Building Your Own Firearm (Part 3 – AR-15 Lower Parts)

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Last time, we finished our look at the legality and mechanics of building your own firearm, particularly the manufacturing of the receiver.  Even though this receiver IS considered to be the firearm, it is not usable as is.  In the next two articles, we look at what is needed to get the firearm functional.  In particular, we will concentrate on the AR-15 style rifles and pistols.  This is because these (and the very similar AR-10 and AR-308) are the most common firearms built from 80% receivers.  This decision will give us the widest choices of parts and tooling and options to experiment with.  By the way, note that the AR in all of these means “Armalite Rifle” (the company which designed this style), not “Assault Rife”.

I let my fingers do the surfing online to find the best combination of apparent quality and price as of February, 2017.  Note that this does not mean this list of parts, prices and sources is still valid, and it was based upon my preferences and budget so may not be the best choice for you and your particular needs.

Once you have a completed receiver which you made for yourself or bought (stripped) through normal firearm purchase channels, you have the basis to easily assemble the firearm.  The bottom part of an AR style firearm, consisting of the receiver, pistol grip, internal Lower Parts Kit (LPK), and the buffer tube kit and stock, is often called the “lower”.  The upper part, consisting of the rest of the firearm, is called, not surprisingly, the “upper”.

The AR-15 80% Receiver

When choosing a receiver, either 80% or finished, the options usually are “cast” aluminum, “billet” aluminum, “forged” aluminum, or some form of reinforced plastic (polymer).  The plastic is likely to be lighter and easier to machine, while the aluminum should be more durable.  There is not that much stress on the lower when firing an AR slowly; the choice between the weakest and strongest lowers can be guided largely by rapid fire requirements and non-firing situations such as dropping it, banging it on something, or applying inappropriate force to the buffer tube or when the upper is hinged open.  The aluminum receivers tend to be of either 7075-T6 (newer, machines better) or 6061-T6 (the previous standard).  The strongest are made by forging, that is, heating the aluminum to where it is softened but not melted, and then forcing it into the desired shape.  Like with knives, this process improves the metallic structure.  Another decent, but often more expensive choice, is taking a billet (block) of aluminum and machining the desired shape into it, potentially providing a more pleasing appearance than forged but with not quite the same strength.   Casting (pouring liquid aluminum into a mold) is the least desirable aluminum choice, not being either the strongest (at least some of them have been known to crack or crumble) or the best looking, and I don’t know why you would choose this option unless you find one dirt cheap, particularly if you are looking for one to practice on.  I couldn’t find any, but I didn’t look very hard.  As for the plastic, there are a wide range of choices; I like the ones which have metal inserts at the points most likely to be affected by stress, and probably will avoid any which do not have those inserts.

Since I plan to try out at least two of the three mechanical methods of crafting a finished receiver (I have a drill press AND a router), and there are three attractive types of receivers to try, I needed to cut down the vast number of choices to a number I could make valid decisions between.  A key factor for me is whether engraving of a serial number and other BATFE recommended text can be provided, as I see following government laws and recommendations to be wise, particularly when the benefits are greater than the costs.

First up would be plastic, which I’m a bit nervous about.  To minimize my concerns, I looked for one which had metal at the stress points, and found the “Liberator” from Tennessee Arms.  It uses fiber-filled nylon with brass inserts at the threaded holes, available in black, dark earth, grey and olive drab; these shades match Magpul stocks color choices.  The cost for the receiver alone is $75, or $99.95 with a one-time use jig which seems a reasonable cost for a single build.  Since the jig is for the drill press method, that is the method I’ll use for this receiver.  Another advantage of this source is that they do engraving of serial numbers and/or a custom image, and I can make an image of the BATFE recommended information.  Their best value is a receiver, jig, set of bits and engraving for $109.  And they were offering a 20% discount plus a quantity discount when I ordered.

Next I looked for forged aluminum, and narrowed it down to a few choices.  From an economy and appearance standpoint, a good choice seems to be the anodized one from www.80-lower.com for $59.95, or $69.95 with FIRE/SAFE marked.  This company offers several multiple-use jigs including the “Easy Jig” so can cover either the router or drill press methods at various price points.  However, they don’t offer engraving.  For that, an alternative is a raw one from Atomic Engraving for $79.99 or an anodized one for $89.99 which include a serial number and BATFE suggested text engraved; they also have an unmarked (except for Fire/Safe) anodized receiver for $64.95.  Another choice is the anodized one from Daytona Tactical for $69.95 with your choice of stock images engraved on one side and for an additional $14.95, they will engrave a serial number and up to four lines of text on the other side.  They also have raw receivers for $44.95 and Cerakoted ones for $69.95, but they won’t engrave either of these because “it does not show up well”.  I’ll use the “Easy Jig” and my router for whichever of these I end up choosing.

Finally, there are a number of billet options.  Since Ghost Gunner has a raw one for $65, which they specifically chose to be used with their machine, this is the one I’d go with if I decide to try that machine.  Although allegedly “any” raw aluminum receiver can be used with the machine, I’d use their specified receiver the first time using this methodology.  As a computer controlled mill, the machine should be able to be programmed to engrave anything desired, so with the machine, I should be able to save money and trouble on any receiver I like since engraving will no longer be a factor in the purchase decision.  If engraving does not enter into the choice, there is a raw one from Daytona Tactical for $59.95 but as mentioned above, they won’t engrave it because it is raw.  They will Cerakote it for $15, though.  And  www.80-lower.com  has one Fire/Safe marked and anodized for $99.95.

One way to improve the search for engraved receivers seems to be to look for “CA Compliant 80%” lower or receiver, which is popping up a lot now due to the new California law.  If the choice is to get a receiver without engraving, but engraving is desired and you are unable to do it yourself, Atomic Engraving will engrave your own 80% receiver (or one of theirs) for $35 to $95 plus shipping, depending on how much engraving you want.

The AR-15 Lower Parts Kit

Basically, this is a set of all the parts which go into the receiver, including hammer, trigger, other controls, disconnector, pins and springs.  Annoyingly, it often also includes the pistol grip, which unless you want a stock (standard) grip, means you end up with an extra.  The trigger guard is not used on receivers which have the trigger guard built-in, typically billet and polymer ones.

The parts are standard from most sources with the major difference usually being quality.  “Mil-spec” (to military specifications) should be the lowest grade you consider, with higher grade parts used as your budget allows or your uses require.  Although a separate hammer and trigger is the norm, you can get a “one piece” drop-in target grade trigger group which can be used instead of the separate parts.  Other high-end parts are available for builds which push the limits of what the platform can do.

For good quality at a medium price, I settled on the Anderson set from Anderson Rifles for $49.70 at a mil-spec level.  If it turns out I need a higher level of quality, there is Spike’s Tactical LPK from Joe Bob Outfitters for $69.95.  And of course there is yet higher quality than that, at a significantly higher price.

Naturally, most LPKs come with the standard right hand only, 90 degree throw, safety selector.  These are adequate but not optimal.  I would prefer an ambidextrous safety, and might as well stick with Anderson who has an ambidextrous selector for $15.75.  It works well, but is wider than the receiver and sticks down enough to be a bit annoying.  A 45 or 60 degree throw safety would be a bit easier to use, and with custom shaped levers, perhaps more comfortable.  But some of these require a notch in the edge of the safety hole (to encourage use only on receivers with the “FIRE” marking appropriately located), which would be difficult to machine at home.  If you use a safety with one of these tabs, it would be easier to grind off the tab than notch the edge of the safety hole.  And if you are really into ambidextrous, magazine releases are also available for $25 and up.

Note that the pins which hold the hammer and trigger in place can, over time, develop looseness, and can even “walk” out of the receiver.  The way to prevent (or fix) this is “anti-walk” or “anti-rotate” pins.  KNS makes a couple of sets for the hammer and trigger pins for up to $40, but you can find their minimal models as low as $20 if you search.  KNS also has spring-loaded takedown and pivot pins for $20 which seems like would be very handy for final fitting, testing, cleaning and changing to different uppers.

The AR-15 Buffer Tube Kit and Stock

The remaining parts of the lower are the buffer tube kit and stock.  Buffer tubes are available in rifle, carbine and pistol lengths.  The buffer tube screws into the back of the receiver, and contains the buffer, which cushions the bolt slamming back, the mainspring, which pushes the bolt back forward into battery, and some mounting hardware.  The stock mounts around or to the buffer tube.  Stocks can be fixed or adjustable (collapsible); complete stock and buffer tube kits can be found as low as $40, but I wouldn’t trust them.  A stock alone for $40 should be adequate though.  On the other hand, you can spend hundreds on the stock, which you should avoid unless you know exactly what you are going for (and can afford it).  Note that there are two common carbine length buffer tube formats, mil-spec and commercial.  Either will work, but make sure all the parts you get are one or the other, since they are not compatible.  Mil-spec may offer a bit better selection of stocks.  Also, be aware that bolts come in different weights, and it is wise to match the buffer weight to the bolt.

For the carbine length buffer tube kit, I’ll get it from the same place as my bolt for $22.95 to get the best match (and attractive price).  If I were building a rifle for which accuracy or long-range was the key factor, I’d get a fixed stock with length and comb adjustments (like the Magpul PRS), which might have its own buffer tube built-in or more likely, requires the rifle length tube.  But “tactical accuracy” is good enough for me, although being big; I’d want it to be big and sturdy, like the Magpul MOE SL-S or MOE SL.

You do not want to use a carbine or rifle buffer tube on a pistol (due to a risk of being charged with possessing a Short Barrel Rifle), and Red Barn Armory seems to have good pistol specific ones by Phase 5 Tactical or Spike’s Tactical for $69.95.  As a cheaper option, Delta Team Tactical has one for $32.99.  And if you want a wrist brace system with the tube, there is the SIG system from Delta Team Tactical for $119.99.

Tune in next time for the upper parts and some useful accessories.

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