We Have An AR At Home, Part 3: Parts

Addendum to my previous article: I forgot to mention this punch set, which includes pin guides and extra-long flat sided roll pin punches.

Now that we have the basic tools we need for assembly, it’s time to choose parts for our rifle. There are a bewildering variety of options, everything from triggers and furniture to barrel length and caliber. Everyone has their own preferences, and if we ask for suggestions the responses would be overwhelming. Therefore I’m going to try and keep this list fairly generic, but I’ll toss in additional options here and there.

Receiver, Upper & Lower

The heart of any firearm is the receiver or frame. This is the part the ATF considers the actual firearm, with everything else being just parts. If getting a completed lower (we discussed 80% lowers in a previous article), it will be necessary to purchase it through an FFL and fill out a form 4473 or, if it’s legal in your jurisdiction, through a private sale from an individual.

AR-15s have a two-part receiver, commonly called the upper and lower receivers. The lower receiver is the regulated part, so I’ll start with that one.

Lower receivers can be purchased as stripped (meaning no other parts) or complete (all required parts already assembled) components. A stripped lower may cost less, but there is more labor involved; a complete lower is less labor intensive, but can be more expensive and limits what parts are used. Personally, I prefer stripped lowers.

With upper receivers, the industry has pretty much standardized on a flat top Picatinny rail upper with Brunton bump (brass deflector) and forward assist.

A standard flat-top upper receiver

Fire Control Group,

Upper and Lower Parts

The Lower parts kit (LPK) and fire control group (FCG) are where a large portion of the variables start to come into play. The FCG alone can easily cost more than a barrel, so determine the purpose of the rifle before making this selection. For example, a long range target rifle will have very different trigger requirements over a short range plinker.

As with the lower, an upper parts kit will be needed for completion.

Bolt Carrier Group

Behind only the barrel and FCG, a good Bolt Carrier Group (BCG) is a primary key to accuracy. There are numerous finishes available that advertise different benefits such as corrosion resistance, easier cleanup, and smoother operation; Nickel Boron (NiB) coated is my personal preference as I find it offers a good combination of benefits at a reasonable price.

The author's preferred general purpose BCG

Barrel and Gas Tube

Speaking of barrels, this is the heart of accuracy in any firearm and it's one area where we shouldn’t cut corners. This is also another category with an excessive number of variables, including length, gas system, weight, profile, lining, and chamber specifications. Since this list is focused on rifles, I’m not going to talk about barrels under 16” in length here. 

Barrels are available either with or without a gas block. The price difference is usually minimal, so unless a specific gas block is desired, getting a barrel with the gas block installed can save considerable time and aggravation.

In addition to a barrel and gas block, the assembly will also require a gas tube. This needs to be matched to the gas system length of the barrel.

My personal preference for 5.56 chambered barrels is a 16” pencil profile with a carbine-length gas system. This reduces the weight up front, making the rifle a bit more handy, and uses a well-tested gas system that is compatible with most factory loaded cartridges.

Buffer Tube, Weight, and Spring

The buffer tube, also called a receiver extension, is part of the operating system as well as a convenient place to attach a shoulder stock. 

Most ARs today have a carbine-style, or collapsible, stock, but when choosing a carbine buffer tube, be aware of the two different styles: the commercial type is slightly larger in diameter, and the Mil-Spec style is slightly smaller. This means a commercial stock will rattle on a Mil-Spec tube, and a Mil-Spec stock likely won’t fit on a commercial one.

However, some people prefer the fixed rifle style. If so, the appropriate buffer tube, weight, and spring will be needed.

Furniture

A number of Lower Parts Kits come with a standard A2 type pistol grip included, but there are many, many other styles to choose from. This is one of the more personal elements of a firearm, so each person will have to choose for themselves which pistol grip has the right combination of features.

Forends are yet another area where the list of options is nearly infinite. Everything from the classic triangle to modern aluminum or carbon fiber handguards are available. This is also an item that needs to match the barrel and gas system properties.

My house AR is currently clothed with this aluminum, free float handguard with a top Picatinny rail and M-LOK slots. I like its combination of versatility, light weight, and rigidity.

Current forend on the author's house gun

Muzzle Devices

Depending on jurisdiction, options in this category may be limited to no muzzle device at all, compensators and/or muzzle breaks only, or all of the above plus flash hiders.

Regardless of which attachment is chosen, make sure to get either an early-style lock washer, or a more modern crush washer to keep the muzzle device in place.

I picked up this Flash Redirect Can that funnels most of the muzzle blast down range, which is a good choice for home defense firearms.

Sights

AR sights fall into three general categories: irons, electronic sights such as red dots, and magnified. Many people who run electronic sights will also have back up iron sights (BUIS) on their rifles.

So there we have it, a reasonably thorough breakdown of the parts needed to assemble an AR. In my next post, I’ll get into the details of assembly.

In the meantime, have fun and safe shooting!