Welcome to the Rabbit Hole
So you have decided to buy or maybe even build your first AR! You have decided to purchase one of the most versatile rifle platforms in history. You have also stepped in front of a fire hose of information. There is an almost endless array of configurations, cartridges, manufacturers, and aftermarket options for the AR-15. Where do you even start?
The first thing is to decide if you are going to buy a complete rifle or try to build your first AR. Then we can dive into the features of rifles and what they really mean.
Buy VS Build
I would strongly suggest buying a complete rifle for your first AR if you fall into any of the following categories:
– Working on a tight budget: an expert builder can usually build a superior rifle for the same price as a factory built rifle. But the tools you will need and the numerous mistakes you can make on your first try will likely result in you getting a lot less value from your first build than a factory gun.
– Hardly ever used a set of tools in your life: If you are not a handy person, firearms aren’t the place I would recommend starting to learn. Yes, an AR-15 can be extremely simple to assemble, but things like correct torque, alignment, and proper safe tool use, can turn it into an expensive, or even dangerous, project if you don’t know what your doing. It can be done but a lot of extra learning and precaution will be necessary.
– This is your first firearm: When you are just learning firearm safety, don’t complicate it by adding on all of the extra considerations that must be taken with an unproven and home built firearm.
– It will be used for self-defense: As a general rule, I would never suggest using a customized or home built firearm for self-defense because reliability is so crucial in defense. It is almost guaranteed that you will never do as much testing and quality control as a major manufacturer. I am not saying that every factory gun is perfectly reliable, but most major manufacturers are pretty darn good. DIY builds suffer less stellar records, especially from first time builders.
If you don’t fall into one, or all of those categories then you may consider building your first AR.
Building an AR can be extremely rewarding, and because of how modular the AR platform is your rifle can grow with you and adapt to your needs. Building a rifle from the ground up is the best way to learn about how the rifle works. You will also make mistakes and figure out what does not work. Understanding your equipment will always make you a better shooter overall.
But to build a quality rifle you will spend several hundred dollars on tools and tens or hundreds of hours researching components. Down the road these tools can be reused and you will find that building a rifle will often get you a much better value and exactly what you want. Most factory rifles on the market make at least some compromises to meet the demands of multiple markets. If you build a rifle it can be tailored to your purpose and there likely wont be a better one in the world for your specific goal.
Whether you are building or buying a rifle, understanding the system and its components is important. I will break down all of the components of an AR-15 and then talk about how you make them work together.
If you are buying a rifle, you are really building one in your head and then searching for the one that most closely matches your budget and requirements.
For the sake of simplifying conversation into the breadth of one article I am going to make an educated choice for you on your first rifle. The rifle will be chambered in 5.56x45mm. “Chambering” is the reference to the size of the hole in the barrel that holds the cartridge when firing. The 5.56 cartridge is what the AR-15 is designed to fire and it is what the all-important “milspec” is designed around. The 5.56 round is extremely versatile and cheap. It is a great round for self-defense, small game, and target shooting. Many other cartridges will meet the requirements of a different niche better, but every cartridge will require its own set of careful modifications in order to operate correctly. That is discussion for another day.
One confusing thing you may find is the discussion of two very similar cartridges; 5.56X45mm and 223 Remington. These are often used interchangeably as the rounds are extremely similar to each other. I won’t go into the details on these two cartridges as it is another long conversation. If you do care read THIS. The important thing to understand is that a 223 round is safe to shoot from a 5.56 chambered rifle, but a 5.56 round should never be fired from a rifle chambered for 223. The pressure generated from a 5.56 round in a 223 chamber has a small potential to cause catastrophic failure of the rifle. A “223 Wylde” chamber can effectively be considered a 5.56 chamber and can fire either round safely. This is why most AR-15s are chambered in 5.56×45 or 223 Wylde.
The reason that ARs are so easy to build or customize is that, for the most part, they all follow a specific set of dimensions for every part. This set of crucial dimensions is often referred to as mil-spec (short for military specification). This means that, regardless of manufacturer, mil-spec components should work together. But beware, there is a tolerance (allowable range) within milspec, and one manufacturer may make components on one end of the spectrum or the other.
To put numbers to this; the milspec for a component may define the diameter of the hole as 1.00” +_ .01”. Hole A of diameter 1.01 and hole B of 0.99” are both within mil-spec. A pin that is .99” wide will fit loosely in hole A and very tightly in hole B. Usually this is still OK, milspec was defined with this in mind and the rifle will still work. But this “tightness” can influence many things. The rule of thumb is that; the tighter the parts fit together, the more accurate a rifle will be, the looser they fit, the more reliable a rifle will be.
For this reason, it is often best to purchase parts that need to fit together with a specific tightness from one manufacturer. Moving parts often fall into this category.
It is also worth noting that most parts designed for an ar-15 will work together regardless of being labeled as mil-spec. Manufactures understand that most of the value in an AR lies in its modularity and do their best to make parts that will work on as many rifles as possible. Again, this does not apply to AR-15 style rifles chambered in any other cartridge!
Building an AR
So lets begin with a complete list of all parts, and then I will dive into each part.
- Lower Receiver
- Lower Parts Kit (LPK)
- Take down pins
- Take down pin x2
- Take down pin spring x2
- Take down pin detent x2
- Magazine release
- Magazine release
- Magazine release spring
- Magazine release plunger
- Bolt catch
- Bolt catch
- Bolt catch spring
- Bolt catch plunger
- Trigger assembly
- Trigger Pin
- Trigger Spring
- Hammer Pin
- Hammer Spring
- Disconnector Spring
- Safety selector
- Safety selector
- Safety selector spring
- Safety selector detent
- Buffer catch plunger
- Buffer catch spring
- Trigger guard
- Trigger guard
- Trigger guard pins x2
- Take down pins
- Receiver Extension
- Castle nut
- End plate
- Buffer tube
- Buffer spring
- Pistol grip
- Pistol grip screw
- Upper Receiver
- Upper Parts Kit (UPK)
- Forward Assist
- Forward Assist Plunger
- Forward assist pawl helical compression spring
- Forward Assist Pawl
- Forward Assist Pawl Detent
- Forward Assist Pawl Retaining Spring
- Ejection Port Cover
- Ejection port cover
- Ejection port pin
- Ejection port spring
- Ejection port retaining clip
- Charging Handle
- Charging Handle
- Charging handle latch
- Charging handle latch spring
- Charging handle latch pin
- Forward Assist
- Bolt Carrier Group (BCG)
- Bolt Carrier
- Bolt Cam Pin
- Gas key
- Gas key
- Gas key screws
- Firing Pin
- Firing Pin Retaining cotter pin
- Extractor pin
- Extractor spring
- Ejector Spring
- Ejector Retaining Pin
- Gas rings
- Barrel extension (usually pressed on the barrel and sold as single piece)
- muzzle device
- Gas block
- Gas block
- Gas block retaining screws
- Gas tube retaining pin
- Gas tube
- Hand guard
- Hand guard
- Barrel nut
Yes over 60 parts, that is a lot of little pieces you have to round up to build a rifle! Don’t worry, all of these assemblies can be bought as a complete unit. You can even buy a complete lower and complete upper, and with two pins put them together. This can be as simple or detailed as you would like.
The lower receiver is technically the “firearm” part of the rifle. It is the only part serialized and the only part that must follow firearm laws for purchase and transfer. You cannot have this part shipped to your house, and you will have to undergo the standard background check like any other firearm to purchase it. The lower receiver is what all of the other lower parts attach to.
Lower receivers generally fall into one of four categories; polymer, cast, forged, and billet. Most of this information also applies directly to upper receivers.
Cast receivers are almost non-existent now. Casting is pouring molten aluminum into a mold that shapes the receiver. They are the cheapest way to make receivers quickly but they are not nearly as strong as polymer or slightly more expensive forged receivers. I would just stay away from them on ARs.
Polymer receivers are often the cheapest and lightest option because they are mostly made of…polymer. The good ones will have aluminum or steel inserts in threaded and very thin areas. Polymer lowers are not as strong as the other types of receivers and have a rough history of being labeled as fragile. They have gotten much better in recent years, and like the polymer framed pistols of today, are very strong. I don’t think I would pick a polymer lower to go to war, because a forged lower is still much stronger and not that much more expensive. But for most people’s uses they provide a lot of value and very little weight. Polymer receivers can also have very unique features because of the freedom of design their manufacturing method provides.
Forged receivers is where the lion share of the market is. Most standard receivers are forged. Forging is a process where metal is heated and then hammered or crushed into the correct shape. The final details and dimensions are machined from this pressure formed shape. The crushing action compresses the metal and makes it stronger than the original metal was. It is also a very efficient way to get the rough shape of whatever you are making. This all means that a properly forged receiver is strong and affordable. It is worth noting that only a handful of places in the US actually do the “forging” part of making the receivers. Hundreds of companies do the final machining, apply a coating, and brand it as their own. The metal in a dozen different receivers is likely to be exactly the same, the quality of a receiver is often defined by the accuracy of the machining and the quality of the coating or finish.
Billet receivers are the boutique receivers. Billet means that they started with a solid block of aluminium and machined out all of the dimensions. This gives manufacturers much more freedom in design and material choices because they aren’t limited to those supplied by the forgers. Billet receivers often have very unique designs both aesthetically and functionally, having features you cant find on any other type of receiver like ambidextrous controls or integrated hand guards. This all comes at a cost, both literally and structurally. Billet receivers can cost 3-6 times what most forged receivers do because a lot more aluminum is wasted in the process and it takes much longer. And generally billet receivers have a lower strength to weight ratio because they don’t get the strength improving crush of forging. But man do they look good!
My Personal Favorite Receivers
For a forged receiver, Aero Precision makes fantastic receivers. They make both upper and lower, as well as both mil-spec and improved versions. They are a large and well respected operation and many other companies use their receivers in their complete rifles. Because of the volume of work they do and their modern manufacturing equipment their receivers are also hard to beat on price. Usually in the $50-120 range depending on model. A win-win.
If you are stepping up to the more customized billet receivers I would take a look at companies like Seekins Precision, SLR Rifleworks, and Gibbz Arms. Each company’s receiver sets have unique features and are executed to an extremely high standard of quality. Seekins and SLR both have billet lowers with ambidextrous safety selectors and slick right handed bolt releases. Gibbz Arms also has an ambidextrous lineup, but their hat trick is a very nice non-reciprocating side charging upper receiver. None of these are budget options (a receiver set will put you back $500-900), but they all have innovative designs that are superior to a mil-spec receiver.
Lower Parts Kit
The lower parts kit contains 28 pieces, most of them pretty small. The lower receiver of an AR-15 is where most of the moving parts are. I wont dig into some of the parts because as long as they are made of quality material and to mil-spec tolerances there is no reason to go crazy here except for aesthetics. Pickup a LPK from any quality manufacturer like Aero Precision, RRA, or CMMG and your good to go.
The parts you do want to pay close attention to, and are often not included in LPK packages, are the trigger assembly and trigger guard. A quality trigger is the number one upgrade you can do to improve the practical accuracy of any rifle. A milspec AR-15 trigger will have a pull weight of 5.5-7 lbs, be of moderate pull length, fairly crisp break, with an average length but tactile reset. This works great for combat or rapid fire shooting, but leaves a little wanting for any sort of precision work. If you want a rifle that is easier to shoot accurately there is an amazing array of trigger upgrades available ranging from $10-$300. This really brakes down into two categories of trigger assemblies, improved milspec and drop-in.
Improved milspec triggers still use the original design and function of a milspec trigger but with higher quality springs, finely finished surfaces, and tighter tolerances. Improved milspec triggers will usually reduce the overall trigger weight, shorten the pull, and enhance the reset.
Drop-in triggers use some form of “cage” to hold all of the pieces together and do not require assembly. This gives more freedom to change the function of the trigger and removes reliance on perfect trigger pin spacing tolerance in the lower receiver. The drawback is cost, they are more complex and have more parts than a milspec trigger assembly.
My personal favorite triggers are of the drop-in variety from TriggerTech triggers out of Canada. Check out my full review of their entire AR lineup HERE. There is a reason most PRS shooters use them.
The receiver extension houses the buffer and buffer spring, therefor it is often called the buffer tube.
As the gun fires, the bolt carrier group is launched rearward by the gasses from the explosion. The bolt carrier moves rearward from the upper receiver into the extension, pressing into the buffer and compressing the buffer spring. Watch this great video for better explanation.
There are 3 types of receiver extensions and they are referred to by length; rifle, carbine, and pistol.
Rifle length was the original. It is the longest and is usually associated with an A2 or fixed length stock.
Carbine length was the “update”. It is shorter than rifle length and has a rail with detentes on the bottom of it to accommodate adjustable length stocks. It is by far the most common type of receiver extension and probably the most versatile.
Pistol length receiver extensions are extremely short, and named so because they are not intended to use with any stock at all. If it has a stock, the ATF considers it a rifle.
Despite common misconception, interior movement space on a rifle and carbine length buffer tube are within 1/4″ because a longer rifle length buffer is required to stop the gas key on the BCG from contacting the back of the receiver. The length of receiver extension you want is really driven by the type of stock you want. Pick your stock, then buy the buffer tube length it is made for.
Pistol length buffer tubes are a different story, they shorten the buffer movement to an absolute minimum to make the “pistol” as compact as possible, usually in tandem with extremely short barrels. Pistol length buffer systems can be very finicky and usually produce extra recoil when compared to a carbine/rifle length system because the spring doesn’t have as much time to slow down the buffer before it smacks into the back of the extension. Only use a pistol length buffer system if you have to for legal reasons or your willing to sacrifice all other areas of performance to shrink the weapons overall length.
Stocks provide two of the four points of contact on the gun; shoulder and cheek. Therefor its selection is an extremely personal endeavor.
The major factors that drive selection of a stock are adjust-ability, weight, and quality.
Adjustment of a stock is all about comfort and cheek weld.
Cheek weld (how your face meets the stock) is important for accuracy. If your eye isn’t in exactly the same spot every time you shoulder the rifle, the way your eye lines up the sights will change, and your point of impact will change.
An adjustable cheek rest allows you to relax your neck and rest your cheek on the rifle and be perfectly lined up with your sights every time. An adjustable cheek rest makes a rifle much more versatile because every combination of sighting system, shooter, and shooting position will require a different cheek rest height. In every shooting position your head/neck angle will change and move your eye. Every optic or set of sights will have a different height of alignment. Every person has a different cheek shape and different cheek to eye distance.
Quality shows in a cheek rest that has solid repeatable adjustments and multiple degrees of adjustment. Settings shouldn’t change when you don’t want them to, but be quickly adjustable between multiple settings for different shooting positions. Some rests have adjustable cant forward-to-back and side-to-side in addition to height.
The other area of adjustment a stock may have is at the shoulder contact. The primary adjustment will be to the overall length of the stock, commonly referred to as length-of-pull (LOP). Adjusting LOP allows a comfortable distance from shoulder to cheek rest. Again the correct LOP may change depending on position and optic. Gear can throw another level of LOP consideration into the mix, a heavy winter jacket or a plate carrier can reduce the distance from your shoulder to your neck considerably. In addition to LOP, the most adjustable stocks will also allow the butt plate, the part that actually touches your shoulder, to cant and move up and down. This allows the stock to firmly contact your shoulder independent of other adjustments. .
The more degrees of freedom you have the more comfortable and repeatable your cheek weld will be, but every added layer of adjustment will add weight, cost, and fragility to the stock.
If the rifles purpose is close range combat and knocking down doors, then cheek weld and comfort probably take a back seat to weight and strength considerations. A completely fixed stock like an A2 or Magpul Rifle, or an adjustable Carbine stock might be a good fit.
For a custom cheekrest on an A2 check out my DIY post here.
If you are building a DMR, varmit, or precision rifle, then a much more adjustable stock will really help the practical accuracy of your rifle system. My personal favorites in this realm are the XLR industries AR stock, and the Magpul PRS stock.
The Magpul is very robust, and has the basic types of adjustment, but is heavy, expensive, and I personally didn’t find it very comfortable. The XLR is much lighter, more comfortable, and has more adjust-ability, but its adjustments do require tools, and its still pretty expensive. The budget option, which I actually found to be very comfortable, is the LUTH AR MBA.
It just isn’t as robust as either the XLR or Magpul and I have heard mixed reviews on how well it keeps your adjustments locked in, but the price is hard to argue.
The third point of contact on the gun!
The pistol grip serves two purposes, lets you grip the gun, and holds the lower receiver takedown pin detent spring in place.
Good news is, pretty much every pistol grip made will correctly hold the detent spring in the gun, so we can focus on the grip part.
Like stocks, the comfort/ergonomics of the pistol grip are extremely personal, but there are some common considerations to make.
One of the most important aspects of “fit” on pistol grip is the distance from the back of the grip to the face of the trigger. The goal is that the grip allows you to place your trigger finger, middle of the last digit, on the trigger without contorting your hand or wrist. For most men (large hands) the standard A2 grip is too thin and promotes a trigger finger placement somewhere between the second digit and first joint – not ideal. A slightly thicker grip that extends up behind the lower receiver, called an extended beaver tail, will help pull your hand back from the trigger. Be aware that this extended trigger reach also moves you away from the fire/selector and magazine release, so don’t go too far or you will have to release your grip to activate these controls. These controls being accessible without moving your hand is one part of what makes AR ergonomics so great, so try not to screw it up.
The other aspect of grip to consider is the angle. Grip angle is mostly correlated to shooting position and stance. The more bladed or angled your stance the more comfortable a swept back grip will be. The more square to your target that your body is, the more vertical your wrist will be comfortable. Most grips in the market strike a middle ground so that they are moderately comfortable in any shooting position.
Beyond these two aspects, the most important and intensely personal selection criteria for a grip is comfort. This is different for everyone, the only way to make this decision is to actually go out and hold them for yourself.
If you are looking for an amazing vertical grip for an all out precision rifle check out the Master Piece Arms Enhanced Vertical Grip (does require a hole be drilled for an AR). Another highly ergonomic grip with a large beaver-tail, that happens to be only $12, is the Larue A-PEG. The Magpul MOE series of grips is the gold standard in simple, robust, and affordable grips that will work for almost any situation.
Upper receivers have all the same material considerations as the Lower Reciever but they do have a few other features that are worth noting; charging style, ejection port, and forward assist.
The milspec charging style is an ambidextrous handle that protrudes from the rear of the receiver. This works very well, but forces both hands to come off of the rifle, in an awkward way, to lock the bolt open. In a self defense or fighting scenario this is almost irrelevant, you shouldn’t ever have to lock the bolt open yourself, the bolt locks itself back after the magazine is empty. For the rest of us who spend more time at the range than on the battlefield, and regularly have to clear our rifle for safety reasons, this is kind of a pain. The charging handle also has to have room to move rearward over the top of the stock, which greatly limits the options for an adjustable cheek rest. The charging handle opening in the upper receiver can also be a major leak point for gas during suppressed fire. Gas in your face is never a good time.
Welcome the side-chargers. A side charging upper relocates the charging handle to the side of the receiver.
Reciprocating charging handles are actually mounted on the bolt carrier group and move with the bolt as the gun is fired. This bolt mounted handle typically sticks out of the ejection port. The upper receiver has to have a recess cut that allows the handle to move back beyond the ejection port. These are the simplest and cheapest form of side-charger, but don’t get your hand or face in the way of that handle!
Non-reciprocating side-chargers have handles that are disconnected from the bolt and stay in place while the gun fires, only catching the bolt when pulled rearward. They are usually on the opposite side of the rifle from the ejection port.
Side-charging uppers allow a lot more freedom in the mounting of cheek rests, allow your strong hand to stay on the pistol grip while locking the bolt open, your cheek can stay welded while clearing a malfunction, and since they don’t have an opening at the back of the receiver they have a tendency to contain gas from a suppressor much better.
Downside of side chargers – cost. They have a tendency to be 2-3x more expensive than a standard mil-spec upper. But when pricing one out, don’t forget that a side charging upper receiver also includes the price of the charging handle! My personal favorite side charger is from Gibbz Arms.
The ejection and forward assist are more of a personal decision than charging style. The ejection port can be over-sized to allow large cartridges like 450 Bushmaster or 50 Beowulf to eject cleanly, or shrank to aesthetically match small cartridges like 9mm or 22lr. But the most common alterations to the ejection port are the removal of the brass deflector to match the removal of the forward assist, making what is called a “slick sided” upper.
The forward assist is not necessary for 99% of shooting situations. It was added to the AR during Vietnam when soldiers complained that the charging handle offered no way to slam the bolt home when a dirty round resists complete chambering. The forward assist can be slammed to send the bolt into battery on a problematic round, but best hope that it doesn’t get stuck during ejection! The forward assist can also jam up the gun badly if the retaining pin comes out and allows the forward assist to slip too far into the receiver.
A slick sided receiver has no forward assist, ejection port cover, and no brass deflector. It usually shaves a few ounces off the receiver and cleans up the look of the rifle, while saving you a few dollars. But it leaves the ejection port open to dirt/gunk, flings brass right in the face of left-handed shooters, and doesn’t allow forcing the bolt closed. Perfect for range toys, less-so for defensive weapons.
Upper Parts Kit
Depending on upper receiver, you may not need an upper parts kit at all. But if you go the milspec route you will need a forward assist assembly, ejection port door, and a charging handle.
Forward assists and ejection port doors are pretty much identical from all manufacturers aside from color and engravings. Just pick what you like.
Charging handles require more careful selection. A cheaply made charging handle may bend or not unlock when pulled aggressively on a stuck round. Quality handles like the ones from BCM, Radian, and Aero Precision are expensive, but they lock firmly onto the receiver when closed, are ambidextrous, open easily, have extended handles to get your hands away from optics, do not flex when pulled aggressively, and seal better with the receiver to reduce gas escape. Its just a handle, but get a good one.
Bolt Carrier Group
The bolt carrier group could be its entire own article, but to keep it simple the important factors in a Bolt Carrier Group or BCG are; bolt test and material, bolt head-space tolerance, coating, and weight.
The bolt is the part of the BCG that does all the work. It grabs a round from the magazine, locks into the barrel extension, contains the forces from cartridge ignition, and then rips the spent cartridge from the chamber and ejects it. The rest of the BCG just assists the bolt in these functions. Because the bolt is subjected to some pretty brutal forces, its construction and material selection are critical for a long life. Two common labels that you will see as proof of a bolts integrity are MPI (magnetic particle inspected) and HPT (high pressure tested). Although not a guarantee of quality, a bolt that isn’t marked as MPI and HPT should be avoided.
The material of the bolt is also important. Mil-spec for the bolt is Carpenter 158 grade steel, and should be the minimum acceptable grade of steel for a bolt. Some companies have started using superior metals, like 9310 steel, for their bolts to increase their life. But regardless of type, steel has to be properly treated and coated to retain its strength, which isn’t necessarily a fast or easy process, so buying a bolt from a reputable manufacturer like Faxxon, Aero Precision, or BCM is a good investment.
Bolt head space is something that is difficult to know when purchasing, and is effected equally by bolt and barrel extension dimensions. It’s not so much something that is a feature for buying criteria as it is one more very important reason to buy a quality bolt. A rifle with too much head-space may not correctly support the cartridge during firing and cause a catastrophic case failure. A rifle with too little head-space will not fully chamber on max-spec cartridges. Again, there are areas where you can save money on a build, but buy a quality bolt!
Coating of the bolt is where most of the marketing hype starts. The better the coating the less friction it creates, the easier it cleans, and the harder it wears. There are lots of articles on why and how each coating is the best, but to keep it simple the ranking of coatings from worst to best is as follows:
- NiB -Nickel Boron
- DLC -Diamond Like Coating
- TiN -Titanium Nitride
- NP3 (Robar)
- QPQ – Quench Polish Quench
Keep in mind that as you go down the list the treatment process becomes more difficult or time consuming to do correctly and therefore becomes more expensive. A very high quality Parkerized coating will perform better than a shoddy Nickel Boron one, so always pay close attention when the coating-price seems too good to be true.
The last thing to consider when selecting a BCG is weight. If all else is the same and the gun has a properly tuned gas system; a heavier bolt carrier group will cause more felt recoil but be more reliable, and a lighter bolt carrier will have less felt recoil but will be more finicky. If you are building a heavy duty combat rifle or value reliability above all else then you want to look for a “full auto” profile bolt that is all steel, or something with intentionally added weight like the JP VMOS heavy BCG.
If you want a rifle built for speed consider a reduced weight or a “semi auto” profile. JP, Faxxon, Cryptic all make “race” BCG that are extremely light weight to reduce felt recoil. These BCGs reduce felt recoil by reducing reciprocating weight and needing less gas to cycle. But they are extremely responsive to changes in gas pressure due to ammunition and are less robust in general which usually results in a limited lifetime.
Its also worth noting that a “full auto” profile BCG will not make your rifle full auto. The full auto profile is the original design used in the M16. As an additional precaution to make it more difficult to convert an AR-15 to an M-16 some civilian focused companies shaved off part of the BCG that trips the extra sear in a full auto gun. An AR-15 doesn’t have that extra sear and cannot be made to run full auto without additional machining and a completely different firing control group. The ATF doesn’t care what profile your BCG is as long as you don’t complete the rest of those modifications.