Building an EP Armory 80% AR Lower

I have been wanting to build an AR from an 80% lower for quite some time, but the added cost of the jigs required for most of the 80% lowers on the market was a tough pill to swallow, especially since it could very well be one of those tools I would use once and then it would begin gathering dust. That all changed when I watched Jeff Zimba’s (the Bigshooterist) video of him building an EP Armory Kevlar reinforced polymer 80% lower.

That video made the task look ridiculously simple, and gave me the confidence to undertake it. At $75 a lower, I figured it was worth a shot. I must admit, I have a fairly well equipped shop that happens to include an upright milling machine, so I would have the added benefit of using that instead of a regular drill press, but even if I did not, I would still have undertaken this if for no reason other than the challenge of doing it myself.

First pass

First pass

Knowing that Christmas was approaching, and I wanted to add something extra for the wife under the tree (whether she wanted it or not), I decided to build her an AR all her own, and thus I ordered the pink lower. The wife type person is new to shooting, having gone through a handgun training class only a year ago, and while she enjoyed the class, she has been reluctant to try any of my rifles, so I figured having one built just for her might be the added nudge to get her to try it. Coincidentally, the pink lower made it much easier to see what was happening in the photographs I took.

Tools Needed
The tools you will need to complete the machining will vary depending on which method you choose to use. I will be using my milling machine so I will need the proper end mill bits (3/8” and 5/16”), but others have completed the building using nothing more than a handheld drill and a rotary bit tool (Dremel tool) with a large bit, that is not so coincidentally sold by EP Armory on their website. While either method will work, I prefer the drill press/milling machine route because I do not trust myself to not destroy the lower with my Dremel tool.

No matter which route you choose to machine out the trigger pocket, you will also need a few drill bits to drill the trigger/hammer pin holes (5/32” drill bit) and the selector hole (3/8” drill bit). If you are going the Dremel tool route, you will also need a 5/16” drill bit to complete the slot for the trigger. If you are in need of the drill bits, those needed are available from EP Armory in a convenient “completion kit” that also includes the “bur monster” Dremel bit.

Additionally, I suggest a ¼”x28 tap as a needed tool, for tapping the pistol grip mounting screw hole, but some people say it can be done by forcing the screw in as it is turned. I feel better about actually cutting the threads with a tool designed for that, but that is a choice the builder is left to make on their own.  Finally, a #42 drill bit can be useful.

Machine Work
The first task for me was to figure out how to clamp the lower into the vise so that it was not only level, but was also secure so that when I moved the table to begin the milling process, the receiver would not shift. I found that setting the trigger guard/pistol grip mounting area gave me enough surface area to securely hold the lower while also allowing me to adjust the level so that the machining process would produce a level bottom in the trigger pocket.

Close to the bottom

Close to the bottom

Once I had it mounted in the vice, and my 3/8” end mill bit was installed, I set about milling out the trigger pocket. First I took off the top part that was protruding from the receiver (black portion in the photo). Once the extra top portion was removed, I carefully lined up the bit to begin cutting down into the receiver. I dropped the bit into the work about ¼” at a time, and using the X/Y table controls on my milling machine, I moved the table in the appropriate direction and removed the black material, leaving just a sliver thin portion of it on the perimeter of the pocket. As you work your way down into the trigger pocket, pink colored bars (or whatever color your receiver is) begin to appear and extend from one side of the receiver to the other.

Side note: Those colored bars serve a dual purpose. They act as a guide for you as you machine the receiver and they prevent someone from being able to force out the black plastic to easily open the trigger pocket. Those bars actually necessitate the machining of the trigger pocket.

I continued this process, dropping a ¼” at a time, until I reached the bottom of the trigger pocket. You know you are nearing the bottom because the colored bars that extend through the trigger pocket will disappear leaving nothing but the black material. Once I had solid black, I slowly worked the bit down until I began to see the pink bottom of the trigger pocket and I made my final pass with the mill.

At this point, it was time to machine out the trigger slot in the bottom of the receiver, so I swapped out my 3/8” end mill for the 5/16” end mill. I had no instructions telling me how long the slot was supposed to be so I grabbed one of my AR’s out of the safe and measured it. It turns out that by using the 5/16” mill (or drill bit if you are using that method) and drilling a hole on either end of the black square protrusion, so that the hole just touches the black, provides you with the correct length trigger slot. In my case, I just used the mill bit to machine the slot, but if you are using the drill bit method, you will need to have a hand file available to clean up the slot.

Now that I had a place to drop the trigger through, I decided it was time to test fit the trigger (I was using a mil-spec DPMS lower receiver parts kit to complete this lower). I found that even though I had removed all but the thinnest sliver of the black material, I still had a pretty decent amount of material left to be removed (just a guestimate, but I would say almost 1/16” more to go). I used my calipers and measured the width of the trigger and using the calipers as my guide for the required width, I removed the additional material a little from both sides so that the trigger and hammer would be centered in the trigger pocket and so that both sides of the pocket would be equally thick (or thin).

Drilling the holes

Drilling the holes

The final machine work step necessary is drilling the holes for the pins and the safety selector. The holes are drilled from each side separately and both sides have a raised nipple that has a concave dimple in the top of it to locate the holes properly.

I chose to use the final drill size when completing this step, but in hindsight, I think using a smaller bit to drill a pilot hole would have been a wiser first step. While the nipples provided are useful in locating the starting point for the drill, as you drill down the nipple disappears and could (and in my case did) allow the drill bit to drift slightly making the holes not align perfectly. Drilling a small pilot hole first would have prevented that, and that is what I will do on any future EP lower builds.

Once the trigger pocket was fully hogged out and the holes are drilled, the assembly stage begins. Early in the assembly stage, I found another thing worth noting. While all the remaining spring and detent holes are already there, it is worth running a properly sized drill bit (by hand) through them just to make sure they are properly sized and free of obstructions. As I began to assemble this lower, I started with the front pivot pin and the associated spring and detent. While the spring dropped in freely, the detent actually got stuck and would not function properly. Removing the stuck detent was no simple task, but I managed it without damaging anything. Once removed, I ran a #42 drill bit in the hole by hand and reassembled it without any further issues.

As I previously mentioned, the drill bit wandered just slightly when drilling one of the holes, which happened to be one of the trigger pin holes. This became visibly apparent when installing the trigger. The trigger is very slightly cockeyed in the trigger pocket due to the misaligned holes. Not knowing how this might affect function, I continued with the assembly. Once all the trigger components were installed, I dry tested the trigger function extensively. Despite the slight alignment problems, the trigger functions fine. It is not nearly as clean as my other AR triggers, but it is functional none the less. That said, I will be keeping a close eye on the trigger/hammer interface to watch for any weird wear issues.

Once I had the lower parts kit installed, I continued with the assembly of the rifle and used the pink Magpul MOE furniture on the lower (unfortunately, the two pinks are different shades). As I planned on assembling a lightweight upper for her rifle, but did not yet have all the parts, I threw one of my uppers on her lower and put some pink Ergo brand rail covers on it so that it would somewhat match.

The wife and I shooting our presents on Christmas morning

The wife and I shooting our presents on Christmas morning

My wife, in addition to liking all things pink, is a girl raised in the ‘80s and as such, loves Hello Kitty. So, in addition to the pink parts, I located a Hello Kitty Punisher charging handle and had a custom engraved ejection port dust cover with Hello Kitty made. Come Christmas morning, she was actually excited to see her new rifle under the tree (if she was faking, she did a convincing job). Later Christmas morning, we took her new rifle out and shot it for the first time (I fired the first magazine through it just to make sure the trigger would continue to function properly). This was the first time she fired a rifle (other than a .22lr bolt gun) and it was amusing. She was totally safe and kept the rifle on target the entire time, but she was not used to the muzzle blast generated by a .223. After each round fired, she would let out a little scream, followed by a giggle. Her only complaint was that the gun was a little on the heavy side, and that will be addressed once the remainder of the parts for the new upper arrive.

The EP Armory 80% lower was not only a fun project to undertake, but it was also a challenge and rewarding. I have assembled AR’s before from stripped receivers and parts kits, but this is the first time I have actually manufactured a rifle. I can see this becoming a habit over the long run. If you have contemplated attempting a 80% lower in the past but were not sure if you could do it, this might be the lower for you. Right now, as I write this, the price for an EP Armory 80% lower is $65, which is $10 less than I paid. That is a hard price to pass up. In fact, I think I am going to go order another couple…

As always, your questions and comments are welcome.
Be safe out there,

The finished product

The finished product

Rifle Update: This morning, I finished assembling the new lightweight upper for the wife’s Christmas present. It consists of an Aero Precision upper receiver, a BCM mid-length gas system, 16″ lightweight barrel, a Yankee Hill free floated mid-length handguard with a low profile gas block. And, to top it off, a nice pink rifle sporting bits of Hello Kitty just would not be complete without a BattleComp 1.0!

Total weight of this gun (no optic or magazine) is 5lbs, 1.4oz according to my postal scale. It is crazy light weight!


Matt is a full time Deputy Sheriff that has been on the job since 1996. During his time as a LEO he's attended countless training classes and is a court recognized firearms expert. Matt brings a unique perspective to TBS given his LEO experience and life time appreciation of firearms and our 2nd Amendment rights.

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  • peterk

    You have a milling machine!? I am jealous. I’d have done this ages ago if I did. :)

    • Matt

      I used to run a small business on the side producing some specialized motorcycle parts. The milling machine is one of the many tools I still have after closing my doors when the economy went in the toilet and the motorsports industry took a big hit.

  • Mario Lara

    That’s awesome, Matt. When building my first AR I debated with going with an 80% lower, but as at that point in time didn’t even own a Dremel of any kind, it would have been a no-go for me. Thinking about doing it for my next build, though.

    A bit hard to tell, but what optic did she wind up with? Thinking about building a .22LR variant for my wife as well, in pink camo. =)

    • Matt

      That is a Bushnell TRS-25. It has been on that upper for a while and has been very reliable. My only real complaint has been battery life.

      Later this month, I will be writing up a fairly thorough budget/affordable red dot optic comparison. Thus far, I think I have 7 optics lined up for the test varying in price from $50-200. That Bushnell (another just like it actually) will be included in the test. If you are shopping for a red dot in that price range, you might want to hold that thought for a few more weeks.

      • Eric Blank

        I am looking forward to that optic comparison from you. Good job on the build

      • DV

        I thought about buying an optic in that range, until I learned about the PRO. Save the $ and go with the Aimpoint (its an aimpoint and comes with mount, spacer, lens caps).

        • Matt

          I have an Aimpoint PRO, but not everyone can afford one, and not everyone can afford more than one if they have multiple rifles. I think you might be surprised by some of these optics. I know a few have impressed me quite a bit thus far.

      • Mario Lara

        Perfect, thanks Matt! Yes, in the market for one and can wait a few weeks. Heard good things about the TRS as well. =)

  • arcaneadam

    So the price on them today is actually $50 (I just bought one hours ago) and if you use the ‘calguns’ coupon code you get free shipping. For $50 bucks it’s almost too good to pass up.

    • Matt

      Crap, I might need a few more…

      • Leigh Rich

        I’ve got 3 already.

    • ct guy

      that code has never worked, I’m in ct so it might only be for cali buyers. my blem had a chip on the inner pin holding the upper and nothing else and my other one looks like it got suck in the mold has a little f’ed up finish. haven’t built yet but looks like the blems are fine

  • AR Shooter

    All I wanted add is . WAY TO GO I am quite sure you and your lucky wife are very proud . When you hit a crowded range wear a very large shirt because YOUR CHEST WILL BE PUFFING OUT ! !! !! !!

  • Edward Halfhide

    The only problem is that you are the one that machined the lower so you are the owner and you can’t give the lower to anyone and it has to be destroyed when you die by law. Your wife will have to machine the 80% herself to legally own it with your help.

    • Matt

      Everything I have read says you cannot manufacture them with the intent to sell, but if after some period of time, you decide you no longer want it, if you mark it and serialize it, it is legal to sell it. If that has changed in recent years, someone please tell me.

      That said, I don’t sell guns. I still have every gun (except 1 that I bought but never liked) that I have ever bought. Since we are married and everything we have is joint property, I’m not too concerned about who the “owner” is, they all live in the same safe. ;-)

      • Edward Halfhide

        I’m definitely no expert on this and I also have many AR builds. My wife and daughter did a 80% build with my help so they know how everything works internally. Just be cautious they are waiting for someone to cross the line to use as a target. Keep on trucking brother.

      • mig1nc

        Hey, you could always put it on your trust and say the trust made it :) Corporations are people too.

        • Edward Halfhide

          Hopefully it stays that way

      • Brian Slane

        It’s not just selling it where it is required to be serialized. If it is transferred at all, it has to have a unique serial number, and must be registered with the BATFE. The information is on their website.

        • J.Jarvis

          It does NOT need to be “registered” with the BATFE. It will still need a serial #. Many states allow for transfers between parties without any paperwork. If you live in a state that doesn’t allow FTF transfers, you’d need to go through an FFL where the paperwork is filled out.

          Know the law before making a blanket statement.

          Selling and transferring a homebuilt gun is legal, as long as you follow the marking regulations by the ATF, and it wasn’t manufactured with the intent to sell or transfer.

          There is so much baloney spouted on this by people “who heard” or “were told”, it’s unbelievable. The rulings are public, read them. The information is freely available.

  • Thor

    I just wish I could afford a drill press, or better, a CNC mill.

    • Matt

      I am lucky and have lots of machinery, but my go-to drill press, if the piece I am drilling will fit, is a small benchtop Ryobi that I bought years ago at Home Depot for $100. For this job, it does not need to be a large, full height drill press, the important part is that is runs smooth. A cheap X/Y table from Harbor Freight and that Ryobi drill press would do the job.

  • Daniel

    Ive been looking at the James Maddison Tactical or JMT polymer 80%s for a possible build since they come with a jig. Did you look at those as a possible option?

    I only own a dremel and a hand drill so I am hesitant to undertake the project with just those tools…

    • Matt

      Prior to yesterday, I was not aware of them. Someone on Facebook pointed theirs out and I gotta say, it looks like a good setup. I am seriously considering picking up one of them to do a comparison.

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  • Geoffry K

    I bought one of the Desert Sand lowers a couple weeks ago at $65. Might buy another now with the lower price. Will be buying a drill press and milling vise from Harbor Freight later this month. I already have a DPMS Sportical, but want to swap out the lower and stock.

  • Edward Halfhide

    I done a few 80% builds in aluminum but none in polymer. Would you trust the polymer lowers as your SHTF gun?

    • Matt

      If that was all I had, yes. Given the options I have, no.

      But let me clarify why. I don’t see the polymer being an issue in normal, or even extreme shooting. Much of the recoil stresses in an AR are handled by the bolt/bolt extension lockup, which is all in the upper receiver. Once unlocked, they are transferred straight back into the long travelling, recoil absorbing buffer system. With conventional loads, at least in my opinion, there is not a ton of stress on the AR lower, just from shooting.

      Now, throw that gun into combat where it is going to get dropped, beaten, used as a battering ram or a device with which to beat someone, then you start having strength problems that are not encountered during normal shooting. Weird leverage issues on the buffer tube mounting area, that are not encountered during normal shooting, could with a polymer lower, more easily break that mounting point rendering the gun useless.

  • Curious2Know

    Could you use this or another polymer lower to build a .22 lr AR?

  • isaac

    How did you remove the stuck front takedown detent pin? I just ran into the same problem and came across your site. wish I’d have known… thanks and great review!

    • Shawn

      This please! I’m sitting here staring at a stuck pivot pin detent as I type this.

      Thanks for the great article!

    • Shawn


      Here’s what I did. I found that I was able to compress the spring a little bit by pressing down on the detent with a small allen wrench (a punch would be better). It would compress about 1/8″ with little effort.

      I sprayed some WD-40 in the vent hole and on the detent. I’m honestly not sure this step was necessary, but I did it so I’m including it here.

      Then I just really pushed down HARD on the detent using my allen wrench. I compressed it maybe 1/2″ and as I released the force and removed the allen, the detent eased out far enough to grab easily with my needle nose pliers.

      I still had to use a needle in the vent hole to pry the spring out, but once the detent was gone that came easily.

  • Will P.

    Thanks for posting this up! After reading this I purchased 2 of these 1(a blem) to see how easy/hard it would be and the second(pink) to build for my wife. I trust composite lowers, my first AR was a New Frontiers poly lower and is still running strong after lots of field use. But I have seen mixed reviews on the EP lowers after purchasing these. I just completed the blem and am awaiting the LPK for it so I can give it a run. Upon completing the second one I am wanting to put a 6.8SPC upper on it, so my wife can also use it for deer hunting. But it kind of worries me if it will be able to handle the higher pressure/power cartridge, and have been unable to find information of anyone that has done this or tested it. Honestly there is little information of performance post build period. How is yours holding up?

  • WeaponryEnthusiast

    I recently purchased one of the EP Armory lowers and spent a lot of time being very careful machining it. After a diligent hour and a half of Dremel time, I got the pocket cleaned out and pin holes drilled. Unfortunately, it wasn’t until I tried to mate the lower with a RRA upper did I find out the pivot pin hole (that was pre-drilled) was off slightly and the rear take-down pin was off substantially. So much so, the pivot pin did not go in and hit the buffer tube extrusion. I’ve decided to attempt installing metal bushings into the lower and re-drill it to spec. I think the lowers are a great idea and they are going the right direction but their molds need work and whoever drills the pivot and take-down pins should be more diligent.

    Additionally, the buffer tube extrusion was too tall and as a result, the charging handle hit it while pulled backwards. This was an easy fix and part of what I would call “typical required fitting”. The bolt hold open slot was also pretty far out of spec, the bolt catch wobbles on the pin so I’ll be installing nylon washers to compensate.

    I’m not sure if I’ll bother buying another, it was cheap, but you get what you pay for I guess. I’d rate this a 2 out of 5 just because of the lack of attention to detail on their end. Easy to work with, but a pain if your a stickler to details.

  • Martin

    How is the rifle holding up ? Have you taken it out again?

  • KenD

    I bought two of these lowers, one pink and one black the first week of January. When I went to complete them they were defective. The company seems very hard to reach. I’ve called and left messages and today I sent and e-mail and left messages. The rear take down pin hole is not in the right place on either of these receivers. I checked it with the company’s own set of “stickers” that act as a drill jig template, and three different production uppers don’t fit properly onto the lower receiver. The predrilled rear takedown pin hole is off by quite a lot, and it’s visibly crooked. Hopefully they stand behind their product and replace them with correct receivers.

  • camosoul

    Bought the Bakers’ Dozen pack… And I don’t even like AR-15s.

  • Bill

    Nice-looking gun I think my wife would also like one but how many rounds have you had through that gun so far cause I’m kind of leery about using a polymer lower and then giving it to my wife. Failure is not an option. Thanks,

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  • Jimmy Ogletree

    wow that is a stupid lite AR, I may just have to go after one after all….and am thinking the new BCM key mod rail that is 7.7 ounces with all parts (barrel nut, and supplied rails). BTW, stay safe out there on the streets, will be going through the academy soon, and be a reserve officer while finishing college

  • austin

    Matt, great article and I enjoyed watching the progress of your build. Have you seen the news about EP Armory being raided by the ATF this past week? Looks like they might have changed their minds about these polymer 80% lowers being legal.. No arrests were made at EP but they took the customer records and confiscated their inventory. What’s your take on it? Its a shame, I really wanted to build one after reading your post

  • blake

    i got a few of these and a really experianced knowledgable smith told me that i shouldnt have bc they are notorious for breaking by the rear take down pins? any truth int his or any tips that can help me to avoid it.

  • Mark Markson

    dude this is totally illegal as you manufactured a firearm with the expressed intent to transfer possession of that firearm to another individual. i’m pretty sure you didn’t bother to mark it correctly either.

    • Charlie

      He gifted it. There’s a difference.