Props to Craptacular on SomethingAwful for this post


So you want to buy what's arguably the most popular semi-auto rifle in the United States, but have no idea where to start? This is where to start.

First, you might want to view this video that explains the operation of the AR15/M16. It's from the 60's, but the information in it is still valid. Then once you're done, you can play around with this flash animation.

Then, take a look at the M4 Chart. Some people don't like this chart for whatever silly reasons they have, but it does explain differences in assembly and parts quality that may not be readily apparent.

Lowers:
The AR15 lower receiver is the part that has the serial number, so it's what's legally considered the "gun". For the most part, a lower is a lower is a lower. Just pick a brand that's cheap and that has a rollmark/logo that you like. Most lowers are forged, which just means that molten aluminum was pounded into the rough shape of a receiver, then the rough receiver was milled and drilled to get the finished product. Some companies make billet receivers, which are just machined from a much larger block of aluminum, but there's really no advantage to these other than whatever e-penis waving pride you get out of them.

Lower parts kits:
The lower parts kit is all the small parts in the lower. For the most part, a parts kit is a parts kit is a parts kit too. DON'T USE M16 FIRE CONTROL PARTS (TRIGGER/HAMMER/SELECTOR/DISCONNECTOR) IN YOUR GUN. You'd have to purposely do this though, you won't do it by accident.

Triggers:
You don't need a 2-stage trigger unless you're building some fancy scoped target rifle.

Colt is retarded and uses non-standard oversized trigger and hammer pins, so if you buy a Colt and later want to put a different trigger in the gun make sure to buy the right size. Edit: apparently they're now using standard-sized pins on newer Colt 6920s, but not other models (yet?).

Stocks/receiver extensions/buffers:
You have two options, a fixed or collapsible stock.

Fixed: Not surprisingly, the A1 stock was originally used on the M16A1 and the A2 stock was originally used on the M16A2. If you're going to go this route, the A1 is probably a better choice as the A2 is too long for most people. Both A1 and A2 lowers use a rifle length receiver extension and a rifle buffer weight.

This didn't fit in the first post.

Collapsible: The collapsibility isn't for ease of storage or transport so much as it is to change the gun's length-of-pull for different shooters. I suggest using a collapsible over a fixed stock for this reason. The typical collapsible stock is the M4 style, originally used on the M4 carbine. They use a carbine-length receiver extension and a carbine buffer weight.

Despite the names, there's no reason why you can't use a 20"-barreled rifle upper with a lower that has a collapsible stock and buffer from a carbine, or use a 16"/14.5"-barreled carbine upper with a lower that has a fixed stock and rifle buffer from a rifle. Either will function just fine. You can get other fancier stocks from Magpul or LMT or Vltor that have better cheek welds and onboard storage space but the M4 stock really isn't that bad.

Uppers:
There's three types of upper receivers, only one of which you should buy. The type of upper that you do want to buy is a flat-top upper, which has no integral rear sight, just a picatinny rail to which you can mount scopes and/or detachable iron sights. Different manufacturers will call their flat-tops either "A3" or "A4" uppers, but they're the same thing.

The A1 and A2 uppers were used on the M16A1 and M16A2, respectively. They both have permanent integral rear iron sights as part of the upper. Unless you want to build a retro-style rifle there's no reason to buy one of these. The main reason not to is because they make mounting a scope much harder. They don't make it impossible, but they're very limiting and therefore non-optimal. Even if you never mount a scope on a flat-top you can still use a detachable carry handle sight and get exactly the same types of iron sights you'd get on an A1 or A2 with the flat-top.

The only other major difference between uppers is whether they have M4 feed ramps or not. These are just feed ramps that extend onto the inside of the upper instead of only being part of the barrel extension. They were developed due to problems in M4 carbines feeding in full-auto, so they're not really necessary but there's not really any downside to them either.

Barrel lengths & gas system types:
20": This is the barrel length on the M16. It uses a rifle-length gas system.
18": These use either intermediate-length or rifle-length gas systems.
16": These use either carbine-length or mid-length gas systems.
14.5": These either use carbine-length or (rarely) mid-length gas systems. Also, because these barrels are under the 16" NFA requirements there must be a muzzle device that's permanently attached to the barrel to bring the whole combination of barrel & muzzle device to more than 16". Because it's permanently attached, this may cause problems later if you want to put a free-float rail on the upper, because many times installing a rail requires that the gas block be removed, and removing the gas block requires that you remove whatever muzzle device is on the gun.
<14.5": These use carbine-length gas systems (11.5" or 10.5" barrels) or pistol length gas systems (7.5" and shorter). Typically you'll only see these barrel lengths on a short-barreled rifle or an AR15 pistol, but occasionally you'll see an abortion like a 11.5" barrel with a permanently attached 5.5"-long flash suppressor that keeps it legal for use on title I guns.

Generally, the best combinations are ones that have approximately 7" between the muzzle and the gas port in the barrel, which results in an optimal gas pressure which is strong enough to reliably operate the gun but not overpowered. So the optimal uppers would be either a 20" barrel with a rifle-length gas system, a 18" with an intermediate system, a 16" with a mid-length system or a 14.5" with a carbine system. It's not the end of the world if you end up with something else though.

Buy a chrome-lined barrel. They last longer and are easier to clean. "Chrome-moly" is not the same as "chrome-lined". Chrome-moly is just describing the type of steel used in making the barrel.

Charging handles:
Get a forged charging handle rather than an extruded charging handle; a forged handle will be stronger and less susceptible to torsion. If you want to spend the extra money, an extended latch like the PRI Big Latch will make it easier to grab onto the charging handle.

Gas pistons:
Instead of the normal direct-impingement gas system, the new rage is for systems that use a gas piston to cycle the action. Because they vent the dirty carbon-laden gas out of the rifle instead of into the action they will make the gun run cleaner. However, they do add some weight over a direct-impingement system, and the reliability of gas-piston systems isn't nearly as proven. If you do get one I'd suggest getting one that's designed as a complete upper, like LWRC or LMT instead of a kit that you add to your own upper. There is no standard gas piston system, so parts that are designed for one brand of gas piston upper may not work with another gas piston upper.

Iron sights:
If you're not planning on shooting people with your gun, then you probably don't need a set of backup iron sights (BUIS). But if you are planning to or you want to increase your e-peen length then you have two basic options, fixed or folding. If fixed, get a Larue or LMT rear and use the factory front iron sights. If folding, the Troy Battlesights are what you want.

Scopes:
Red dots:
Aimpoint ML2, ML3 or T1 Micro - They use small watch batteries which can power it for years at a time. The reticle is just a simple dot. The T1 is lighter but has a smaller lense.
EOTech - They use CR123 or AA batteries. The reticle is larger (circle w/dot) which some people complain is more cluttered.

If you're on a budget and the gun's just going to be for shooting paper at the range I guess you can buy something cheaper but when it breaks don't come crying to me you big baby.

Magnified scopes:
Trijicon ACOGs are a good fixed-power magnified optic that you might want to look at. If you want a variable-power, try a Trijcon Accupoint. I've got a Nightforce NXS that I like on one of my guns but it's expensive so I'll probably get yelled at for recommending it. Any other suggestions on magnified optics welcome.

Free float railed handguards: They make it so the barrel is only connected to the upper receiver, not the handguards. This means they're not affected by pressure on the handguards, making the gun more accurate. It also makes it easier to attach lights/lasers/sling mounts/grenade launchers/big floppy purple dildos/whatever to your gun. Don't buy cheap crap like YHM, save up for something like a Larue or a Daniel Defense or Troy rail. Cheap railed handguards are heavy, have out-of-spec rails that may or may not work with whatever you're trying to attach to it, most likely don't free-float the barrel and can rotate and/or loosen a lot easier. You get what you pay for.

Lights:
If you're planning on using your gun to shoot things that go bump in the night a light is a really good idea, whether the gun is an AR or not. A railed handguard will make it easier to attach a light. If anyone has any specific suggestions on lights let me know.

Slings:
Get a Vickers 2-point. If not that, get another brand of 2-point sling. 3-point slings are too complicated and single-point slings let the gun swing around too much when slung.

Magazines:
You have two options, USGI (aluminum, from any number of manufacturers) and the plastic Magpul PMAG. Well, I've heard good things about the plastic Lancer L5 magazines too but I haven't used those myself. USGIs are usually a few bucks cheaper per mag. If you buy USGI magazines make sure they have anti-tilt followers (Magpul or CMMG SSAT). Older followers can tilt inside the magazine causing a failure, and the newer anti-tilt followers aren't much more expensive. All PMAGs come with anti-tilt followers.

And this didn't fit in the third


Building an AR15:
Since there's so many different manufacturers of AR15s, many of which sell their parts separately, it's possible to build one from parts instead of having to buy a complete gun from the factory and then modifying it to your preferences. Assembling the lower only requires a punch set and a hammer. Also it's a great way to build an AR15 on a budget because you can spread out the purchase of the parts over time instead of having to buy a complete rifle in one $1000+ chunk. Protip: when installing the lower parts that use springs, put the lower in a big clear plastic bag like an oven bag so when some small part springs out of place it won't go across the room never to be seen again.

Building an upper isn't much more complicated than building a lower, but you need more specialized tools (at the minimum a barrel wrench and a vice block to hold the upper while you torque the barrel), so unless you want to do it for the experience it's probably not going to save you money if you're building just one upper.

Neither a stripped lower nor a complete lower sold without an upper are legally rifles so if you're under 21 you can't transfer one from an FFL. If you're between 18 and 21 and want to build up a lower you'll either need to buy a stripped lower from a private seller (if legal in your state) or wait until you're 21.

AR15 pistols:
The AR was designed for use with a stock. Trying to use one without a stock is stupid. If you want a short barrel either register the gun as a short-barreled rifle and then put a stock on it, or if you can't do that due to state law then move to a state where you can get an SBR.

Caliber conversions:
One advantage provided by the AR15's removable upper receiver is that it makes changing the caliber as easy as swapping on a new upper. The biggest calibers that fit through the magwell are in the 7.62x39/6.8 SPC/6.5 Grendel size range. If you want an AR that shoots a larger caliber you either need to buy an AR10 (which has a larger magwell) instead of an AR15, or get a single shot upper like certain AR15 .50BMG uppers that don't feed through the magwell.

.22LR:
The best caliber conversion to get is for .22LR because it allows you to shoot so cheaply. There's several different types, but the defacto standard is the Ciener-style conversion. There's nothing wrong with the Ciener conversion kit itself, but don't buy directly from the manufacturer, as they have arguably the worst customer service of any store ever. Either buy a Ciener conversion kit from another vendor or buy a Ciener-compatible conversion kit from some place like Model 1 Sales or Spike's Tactical.

Because .22LR and .223 Remington/5.56x45mm are almost the same caliber (.223 vs. .224), you can shoot .22LR out of a .223/5.56mm barrel. The conversion kits all include a chamber insert which allows the .223/5.56mm barrel to chamber .22LR. The main downside to using the .223/5.56mm barrel is that usually the barrel's twist rate is too fast for .22LR, which can result in keyholing and poor accuracy. The best solution is to get a dedicated .22LR upper that has a barrel with the proper twist rate, such as one from Tactical Solutions or Spike's Tactical. You will also need to buy magazines that hold .22LR. The only real choice is Black Dog Machine.

9mm:
The next most popular caliber conversion is for 9mm, again because of ammo lower cost. There's two types of conversions, Colt-style and Olympic-style, which are incompatible with each other. You want Colt-style. For 9mm, you'll need a new upper with a 9mm barrel and bolt, a 9mm buffer weight, 9mm magazines and a magazine well.

Rock River Arms and Spike's Tactical are two popular 9mm parts vendors. Because 9mm uppers are blowback operated versus direct impingement, they need a heavier buffer weight. It's possible to use the heavier 9mm buffer weight when shooting .223/5.56mm as it slows down the rate of fire and some people feel it shoots smoother.

There's two options for magazines, depending on whether you want the bolt to lock back on the last shot or not. If you do, you want to buy C Products magazines which have a follower that holds the bolt open. Uzi magazines will also work, but they don't hold the bolt open. Since the magazine catch slot on Uzi magazines is in the wrong place for an AR15, a second slot must be cut in the proper place on the magazine body for them to function. In order for the magazines to fit in the AR15 lower's magazine well, you need a smaller 9mm magazine well which fits inside the lower's magazine well. Rock River Arms, Hahn Precision and C Products all make magazine wells that use the standard magazine catch. However, if you're using Uzi magazines, you have another option. You can use cheap unconverted Uzi magazines with a VM Hy-Tech magazine block, which has a second magazine catch as part of the block, which uses the original magazine catch slot in Uzi magazines. However, this means that the magazine catch is in a non-standard location, which may or may not be an issue for you.

Links:
ARFCOM, the 800-lb gorilla of AR15 message boards
M4 Carbine.net, not nearly as big as ARFCOM but IMO a higher proportion of posters there know what they're talking about.
Weapon Evolution, not so much discussion but some good photos/reviews of new parts & accessories.

Manufacturers:
Colt Mfg
Colt Defense (Sold as LE-only but their semi-auto rifles are legal for anyone to own in states without a state AWB).
Noveske
LMT
Charles Daly
Smith & Wesson
Sabre Defense
Armalite
CMMG
Stag
Bushmaster
Rock River Arms

Vendors:
There's plenty of vendors out there selling AR15 parts, but I've bought stuff from all the stores listed below and would recommend any of them.

G&R Tactical
MSTN
Bravo Company
Ranier Arms
JTAC Supply
ADCO
Global Tactical

Good find!