tube the rest of the way.
Now you can install the buffer spring and buffer. You do this by using our tape wrapped screw driver to hold the detent down, first slide the spring in (make sure the hammer is back so the buffer can clear it), then put the buffer in sliding it in past the detent and then releasing the detent to hold it in place.
Now before we install the stock, install the rear take down pin, detent, and spring – the spring and detent are held in by the stock.
Now slide on the stock and be careful, you don’t want to bend the take down pin detent sping as you slide it on. The stock has a knob that sits in the receiver.
Now use the screw to attach the buttstock to the buffer tube.
The lower Receiver is now complete!