How To Assemble An AR-15 Lower [Complete Guide]

CAN WE BUILD IT? YES, WE CAN!

One of the best things about the AR-15 is how easy it is to work on yourself. While assembling a lower isn’t exactly hard, it is a step above what the average gun owner is normally willing to do.

With some basic tools, a little time, and this guide – you can rise above and get a more intimate look at your rifle.

NEED SOME TOOLS

Good tools are hard to find these days, but thankfully there are still a few options out there.

If you’re looking to do this as cheaply as possible, just get whatever crap is the lowest price on Amazon and call it a day. Honestly, it will probably be good enough for one build, but very likely only one.

If you think you might do this more than once, or you just don’t want to make yourself suffer while you build, invest a little more money and get tools from reputable firearm brands like RealAvid and Wheeler.

Personally, RealAvid is normally my go-to pick. Full disclosure, they’ve sent me a number of products to review over the years, including most of the ones from them in this article. 

But I also put my money where my mouth is, I’ve spent a lot of my own money on their tools over the years because I like the product. 

If you want a one-stop option, RealAvid’s Armorer’s Master Kit is pretty amazing but pricy. And it's totally overkill for just one lower. If you want to go hard and build a lot, it’s worth it.

My must-have tools are pretty simple, a Bench Block, a hammer and punch set, and an armorer’s wrench.

The bench block can be just about any model. The Smart Bench Block is simple, and I’ve been using mine for about 5 years, love it. The Master Bench Block is the one I used for this article, and it’s pretty nice but not a must-have over the normal block.

I really, really recommend a punch set that is designed for guns, especially the AR-15, since those sets are normally sized right, and some include punches built for the bolt catch – they make life easier and make not scarring your lower a lot easier.

RealAvid’s set comes with all the punches you need, a small hammer, and a nice case. Wheeler’s set is decent for most guns but doesn’t have the AR-15 bolt-catch punches. The AR-Stoner set I owned broke the first time I used it. YMMV.

The wrench is the tool I care the least about; literally, anything off Amazon will be fine. I’ve used the $5 one for about 4 or 5 lowers, and it’s fine. Wrap the handle with a shop rag for a better grip. If you want something a bit fancier, Magpul, RealAvid, Tapco, Brownells, and Wheeler, all make good options.

Those are your basics. If you have a bit more money or just want to fill out your DIY tools a bit more, I recommend a vise, a set of hex drivers, and a pivot pin tool.

I used a $40 no-name off Amazon vise for about 6 years on I don’t know how many dozens of firearms. They work, and they’re cheap. Again, if you want something nicer, the RealAvid Master Gun Vise is pretty outstanding. I’ve only recently started using it, and wow, it’s a keeper.

Hex drivers, get whatever. The cheap set from Harbor Freight is what I normally get because I lose or break them enough that I don’t want to invest real money in it.

The pivot pin tool is $12, and you really don’t need it. I still recommend it because the pivot pin is kind of a pain in the ass, and I hate it. This tool makes it stupid easy.

PARTS LIST

Most people just get a lower parts kit these days, but not all lower parts kits have the same thing. Either way, this is what you NEED for a complete lower.

• Trigger spring
• Trigger pins
• Trigger
• Hammer spring
• Hammer pin
• Hammer
• Disconnector and spring
• Takedown detents (two)
• Takedown springs (two)
• Front and rear takedown pins 
• Buffer retainer and spring
• Magazine catch
• Magazine catch button
• Magazine catch spring
• Safety selector
• Safety selector spring
• Safety selector detent
• Bolt catch
• Bolt catch spring
• Bolt catch roll pin
• Bolt catch plunger
• Trigger guard kit (if your lower doesn’t have a trigger guard)
• Pistol grip
• Buffer tube
• Buffer spring
• Buffer weight
• Stock

Most of the time, you’ll find a lower parts kit and a buffer assembly separately, but not always. When getting your buffer assembly, make sure you get the right kind (carbine or rifle) that fits your stock of choice.

Since the kit I'm using is a build kit from PSA, it comes with everything in the box. Just open it, check it, and get ready to build.

HOW TO BUILD AN AR-15 LOWER

Where to start? Well, it doesn’t matter that much, but I like to start with the magazine catch because why not.

Magazine Release

Three parts, the magazine catch, the magazine button, and the spring. 

Put the round arm of the magazine catch through the lower from the left side – you’ll see the groove for the flat part of the catch to sit in, and it’s pretty hard to miss.

Spring goes over the round arm once it’s in the lower.

Take the button, and with the grooves of the button facing out, screw the button onto the magazine catch. You’ll fight the spring, so push hard and power through. Once you get it on a thread or two, it’s a lot easier. Screw it down until it gets close to the lower.

Once it’s that far you’ll need to push the button so that the flat arm pops out of the lower on the other side. Twist that to keep tightening the magazine catch.

You’re done when the flat arm sits just inside of the magazine well cut out when the magazine button is depressed flush with the lower. That’s a lot of words, so just look at the picture.

Test fit with a magazine to make sure it works.

Bolt Catch

Four parts, three tools. Bolt catch, bolt catch plunger, bolt catch roll pin, and a bolt catch spring. Two punches, the starter, and the punch. Plus, a hammer.

Assuming you got a decent punch set, you should have at least two options – one is a starter, and the other is a punch. If you got the RealAvid set, these are the two long punches with scalloped sides.

Set the roll pin in the starter punch, that’s why it starts. The other punch is for when you bottom out the starter.

The spring goes on the plunger’s tail. The plunger goes into the hole under the bolt catch area, spring end first.

Bolt catch goes over top, then you just hold it down while you drive the roll pin in.

I like to start the pin before anything else so that it’s one less thing to hold. Don’t put it in very far, or you won’t be able to get the bolt catch in. 

Wack in the roll pin until the starter punch about bottoms out on the lower, then switch punches. Keep wacking until the roll pin is flush.

Wiggle the bolt catch to make sure it moves freely and you’re done.

Trigger Guard

This is a little tricky because not all trigger guards are the same. For lowers like Aero Precision, the trigger guard is normally a part of the lower and you don’t need to install something else. For PSA, you need to install one.

This kit comes with a Magpul trigger guard and is pretty easy to install.

Look at the trigger guard and see that one side has two holes and one side has one hold plus a blind or unfinished hole.

The side with two holes gets a roll pin, the side with one hole uses a set screw.

Look at the lower and again, one side with two holes, one side with one. Align the trigger guard so that the two match. Two holes to two holes, one hole to one hole.

The one hole gets a set screw, just crank it down and lock it in.

The other side uses a roll punch. Normally, you just set the lower on a hammer block, set the pin, and wack it in. But honestly, this is a lot easier than it sounds. Also, you run the risk of breaking those little ears. This method can work, just be careful, don’t hit too hard, and make sure you support the ears as best you can.

Personally, I do this roll pin in the vise. Set the lower so that one ear is against the vice jaw and the roll pin is against the other. Then just tighten the vise until the roll pin is flush.

Super easy, super fast, no chance of breaking the trigger guard ear. Just because you’re ONLY tightening the roll pin. 

Some tape on the jaws helps prevent marring.

Trigger

Two styles of triggers, mil-spec and drop-in cassettes. Even if a trigger isn’t “mil-spec”, if it comes as multiple parts and springs, then it installs roughly the same way no matter what. Double-check the manufacturer, just in case.

Cassette triggers basically just drop-in. Literally, drop them in, line up the pins, and you’re normally done. Some require you to tighten a set screw or two, but not all. Again, check your manufacturer’s instructions.

For normal triggers, this is how they (normally) install:

You have two large springs, one small spring, two pins, the hammer, the trigger, and the disconnector. 

First, the trigger spring and the trigger. The trigger is the part that looks like a trigger, the trigger spring is the spring that is folded back on itself. You want to put the spring on the trigger so that the loop is tucked under the front of the trigger. Look at the picture and make your trigger look like that.

Second, look at the small spring. This spring is actually tapered, but it can be hard to tell. The spring goes into the top of the trigger in a small hole. If the spring is really easy to drop in the hole, take it out and turn it around. You want the FAT end of the spring in the hole, so it takes some pressure to push in.

The disconnector goes in on top and sits in the trigger and over the small spring. Again, look at the pictures.

Once you have it all together, just set it in the lower. Don't throw it in, set it gently. If the disconnector falls out, put it back. Press firmly and line up the holes in the side of the trigger with the LOWER set of holes in the lower body. Push a pin through the hole.

It might take some wiggling, it might take some realigning, it might even take a wack or two from the hammer, but overall they should slide in fairly easily with some firm pressure. If it requires you to beat in, they aren’t aligned, and you need to straighten it out instead of making a new hole.

The hammer is the same idea, set it in and line it up, but a little more annoying. When you set it in, you need to bend it back so you have spring pressure. Look at the image below, that is how it goes in at first with the arms of the spring pointed to the buffer tube end of the lower.

You then rotate the hammer so that it folds back on those arms giving you the spring pressure you need.

Do it right and the hammer should kind of slide into its place and even lock back on the disconnector and sear even without being pinned into the lower.

This also makes pinning the hammer a lot easier, just be careful since there is no safety and that hammer could go boing.

Line up the pinholes, and put the pin in. This one is normally a little hard to align but the same rules apply. If you need to give it a wack, that's okay. But beating on it is not.

Safety Selector & Grip

Safety selectors come in a few flavors, but they usually all install basically the same. If you have an ambi selector, you might need to screw the other handle in or something, but for a normal mil-spec selector, it’s easy.

First, cock the hammer back. Be careful because you don’t want to brush the trigger and send the hammer into the lower unprotected. If you’re nervous, some tape can help.

With the arrow pointed at “fire” just drop the selector into the hole. You might need to wiggle it a little, but it should go in fairly easily. Move the selector to “safe” and keep it there as best you can. 

Hold it in place and turn the lower over so the bottom is facing you. Right side of the lower is a small hole, the safety selector detent goes in that hole with the point going in first. 

Grab your grip and look for another small hole, the spring goes in there.

Turn the lower on its side and line up the grip and the spring, and the detent. Slide them together. 

You should have a screw for the grip, it goes inside the grip and into the lower that way. Most screws can be installed with a flat head or a hex head. If you’re using a Magpul grip, some are packaged so the screw inside the grip with the cover on. 

Tighten the screw until it’s good and tight. Don’t crank down it with the power of He-Man, but make it goodly snug.

Give the safety a flick to make sure it’s nice and clicky. If it feels mushy or loose, you might have used the wrong spring. Double-check your springs and be sure.

Rear Takedown Pin & Buffer Tube

The rear takedown pin, buffer tube, buffer retention, end plate, and castle nut are all one big assembly mess. They all rely on each other, so it’s a little annoying and pretty easy to drop stuff. Go slow, try not to turn anything upside down.

On the back of the lower, under the big ring, is a small hole and a large hole. The small hole is for the takedown pin detent and spring, the large hole is for the end plate.

The takedown pin goes across the lower from right to left, turn it so the groove is facing the rear of the lower.

The detent goes in the little hole, it doesn’t matter what end goes first since they are the same shape.

The spring goes in next. Give the takedown pin a little twist to make sure the pin and the groove are lined up.

Your buffer tube should have the castle nut on the threads and the end plate closest to the lower. The PSA kit I used had this backwards, I think for shipping, so double-check and change it if needed.

Twist the buffer tube into the lower. The end plate will bonk the spring as you twist it, I normally just let it and be a little careful so it doesn’t go flying.

Keep twisting it in until the buffer tube covers about half of the buffer retainer hole. The buffer tube is shaped weirdly, and the part that covers that hole is bulged out from the face of the buffer tube – this is normal.

When half the hole is covered, back the buffer tube out a quarter turn so that the hole is exposed, and drop the retainer spring in, followed by the retainer pin. 

Push them down slightly and tighten the buffer tube so that the bulged part sits over top the retainer pin and holds it in place. 

Push the end plate so it is flush with the receiver, and push the takedown pin spring into the receiver. Make sure it doesn’t kink as you do. Hold the plate in place and hand tighten the castle nut until it’s tight with the end plate.

Best practices are to put the lower in a vice now and give the castle nut a firm tightening using the wrench. If you have a torque wrench, you want 35-40 ft.lbs. Then you can stake the castle nut in place for it to be extra secure. If this is a rifle you intend on abusing, I recommend doing all of that.

For my more fun rifles that I plan on doing more tinkering with, or I just don’t care as much, a SMALL amount of blue Loctite and a firm twist with the wrench have never failed me and are a lot easier to uninstall down the road.

Front Takedown Pin (Pivot Pin)

Okay, this part is annoying. If you have big hands, you might want to just skip even trying the normal way and use the pivot pin tool.

Look at the front end of the lower. See the hole on the right side? The other takedown pin spring goes in that, followed by the takedown detent. You need to hold those in the receiver so they are flush while then getting the pivot pin over them and captured in the groove.

I find using a razor blade or a thin knife blade helps a lot to hold them down while you get the pin in. Just be careful, and if you cut yourself, it’s your fault.

Since I have cut myself doing this, and I hate it, I use the pivot pin tool.

If you go this road, take it apart and put the flat part so the pinhole is over the hole in the receiver.

Push the red pin in through the pivot pinhole from the other side to lock it all in place.

The spring goes in the funnel, then the detent. Use the silver pin as a ramrod to push the two down and flush in the receiver. 

Turn the whole assembly so the flat part is now vertical, and the spring and detent are captured in the receiver. 

Remove the ramrod.

Insert the pivot pin and use it to push the red pin out the other side. Make sure the pivot pin is turned so that the groove is facing where the spring and detent are. 

Do it right, and it should all slide into place, and it’s super easy.

LOOSE ROUNDS

There it is, a complete AR-15 lower. We’ll do another guide soon for building an upper, but those require some more tools.

If you want the easy road, buy a complete upper and slap it on your lower for a complete rifle.