Lately, several people have asked me with help in purchasing an AR-15. The beauty of the AR platform is its modularity. The downside is that the array of options can be almost overwhelming.
The good news is that most manufacturers build components that are 100% compatible with each other, so if you buy a lower from one manufacturer and an upper from another manufacturer it fairly well guaranteed that they two parts will fit together without too much trouble.
The AR platform consists of two main sub-assemblies. These are known as the "upper" and "lower" half.
A complete lower half consists of a lower receiver, lower parts kit, and some sort of stock.
A complete upper half consists of the upper receiver, barrel, front sight base, gas tube, hand guards, bolt and bolt carrier, plus some misc. small parts.
The lower receiver is the firearm. This is the part that has the serial number, and thus this is the part that must be purchased either from an FFL, or from a private party in state. Everything else is just a part, and can be purchased over the internet, mail order, etc.
While it is generally cheaper to buy the upper and lower halves separately, there is nothing wrong with purchasing a complete rifle.
The first decision to be made is the barrel. Barrels come in a variety of lengths, weights, materials, and twist rates. Generally speaking, AR barrels come in 3 standard lengths, 20", 16", and 14.5-14.7". Other lengths such as 18" and 24" are available, but are much less common.
The classic rifle length barrel is 20". This is the length of a Vietnam era M-16 rifle that one might see when watching war movies involving the late un-pleasentness in South East Asia.
A 16" barrel is a "carbine" length barrel. This is the minimum length allowed (without a special tax stamp) in the united states. Any shorter and it becomes an SBR (short barreled Rifle)
The military M-4 carbine utilities a 14.5" barrel. Because the military uses this length, a lot of folks want this length barrel. If one attaches a muzzle device such as a flash suppressor, and has this permanently attached. and the overall length is over 16". Then this counts as being over the magical 16" for legal purposes.
I firmly recommend that the new AR buyer stick with either a 20" or 16" barrel. The velocity loss as one goes from 16" to 14.5" is actually quite significant, and dealing with a permanently attached flash-hider can be a small pain for certain disassembly tasks.
The next question is barrel twist. Early ARs had a 1 in 12 twist, that is one revolution of the rifling every 12 inches. (actually the earliest were 1 in 14, but they didn't work in extreme cold) This was fine for the 55 grain projectiles in use at the time. When the military switched to the 62 grain SS109 projectile, they needed a 1:9 twist to stabilize the heavier bullet. They actually picked 1:7, which was required to stabilize the tracer round, which is much longer, and requires a faster twist rate.
A couple years ago it was very hard to find a 1:7 twist barrel. Most manufacturers used 1:9. Match rifles generally used 1:8 (to stabilize the long low drag bullets used for shooting at 600-1000 yards). Nowadays 1:7 is reasonably common. Again, 1:7 is considered more desirable because it's what the military uses.
I actually think that for most folks, it doesn't matter that much. 1:9, 1:8, or 1:7 will stabilize just about any bullet the average person would reasonably use. A match rifle should be 1:8 or 1:7.
Barrels generally come either Hbar (heavy barrel) or GI profile (turned down under the handguards). You can save around a pound of weight by going with a GI profile.
Again, there are many other profiles. The M-4 profile is a GI profile with an extra turned down "notch" on the barrel to accommodate the M203 Grenade launcher. Since the average AR owner is NOT likely to have an M203, the notch is essentially decorative. Again its "desirable" because the .mil has the notch on their rifles.
TO BE CONTINUED...