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I've had a lot of questions recently about how to build an AR-15 and some request to make an FAQ about it. I'm sure all of us can put together a good how-to for everyone to follow.
How to build an AR-15 rifle
1. Required parts.
Lower Receiver - the lower receiver is the bottom part of an AR15. This is what has the pistol grip, and the magazine well. Most importantly this is the registered/serialized portion of the AR-15 which constitutes the firearm itself and will require an FFL to transfer it to you and complete the 4473 paperwork. Generally all lower receivers are cast aluminum, and most all are cast from an A2 lower design. All lowers by and large are created equal. A more expensive lower will not make your gun more accurate, or more reliable. It is very uncommon to run into a lower receiver which was built or machined "wrong." There are lowers which are billet machined, and not cast - they cost considerably more and for the most part are just for aesthetics. Find a lower which has a design you like, and go with it. Lowers generally run from $80-150.
Parts kit - a parts kit is all the junk that fits inside your stripper lower received to make it work. Basically this includes the pins, springs, hammer, trigger, pistol grip, and mag match, among other things. Not all parts kits are created equal, although all will work. Some more expensive kits such as Colt will have fully hardened parts or better machined faces which could make for a smoother trigger. Some hammer springs are stiffer, etc. If this is your first AR-15 I would recommend the cheapest you can find, if you want a better trigger you can purchase a premium model at a later time. Generally parts kits will run around $70-120.
Butt Stock - there are two major stock types for an AR-15. There is the adjustable/collapsible M4 style, and the A1/A2 style which is fixed. For long barrel hunting rifles a fixed stock such as an A1/A2 or Magpul PRS stock is best suited. For carbines an adjustable stock is probably the best choice.
Buffer Tube - There are two types of buffer tubes. Milspec and Commercial. Milspec tubes have a smaller diameter (very slightly) and are shorter. Commercial tubes have a slighter larger diameter, and some will have a slight cant to the rear. There is no quality difference between the two tubes.
Buffer/Buffer Spring - There are two basic types of buffers - rifle and carbine. Of those there are slightly different weights for fine tuning. For most any standard build a rifle or carbine will be plenty. Carbines buffers/springs are meant to operate in collapsible stock tubes, and rifle buffers operate in full length A1/A2 fixed stocks. The way to identify the two is carbine buffers have a slight polymer taper to the front and have a cylindrical profile all the way down. Rifle buffers have a machined bevel in the side of the buffer, semi-conical. Rifle buffers are also slightly longer.
Upper Receiver - the upper receiver of an AR-15 is what holds your bolt carrier, barrel, hand guard, and optics/sights. Generally people purchase an upper "complete" meaning the barrel and hand guard is installed already, as it requires special tools. There are two different kinds of upper receivers for the most part - standard, and sportical. Standard upper receivers will have a forward assist on the right side of the upper, which allows the shooter to force the bolt into battery in the event it has become dirty and jammed. A sportical style does not have a forward assist and you would be unable to force the rifle into battery. Sportical uppers are typically used in target style rifles which do not shoot a large volume of ammunition.
Bolt Carrier Group - The bolt carrier group on an AR-15 consists of the bolt head, firing pin, cam pin, retainer pin, and carrier. These are sold as complete units and generally range in price from $100-250 depending on features. Some bolt carriers carrier higher testing certifications such as MPI. Generally any bolt carrier will be fine for a semi-auto rifle. Most of the higher certifications are more pertinent to full-auto rifles. Bolt carriers are separated into two major groups - semi auto and full auto carriers. Semi auto carriers have a larger slot on the bottom of the carrier. Full auto carriers have a smaller slot on the bottom of the carrier which is used to trip the auto sear. For the most part either will work. Some carriers such as Fail Zero and Nickel Boron are coated to increase reliability and decrease the need for lubrication. These are usually at the higher end of the price spectrum.
Charging Handle - The charging handle is a simple T shaped piece of aluminum used to pull the carrier back to charge the rifle. There are different styles and latches available but generally all are the same. These run typically between $15-60.
Barrel - Barrels are important, this decides the guns accuracy and capability. Barrels generally range from 7.5"-24" on AR-15's. And twist rates from 1:7-1:12 are common. The most common length is 16" since it is the minimum legal length. A true M4 carbine has a 14.5" length for reference. Target rifles usually start at 20" and go up to 24". Twist rate is an important factor because it will determine what type of ammo you can fire. The standard weight bullet for an AR-15 is 55gr, which the military designates M193. Another popular ammo is 62gr SS-109 penetrator, which the military designates M855 which has a steel rod inside the projectile to pierce sheet metal and windshields. Other heavier grains are primarily used for target shooting, up to 77gr. A barrel twist rate of 1:9" is the most common. This means in 9" of barrel the rifling twists 1 full revolution. So a 1:7" barrel is 1 full revolution of twist in 7" which means it's a tighter twist. The faster or tighter the twist, the heavier the bullet can be stabilized. Shooting too heavy of a bullet can cause loss of accuracy. A 1:9" barrel will stabilize 55-62gr no problem. A 1:7" will stabilize 55gr-77gr. The disadvantage is the quicker the twist, the quicker the barrel will wear. Most barrels will last approx 15-25k rounds before keyholing. Barrels also can come in standard steel, chromoly, chrome lined, or stainless. Steel and Chromoly are generally your standard barrels and are the most affordable. Chrome lined barrels (as you might assume) have a chrome bore and are more resistant to corrosion and slightly easier to clean. Stainless barrels are resistant to rust and corrosion and are typically found on higher end rifles because of the cost. Barrel length does have an effect on the power of the rifle. For instance, a 7.5" barrel will send a 55gr bullet down range at around 2500fps, and a 20" barrel will send that same bullet out at 3200fps - or about a 22% increase in power. AR-15's typically perform best in the 16-20" lengths, with 10.5" barrels being the break point for power. Barrels will typically run from $130-400.
How to build an AR-15 rifle
1. Required parts.
Lower Receiver - the lower receiver is the bottom part of an AR15. This is what has the pistol grip, and the magazine well. Most importantly this is the registered/serialized portion of the AR-15 which constitutes the firearm itself and will require an FFL to transfer it to you and complete the 4473 paperwork. Generally all lower receivers are cast aluminum, and most all are cast from an A2 lower design. All lowers by and large are created equal. A more expensive lower will not make your gun more accurate, or more reliable. It is very uncommon to run into a lower receiver which was built or machined "wrong." There are lowers which are billet machined, and not cast - they cost considerably more and for the most part are just for aesthetics. Find a lower which has a design you like, and go with it. Lowers generally run from $80-150.
Parts kit - a parts kit is all the junk that fits inside your stripper lower received to make it work. Basically this includes the pins, springs, hammer, trigger, pistol grip, and mag match, among other things. Not all parts kits are created equal, although all will work. Some more expensive kits such as Colt will have fully hardened parts or better machined faces which could make for a smoother trigger. Some hammer springs are stiffer, etc. If this is your first AR-15 I would recommend the cheapest you can find, if you want a better trigger you can purchase a premium model at a later time. Generally parts kits will run around $70-120.
Butt Stock - there are two major stock types for an AR-15. There is the adjustable/collapsible M4 style, and the A1/A2 style which is fixed. For long barrel hunting rifles a fixed stock such as an A1/A2 or Magpul PRS stock is best suited. For carbines an adjustable stock is probably the best choice.
Buffer Tube - There are two types of buffer tubes. Milspec and Commercial. Milspec tubes have a smaller diameter (very slightly) and are shorter. Commercial tubes have a slighter larger diameter, and some will have a slight cant to the rear. There is no quality difference between the two tubes.
Buffer/Buffer Spring - There are two basic types of buffers - rifle and carbine. Of those there are slightly different weights for fine tuning. For most any standard build a rifle or carbine will be plenty. Carbines buffers/springs are meant to operate in collapsible stock tubes, and rifle buffers operate in full length A1/A2 fixed stocks. The way to identify the two is carbine buffers have a slight polymer taper to the front and have a cylindrical profile all the way down. Rifle buffers have a machined bevel in the side of the buffer, semi-conical. Rifle buffers are also slightly longer.
Upper Receiver - the upper receiver of an AR-15 is what holds your bolt carrier, barrel, hand guard, and optics/sights. Generally people purchase an upper "complete" meaning the barrel and hand guard is installed already, as it requires special tools. There are two different kinds of upper receivers for the most part - standard, and sportical. Standard upper receivers will have a forward assist on the right side of the upper, which allows the shooter to force the bolt into battery in the event it has become dirty and jammed. A sportical style does not have a forward assist and you would be unable to force the rifle into battery. Sportical uppers are typically used in target style rifles which do not shoot a large volume of ammunition.
Bolt Carrier Group - The bolt carrier group on an AR-15 consists of the bolt head, firing pin, cam pin, retainer pin, and carrier. These are sold as complete units and generally range in price from $100-250 depending on features. Some bolt carriers carrier higher testing certifications such as MPI. Generally any bolt carrier will be fine for a semi-auto rifle. Most of the higher certifications are more pertinent to full-auto rifles. Bolt carriers are separated into two major groups - semi auto and full auto carriers. Semi auto carriers have a larger slot on the bottom of the carrier. Full auto carriers have a smaller slot on the bottom of the carrier which is used to trip the auto sear. For the most part either will work. Some carriers such as Fail Zero and Nickel Boron are coated to increase reliability and decrease the need for lubrication. These are usually at the higher end of the price spectrum.
Charging Handle - The charging handle is a simple T shaped piece of aluminum used to pull the carrier back to charge the rifle. There are different styles and latches available but generally all are the same. These run typically between $15-60.
Barrel - Barrels are important, this decides the guns accuracy and capability. Barrels generally range from 7.5"-24" on AR-15's. And twist rates from 1:7-1:12 are common. The most common length is 16" since it is the minimum legal length. A true M4 carbine has a 14.5" length for reference. Target rifles usually start at 20" and go up to 24". Twist rate is an important factor because it will determine what type of ammo you can fire. The standard weight bullet for an AR-15 is 55gr, which the military designates M193. Another popular ammo is 62gr SS-109 penetrator, which the military designates M855 which has a steel rod inside the projectile to pierce sheet metal and windshields. Other heavier grains are primarily used for target shooting, up to 77gr. A barrel twist rate of 1:9" is the most common. This means in 9" of barrel the rifling twists 1 full revolution. So a 1:7" barrel is 1 full revolution of twist in 7" which means it's a tighter twist. The faster or tighter the twist, the heavier the bullet can be stabilized. Shooting too heavy of a bullet can cause loss of accuracy. A 1:9" barrel will stabilize 55-62gr no problem. A 1:7" will stabilize 55gr-77gr. The disadvantage is the quicker the twist, the quicker the barrel will wear. Most barrels will last approx 15-25k rounds before keyholing. Barrels also can come in standard steel, chromoly, chrome lined, or stainless. Steel and Chromoly are generally your standard barrels and are the most affordable. Chrome lined barrels (as you might assume) have a chrome bore and are more resistant to corrosion and slightly easier to clean. Stainless barrels are resistant to rust and corrosion and are typically found on higher end rifles because of the cost. Barrel length does have an effect on the power of the rifle. For instance, a 7.5" barrel will send a 55gr bullet down range at around 2500fps, and a 20" barrel will send that same bullet out at 3200fps - or about a 22% increase in power. AR-15's typically perform best in the 16-20" lengths, with 10.5" barrels being the break point for power. Barrels will typically run from $130-400.
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