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Finishing 80% AR lowers: part 1.

The fact that you don't actually save money building one doesn't deter hobbyists from building their own AR lowers. Many step up to the challenge just for the fun.

The beauty of the AR-15 pattern rifle is its modularity and ease of assembly. No traditional gunsmithing skills are required to complete an AR from a kit. Yes, you need a few tools, but they are just wrenches, punches and a vise.

Building a bolt-action rifle may require a lathe, reamers and advanced machining skills, but a black rifle is simple mechanical assembly. If I were to inventory the AR-15 pattern rifles I've accumulated over the years, there would be very few factory rifles in the pile.

I enjoy piecing them together from selected parts and pondering my options while browsing the Brownell's catalog for ideas. Assemblers like myself eventually look for new challenges to maintain their interest in Gene Stoner's brainchild and simply assembling a kit from finished parts becomes mundane after the first dozen.

The bored do-it-yourself builder may begin noticing ads for 80% AR lower receivers and questions arise. What is an 80% receiver? Is it legal for a non-licensed person to finish one? What tools do I need? How difficult is it? In the following pages I will attempt to answer these questions and others and you can decide for yourself if an 80% receiver is in your future.

What is an 80% lower? An 80% lower receiver is simply one that has not been fully machined. As such, it is not considered a firearm by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), and does not require a dealer to receive and transfer it if you order one online.

The typical 80% lower has fully finished external surfaces even including anodizing, but the pocket in the rear of the receiver where the hammer, trigger and selector lever normally live has not been machined and the holes for the trigger and hammer pins and selector have not been drilled.

Legally, the 80% receiver is just a lump of aluminum. However, if you mill out the pocket for the fire control parts and drill the holes for the hammer and trigger pins and selector lever that lump of metal becomes a firearm in the eyes of the federal government.

By finishing an 80% receiver you have manufactured a firearm. Manufacturing a firearm for resale requires a federal license and payment of excise taxes. Manufacturing a firearm for your personal use does not. The receiver you completed becomes part of your personal collection for eternity. You cannot legally sell it.

Also, it is subject to all local and state laws regarding firearms. Because there is no serial number (although you may apply one if you wish) the anti-gun media refers to these home builds as "ghost guns" because they cannot be traced through manufacturer and FFL records.

The implication is that a hobbyist would only complete an 80% lower for some illegal purpose. In fact, most firearms that have serial numbers can't be traced beyond the original buyer. There are thousands of face to face private sales in this country every day that are perfectly legal and don't involve an FFL. Is that a problem? I don't think so.

I view creating a functional AR-15 lower receiver from an unfinished chunk of material as an interesting DIY project for the advanced hobbyist and good practice with a milling machine rather than a way to avoid paying manufacturing excise taxes to Uncle Sam.

If you only consider cost, this project doesn't make sense. Currently, I see forged stripped lowers for $49 or a bit more. The 80% lower, fixture and tooling will cost much more than that without even adding man-hours of labor. My point is that you can't justify this project by claiming to save money.

Once you've decided to take the plunge, you will be faced with choices. 80% lowers are available in both aluminum and polymer and in several configurations including 5.56mm, .308 and 9mm. Some are supplied in kit form with a fixture, tooling and even a lower parts kit while some are simply a standalone lower.

If you are finishing the lower on a drill press or with a router, you will need a fixture to hold it in the vise and to serve as a guide for the drilling and milling operations.

There are three methods for finishing lowers that I'm aware of: milling, routing or a drill press. Milling is by far the best method but most hobbyists don't own a milling machine.

Machining the lower with a router seems to be the current popular method. I would rate the drill press as least desirable, although it will work fine if you are careful and use a proper vise. I have tried all three methods. What follows is a detailed description and evaluation of each.

Drill press method: A few years ago I machined a lower using the drill press method for another publication. The end result was a fully functional receiver but I found the method a bit scary. Letting a vise free float on the table goes against standard machining practice and common sense.

The 80% receiver was bolted into a Juggernaut Tactical fixture consisting of left and right side plates and three interchangeable top plates that would serve as guides for the drills and end mill. The machine was a typical floor-standing 3/4 hp drill press with a 12x 12-inch table.

I checked the table for level. I'd be drilling holes, and they needed to be straight. With the receiver bolted into the fixture I clamped it in a drill press vise on its side and leveled it to drill the hammer pin, trigger pin and selector holes.

The jig provided holes in the proper locations to act as drill guides. The hammer and trigger pin holes were drilled with a 5/32" drill, but not all the way through the receiver.

The hole needs only to be slightly deeper than the thickness of the finished receiver wall. Once the trigger pocket is milled out, the inside ends of the holes will be exposed. The selector hole requires a 3/8" drill. After the three holes are drilled 1/4" or so into the receiver, I flipped the fixture over and the same technique was repeated for the other side.

The jig was repositioned in the vise upright and the first top plate was installed. Plate #1 consisted of a series of holes forming the shape of the trigger pocket. A l/8"drill bit through the top plate holes was drilled almost to the full depth (1.249 inches) in the 80% lower. The drill press depth stop should be carefully set to avoid drilling too deep.

Once the pilot holes are completed, remove the top plate. A 3/8" drill bit opens the holes up and removes much of the material in the trigger pocket. When all the holes are drilled, you have a roughed-out cavity.

The opening in the second top plate serves as a guide for an end mill. Insert a long 3/8" carbide end mill in the drill chuck. This is where the problems associated with using a drill press as a milling machine become obvious.

A drill press is designed to exert downward force. The Jacobs chuck is pressed into a Morse taper in the spindle and retained solely through friction. A side load (milling) may loosen the chuck, causing it to drop out of the machine. I experienced this failure twice during the milling of a lower.

The end mill is lowered through the top plate to a depth that allows the smooth shank to be guided by the fixture and the vise with the fixture in it is moved under the cutter so the cutter engages the aluminum in a clockwise direction (move the vise counterclockwise).

WD-40 is often recommended as a cutting fluid for aluminum and that's what I used. Don't be stingy with it. A shop vac is important to keep the chips cleared out of the cavity.

The drill press quill was locked each time the depth was adjusted, so both hands were available to steer the vise. Use a firm grip to avoid throwing the vise across the room if the cutter fetches up. Lowering the cutter gradually in small steps, I worked my way toward the target depth of 1.249 inches.

A note about this dimension is in order. The prints I've seen call for 1.249 .010". Don't get nervous about hitting it right on the money. In fact, I think you could cut away most of the floor of the receiver and the rifle would run fine. Nothing bears on it except the trigger spring legs.

Once the main pocket is finished, change out the second fixture top plate for the third one to cut out the rear section and the trigger slot.

Juggernaut Tactical 80% receivers are provided with the clearance hole for the rear lug of the upper receiver already completed. This means the rearmost section of the receiver doesn't have to be machined all the way to the takedown pin.

I cut it back enough at the full 1.249-inch depth to expose the selector lever and to provide clearance for the tail of the trigger. Note that this rear pocket is slightly offset to provide a thicker section of receiver wall on the right side to house the selector detent and spring. Also, there would be a step beginning just behind the selector only about .630" deep, so a depth adjustment would be required here if I wanted to open it up completely.

The last step is milling the trigger slot. A long center-cutting 5/16" end mill is lowered through the guide slot in the third top plate and bores completely through the receiver floor. Then, simply move the vise to mill the length of the slot and pull out the cutter. Except for some cleanup and deburring, your lower receiver is completed.

Router method: This method requires a router, electric or cordless drill, a fixture and a sturdy vise to hold it. I confess I did not own a router so I purchased one from the local Home Depot for $99.

There are lots of suitable machines available in this price range, but your choice must be small enough to clear the buffer tower at the rear of the receiver when milling the rear pocket. The Ridgid R24012 VA hp compact router I purchased seemed like a solid unit, although I had an issue with the depth adjustment slipping that I'll discuss later.

Standard woodworking router bits are provided with a 1/4" shank. The end mill supplied with the tooling kit for the fixture was 5/16". It will be necessary to find a 5/16" collet for your router or a 5/16" end mill with a 1/4" shank.

5D Tactical ( offers a hybrid three-flute long 5/16" carbide end mill designed for this very task and the shank tapers to 1/4" to fit all routers. I used the 5D cutter for this project and it worked well.

The fixture I used to secure the receiver and guide the cutter is the new Ultimate Jig from Juggernaut Tactical (Brownell's 100-022-572) designed to accept 5.56mm, 9mm or .308 cal. 80% lowers. Updated from the earlier jig I employed with the drill press, it is a hefty unit fabricated from thick hard coat anodized aluminum plates.

Stainless steel drill bushings are provided for locating and drilling the hammer pin, trigger pin and selector holes. Left and right side plates are bolted to the 80% lower receiver and the recessed socket head cap screws are tightened.

The cap screws go through the takedown pin holes of a 5.56mm or 9mm lower. The .308 cal. lowers are located with pins incorporated into the jig walls. An interesting feature of this new jig is an alignment tool that screws into the buffer tube threads and centers the top plates.

It also incorporates an LED light to illuminate the trigger pocket while you cut metal. Also included with the Ultimate jig is a plastic depth gauge for setting the stop collars on the drill bits and the depth of the end mill in the router.

A tool kit, available separately (Brownell's 100-022573) contains a long 3/8" end mill for finishing the trigger pocket and drill bits with stop collars.

I drilled the trigger, hammer and selector holes through the drill bushings in the jig with an (ancient) electric drill. A drill press would keep them square, but my goal with this lower was to finish it with a drill and a router. The holes only had to be drilled about 1/4" in depth. This step could be done last, but the drill poking through the aluminum receiver wall will leave a burr, so doing it now reduces cleanup.

The procedure for finishing the trigger pocket is similar to the steps completed with the drill press. Plate No. 1 was installed with the four screws left loose. I screwed the alignment tool into the buffer tube threads of the lower and mated it up to the half circle cut in the rear of the plate to align it perfectly with the receiver centerline.

This is a nice touch I haven't noticed on other fixtures. The reality is that the trigger pocket can be slightly off center and the rifle will function fine, but why settle for "good enough" when you can get it perfect? Don't forget to tighten the top plate screws once it is aligned.

Top plate No. 1 is a honeycomb of holes that serve as drill guides to drill out a lot of material. The depth of the holes is controlled by a stop collar installed on the drill bit and included in the tooling kit.

I set it with the plastic depth gauge that came in the box with the fixture. Drilling through the floor of the receiver would be embarrassing.

Drilling all the holes with an electric drill is tedious and a drill press would be handier if you have one available. A lot of chips are created, and they will have to be cleared occasionally. A shop vac will help and the fixture can be removed from the vise and simply flipped over to dump chips in the trash.

Once the holes were done, top plate No. 1 was removed, the chips were cleared out and I installed top plate No. 2, making sure it was centered with the alignment tool.

As with the fixture used in the drill press, top plate No. 2 is used to guide the end mill while milling out the main trigger pocket. I installed the 5/16" end mill in the router and adjusted the depth so the cutter was just barely touching the top of the receiver through the top plate.

I applied some cutting fluid and began milling clockwise around the circumference of the top plate slot. The smooth surface of the end mill is guided by the jig.

A firm grip is required to control the router. I drafted my friend Jamie to run the "hand mill" while I took pictures. He adjusted the depth of the cutter in steps of about 1/32" until the pocket was milled to the final 1.249-inch depth.

Toward the end of the operation the depth adjustment on the router slipped a bit and we ended up a little deeper in spots than we had planned, but I think it is more of a cosmetic rather than functional issue.

Top plate No. 3 was installed after clearing chips from the pocket. The forward slot is for cutting the trigger opening and the rear slot is for cutting the rear pocket.

The rear shelf under the takedown pin is already cut on Juggernaut lowers but if you need to mill that area out, make sure to remove the cross bolt from the fixture before cutting and adjust the depth of cut much shallower than the main pocket.

The trigger slot is cut through the floor of the receiver gradually as the depth is adjusted deeper and deeper on the router. All milling operations were performed with a 5/16" end mill.

Blueprints for AR-15 receivers usually show the inside corners of the trigger pocket as 7/16", but that dimension is not important, so you may encounter milling instructions specifying 5/16", 3/8" or 7/16" cutters. All three sizes will work.

For complete video and print instructions detailing the use of the Juggernaut Tactical Ultimate Jig go to their web site at I assembled the router-finished Juggernaut lower with a standard parts kit from CMMG (Brownell's 100-011-594), a Brownell's Al buffer tube assembly (080-000-553) and a fixed Al-length GI-type stock from Fulton Armory I had laying around.

The parts fit fine and no problems were encountered during assembly or test-firing. Current Juggernaut lowers, including the 80% models, are equipped with an ambi bolt release activated by a pin inserted through the center of the bolt release detent spring. If you encounter a pin taped to the side of your lower when you remove it from the shipping box, that is what it's for.

Next time, we will finish a polymer lower and an aluminum lower on the mill/drill.

Caption: Make an AR receiver with a drill press or a woodworking router? You can do it with skill and determination. Just don't do it thinking you'll save money.

Caption: Here's a Juggernaut Tactical 80% .308 lower; the pin holes have been drilled, but the trigger pocket has yet to be machined. It's nicely anodized.

Caption: A firm grip on the vise is required when milling out the trigger pocket. It's important to lower the cutter only a little each time or it will bind up.

Caption: The Juggernaut fixture correctly positions and spaces the pin holes. Drill holes just deep enough, then reverse the fixture and drill the other way for best results.

Caption: The drill press depth stop must be adjusted to slightly less than the 1.249-inch final depth. You don't want to drill too deep and break through the receiver floor.

Caption: Each of the three Juggernaut top plates controls metal removal from one part of the receiver. The plate at right is for clearing the main and rear pockets.

Caption: Drilling the selector hole. Hardened drill bushings locate the holes perfectly. You'd prefer a drill press to this hand drill, but it can definitely be done with care.

Caption: A sturdy bench vise is essential for this job. The lower is mounted in the jig and securely clamped in a vise before the first top plate is installed on the top.

Caption: Drilling holes to remove material from the trigger pocket. Depth is controlled with a drill bushing. This method requires a lot of hand file cleanup.

Caption: Drilling creates a lot of aluminum chips that must be removed before installation of top plate No. 2. Norcross says a shop vac comes in handy here.

Caption: Luckily, aluminum is pretty soft and can be cut even with a compact hand tool like this Ridgid compact router with long 5/16" carbide end mill installed.

Caption: Norcross' lower finished with the router was assembled and test fired with no malfunctions. Makers of 80% lowers don't always make them look exactly like an AR.
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Author:Norcross, Gus
Publication:Firearms News
Date:Aug 1, 2017
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