Building Your Own Firearm (Part 6 – Assembling and Testing the AR-15)

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Written by John Hertig on The Prepper Journal.

Last time (Building Your Own Firearm – Part 5 – Manufacturing the AR-15 Receiver), we completed a receiver.  The “firearm” is complete but not usable.  Now it is time to assemble it and try it out.  As before, I will include a link to a video or videos, and then add my comments.

Tennessee Arms Receiver Notes

Tennessee Arms replaced my less than perfect receivers.  The engraving in the normal location (right side magazine well) was much better, the larger font serial crisp and clear, and the buffer tube socket was fully threaded.  I finished one of them using a router jig, but not the take-down lug pocket part.  For that, I used AC Delco Gear Marking Compound (Dykem may also work) on the lug to find where the existing take-down lug hole needed to be relieved (yellow goo scrapes off onto the receiver to show where it is tight).  It was mostly the sides, which I ground away with a Dremil tool.  The take-down pin detent channel is real close to the take-down lug pocket, so I ground the take-down lug hole at a sleight angle at the top on that side to avoid compromising the channel.  The result of this technique was the best fitting upper yet.  The other replacement receiver appears to have had the take-down lug pocket already widened; I just needed to do a bit of trimming in the back corners.  The take-down detent channel was compromised; a KNS push button take-down pin made this a non-issue.  I made this one with the included jig and end mill bit, and then went over it again with a different end mill bit with the shank the same size as the cutters to get the cavity width correct.  Both still needed the safety plunger tube to be opened up a bit.

Both of these resulted in particularly nice finished receivers, light, and available in a variety of colors.  I also got an additional black one to try out the fiber laser which is better at detail and results in white engraving.  Although it is not really “engraving”, as it does not remove material but rather causes a chemical reaction which changes the color of the surface material.  This was much more precise for detailed images, although the image came out as a “negative” (white areas in the image were unchanged, so were left black and the black areas in the image were converted to white).  As before, a big plus was that the drilling and milling of the polymer was so much less annoying than forged aluminum.  These receivers, when without the earlier flaws and using an end mill bit with the shank the same diameter as its cutters, are a particularly good choice for either the drill press (included jig) or the preferred router (jig purchased separately) methodology.  And the company provides exceptional support.

Easy Jig Notes

I noticed that when drilling the polymer safety and trigger group pin holes with a hand drill, I was getting aluminum shavings, so if you are going to be doing more than a few receivers with a hand drill, I’d suggest getting the slightly higher priced model with the hardened steel drill guides.  Alternatively, using a drill press for the trigger group holes reduces the wear of the jig side walls significantly.

After I invested in the 5D Tactical Jig, I found that came out with the Gen 2 Easy Jig, which seems like it could be strong competition for the 5D jig.  At this point, I don’t think I’ll be buying another jig, and certainly not until it has been out long enough to find and fix any problems it might have.

5D Tactical Jig Notes

I found an alternate jig ( which seemed to be a significant advance in technology over the Easy Jig, so I tried one.  It was on a sale at the same price as the Easy Jig, and 5D also has a solid steel drill guide upgrade available.  The 5D jig is more involved to assemble initially, but once set up, you can change receivers without complete disassembly.  It seems to hold the receiver better and be a bit more universal.  Except they are length constrained and since the Tennessee Arms receivers have about a 1/16″ longer buffer tube socket, they won’t fit.  I made a simple modification to the jig to allow the receiver to fit.

There are three major differences from standard jigs.  Unlike most others, you don’t follow a template with the shank of the mill bit, but have pins in guide depressions on each side of the cavity.  This means there are greatly reduced chances of cutters impacting the jig, as well as reducing the need to look into the cavity while milling.  Of course, you can only use their end mill bit.  Next, there are only two guide holes to be drilled; a boon when doing forged, and less risky when drilling the trigger hole pilot with a hand drill, although using a drill press is still a much safer option.  Lastly, the end mill bit is special, with a 1/4″ shank to allow use with compact routers, expanding to 5/16″ to allow less flex than thinner bits.  The result is supposed to be more accuracy and a smoother finish.  Maybe so in aluminum, but it left “fuzz” in my polymer trigger cavity.  Still, I found this system easier to use and the cavity was the right size, and a sharp knife took care of the polymer “fuzz” left behind.


General Process Notes

In addition to the things which did not quite fit already mentioned, the magazine well on the original second Tennessee Arms receiver was a tight fit.  Because I had so many things which I found “didn’t quite fit” AFTER doing the machining that first time, I suggest you try everything you can BEFORE making the first hole.  Some things you can fix; some you’ll need to return for replacement, and it will be much easier to do that, and waste less of your time, to find anything wrong BEFORE you start cutting.


In most of the receivers I tried, the take-down lug pocket was too small and I’ve seen receivers which don’t have this pocket done at all.  I suggest you consider fitting the upper first, as if this operation gets messed up, the receiver may still qualify as “not a firearm” to aid in getting it replaced.  Unless, of course, the take-down lug pocket is not milled out at all and then you’ll need to cut it as part of the first passes milling out the trigger group cavity.

If price is a concern, I’m now finding a complete parts kit, except for the lower receiver and sights, for around $300, which makes a $400 build possible (in early May 2017, at least).  If quality or performance is your goal, you’ll have to spend more, of course.

Assembling an AR-15

Here is a good look at how to assemble a lower:  and here is another using some “makeshift” tools.

As with any specialized task, there are tools which will make things go much easier and are quite advisable.  You may be able to get by with “standard” tools, but the special tools will often work better and have less chance of messing up your parts.  As shown in the video, a special wrench to tighten the buffer tube castle nut is almost required (be aware that the original nut and the modern M4 nut use different wrenches).   You can get a tool which will do just this, or a “combo” AR-15 wrench which will allow you to work on barrels, hand guards and flash hiders as well.  A “receiver” block allows you to mount the receiver in a vise without damage, which is handy when you need two hands for your task and a third hand to hold the receiver.  An inexpensive set by AIM Sports is available from OpticsPlanet for $33.29 (with 10% discount code); including the wrench, lower receiver vise block, sight tool, and upper receiver vise block.  The latter is useful if you are building or modifying your upper, but with this package, it is essentially “free” so there is no harm in getting it and having it on hand; you will probably need it some day if you deal with the AR-15 platform.  It is highly recommended to get a couple of dummy rounds to test feeding safely, and a brightly colored “snap cap” to test the trigger, hammer and disconnector functions.

My Lower Parts Kit (LPK) came in colored bags, which were not labeled.  The color scheme was (from Anderson Manufacturing, at least) yellow for the trigger group, red for the safety and take-down pins, blue for the bolt catch and buffer retainer, and black for the magazine catch, trigger guard and grip screw.  With some exceptions, installation order is up to you.  I installed the safety plunger and grip first, because that was a known problem with my receivers.  Then I installed the trigger group, so that if there are problems with their functioning, they can be addressed without having to remove any other parts.  For instance, in the original drill press receiver, I found that I had a “bump” which kept the trigger from returning, and all I had to do to fix it was remove the trigger and grind off the bump.  The hammer area needed a bit of grinding as well and I did not even need to remove the trigger until the hammer was working, and then only to clean off the chips.  Other than those steps, I went by parts bag, but any sequence which appeals to you is fine as long as you do the safety and grip in the right order, and the buffer tube and take-down pin, and the trigger before the hammer.

Perhaps the most difficult step is installing the bolt latch roll pin.  I use a pair of channel locks to squeeze the pin into the first side and then the rest of the way after the latch is in place and lined up.  This works very well in polymer and seems to work in aluminum adequately (wrap the jaws in tape to prevent scratching).  If you use a punch, cover the side of the receiver with tape to protect from scratches.

Installing the safety with the pin in place is slightly difficult.  First of all, the hammer must be cocked to get the trigger bar out of the way.  Then, back off the screw holding on the grip until it just barely still catches the frame.  This will relieve the tension on the pin to where the safety will easily get past it.  Then, making sure the spring is still in the hole in the grip and in the frame, re-tighten the grip screw.  If you prefer, you can try to push the pin out of the way with a screwdriver and hold it there with an appropriately sized drill bit shank or pin punch while installing the safety past it.  Or (assuming the safety plunger hole is correct), install the safety first and then the plunger, spring and grip.  Once you get it in place, check it’s operation; it will likely be stiff the first few times.  If it does not loosen up enough, you may need to run the 3/8 drill through both sides of the hole to line them up better.

Installing the front pivot pin is tricky; the way which worked for me was to insert the shank end of a 1/4″ drill or pin punch through the holes backwards, to give the detent something to lean against.  Then (while keeping it pointing in a direction where I might be able to find it if it goes flying), I used the pivot pin to push the detent pin flush and then pushed the pivot pin through the pivot holes, displacing the drill bit/punch.  Make sure the detent pin goes in the slot in the pivot pin.  Or avoid this annoyance by using the KNS push button pivot pin which does not use the detent pin or spring.  KNS has versions with a QD socket or sling stud on the far end of the pivot pin, but these should be avoided unless having the sling attachment is more important than easy removal.  Or, too late for me, I found this trick which makes this task much easier.

When installing the magazine catch, screw in the magazine catch until the threaded end is flush or just slightly below the surface of the magazine release button.  You’ll have to push the magazine release button below the surface of the receiver to manage the last couple of turns.

The way the video guy does the take-down pin detent and spring, as well as the buffer tube, seems more difficult than it needs to be.  Again, a KNS push button take down pin will eliminate much of this annoyance.  Otherwise, there is a bit better way to install the buffer tube and the take down pin detent.  Look at the screw end of the buffer tube.  There should be a slightly extended area around the front edge, perhaps with a small notch in the center if it is a carbine stock tube.  The buffer stop pin will come up into this notch to properly index the stock; pistol tubes may not have the notch, in which case, just put the pin near the center of the extended area.  Put on the castle nut, with the big notches facing towards the rear, and run it all the way to the rear.  Place the receiver plate (or a receiver sling mount plate) over the tube, with the prong in the groove in the buffer tube threads, and the bump on the plate facing forward to match up with the hole in the receiver.  WITHOUT the buffer or spring installed, screw the tube in until the extended edge is right before the edge of the buffer stop hole.  Put the buffer stop spring in the hole, and then put the buffer stop over the spring.  Depress it until the wide part is below the edge of the buffer tube, and screw the buffer tube in until the extended area nearly touches the small pin.  If there is a notch for the buffer stop pin to go into, use a screwdriver or punch to push the buffer stop completely below the buffer tube and tighten the tube until the stop pin is just past where it would spring up into the notch or past the center of the extended area.  This should allow access to the take-down pin detent hole; install the take-down pin, take-down pin detent pin and take-down pin detent pin spring, making sure the detent pin goes into the groove in the take-down pin.  Rotate the buffer tube back to its final position with the receiver plate over the spring, and the stop pin snapped into the notch, if any.  Push the receiver plate against the receiver to hold the take-down stop pin spring in place and tighten the castle nut finger tight.  With the hammer cocked, insert the mainspring and then the buffer and push it in until it passes over the buffer stop pin (using a small screwdriver to depress the stop pin if needed), then let it out until it contacts the stop pin.  Use the correct wrench to torque down the castle nut.

To attach an upper, pull out the pivot pin and take-down pin until they stop (KNS push button pins are completely removed), then put the front lug of the upper between the two receiver lugs, and push the pivot pin through.  Rotate the upper down until you can push the takedown pin through.


There are two areas of the AR-15 design where things can work themselves loose and lock up the firearm, or worse, cause damage or injury.  One of these is the bolt key.  This transitions gas from the gas tube system into the bolt, so there cannot be any gap between the bolt key and the bolt itself.  The key is fastened to the bolt with two screws, which if they back out, leaves a gap for gas to escape or the extended screws can even mechanically lock up the bolt.  Unfortunately, this area gets too hot for Loc-Tite to be an option.  The other place is the castle nut which holds the buffer tube in place.  If this backs off, then the buffer tube can unscrew which disables the firearm or can even result in an explosion.  The solution to both these problems is called “staking”, where some metal around the threaded object is “mushed” into the object, locking it in place.  Doing this to the bolt key is beyond the scope of this article, since I suggest you get a complete BCG (Bolt Carrier Group).  As for how to stake the castle nut, see this video.  Don’t worry; this staking is not truly permanent.  Alternatively, there are a few places offering “enhanced” castle nuts which incorporate a set screw to lock them in place.


Check all functions

– Magazine insertion and catch

– Magazine release and removal (falls free)

– Cycle with no magazine (bolt is NOT held back)

– Cycle when magazine is empty (bolt is held back)

– Insert magazine with round(s) and test bolt release (DUMMY ROUNDS for safety and hammer does NOT fall)

– Cycle with magazine not empty (DUMMY ROUNDs for safety)

– Ejection of last dummy round (chamber the SNAP CAP for safety for the next steps)

– Trigger function with safety on (hammer does NOT fall)

– Safety off after pulling and releasing trigger with safety on (hammer does NOT fall)

– Trigger function with safety off (Double check that the SNAP CAP is in the chamber first and there is NO MAGAZINE installed. the hammer should fall) DO NOT RELEASE THE TRIGGER

– With the trigger still pulled, and the hammer still down, cycle the action (hammer should NOT fall)

– IMMEDIATELY (so there is no chance for a live round to sneak in) release the trigger and pull it again (the hammer should fall)

Head Space

The concept of any firearm is that there is an explosion behind a projectile, and that explosion is contained in every direction except for the direction in which the projectile is intended to go.  This has two benefits: all the energy of the explosion is used to propel the projectile, and none of the energy is used to damage the firearm or the person using the firearm.  Any firearm which uses “rimless” ammunition has something which prevents a round of ammunition from falling or being pushed “too far” into the barrel.  It may be a “ledge” to catch the rim of a straight walled cartridge such as the 9mm, or it could be some place on the bottleneck area common to many rifle cartridges such as the .223 or 7.62×39.  There is a “measure” of the distance between this “stop” and the rear of the cartridge chamber (usually the bolt face), called “head space”.


Why would you care?  Well, if you have fired a firearm, and it did not blow up in your face, and the ejected casing is in good enough shape to be reloaded (that is, not cracked or significantly deformed), then your head space is probably fine.  Since the firearm blowing up in your face, known as “destructive testing”, is not a preferred test methodology, it is wise to verify the head space of a firearm which has never been fired, and even one which has never been fired by you, before firing it (or taking possession).

The way to do this is with a set of “head space gauges”.  This consists of a “GO” gauge, which verifies that the head space is at least the specified minimum distance, by allowing the bolt to close on the gauge.  If the bolt does NOT close on a GO gauge, the head space is too short, and there will be a gap between the chamber and the bolt, which would not be good.  Never fire such a firearm.  If you don’t have a GO gauge, use an accurate dummy round, or with great care, an actual round.  Next is a “NO GO” gauge, which if the bolt closes with it chambered, indicates the head space is longer than the specified maximum.  This is not necessarily immediately unsafe, but it likely will result in unreliable ignition, firing pin damage and/or the empty casings being too stressed to be reloaded a usual number of times.  There is a third gauge, call a “FIELD” gauge, which is the maximum SAFE head space.  Never fire a firearm whose bolt will close on the FIELD gauge, and have at least the FIELD gauge for each caliber you plan to use a lot.  The head space can change on a firearm during it’s lifetime of use.

The easiest time to measure head space is before you install the barrel; otherwise, remove the upper from the lower to make it easier (and safer).  Everybody tells you to take out the ejector, and many say to take out the extractor as well, and that is definitely the “best” process.  But it is a pain and appears to not really be necessary (if your gauge has an extractor groove).  I put in the gauge, push the bolt forward just far enough for the extractor to click into its groove, pull the bolt out slightly (using the charge handle), then test as normal.  Just be careful when pulling the bolt out, because the gauge going flying across the room would be problematic in several ways.

How Did It Work?

The proof is in the pudding, or in this case, the shooting.  After finally finding a rifle range which had not been closed down and was not a couple hours drive away, I put the newly created firearms through their paces.  First thing I learned was LUBRICATE the bolt; the first one locked up after only a few shots and took rather a lot of effort to free up.  Second was to make sure you have the correct tools necessary to adjust the sights, because they were all way off, and without the tools, stayed that way.  The firearm which locked up also seemed to have something wrong with the lock up (later swapping the BCG with another seems to have fixed it), but the other two firearms fired and functioned flawlessly (they were lubricated after the annoyance with the first one).  Not too shabby for something I made with ‘household” tools.

Oh, and if you fire 30 rounds quickly, these firearms get REALLY warm.  I even slightly burned myself on something in the handguard area of one; I looked for what it was I touched, but it had quickly cooled off to where I could not find it.


Given current laws and technology, this is a real option for those who are interested in acquiring a firearm without the concerns that the information they provide to the government will come back to bite them someday and/or people who like doing things like this.  Just be aware of all the current and potential future ramifications of taking this option.  As for methodology, of the two most common, the router is better.  The drill press can work (quite well for polymer), but it is just not as effective as the router in aluminum.  Plus getting the right depth of final cut is more trial and (hopefully) not error.  Whichever way you choose, the key to success is a good jig and a specialized end mill bit (short cutting area and shank the same size as the cutters), as well as lots of patience

As for receiver types, the polymer ones CAN be very satisfactory.  I’d be reluctant to trust one which did not have metal reinforcement.  The forged ones are just as nice in appearance and functionality, but they are a royal pain to machine.  If I were ever to do another aluminum receiver, I’d try a billet and pray it was less tedious than the forged.

This completes the process of building your own AR-15, or at least one such process.

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