So maybe you have an AR15 already, or a few, or maybe you have decided that the time to acquire one is now!
Undoubtedly you have heard of the possibility of building one from scratch. There is a lot of information that is great to know before you begin this task.Building one from components is a very rewarding thing that you will always be proud of.
One of the first things you need to decide is what are you going to be doing with your rifle. Clearing crack houses/Home Defense? Plinking cans in the yard? Target shooting/competition? Varmint control/Long range precision or a mix of some of the above. Different roles call for different components. This role is sometimes called the philosophy of use (POU).
For Home Defense or Close Quarters Engagements
You want something very compact and light weight with most likely iron sights, a red dot type sight or a low power optic (2-4x) for rapid target acquisition. A 16" barrel is the shortest legal barrel you can have without asking the ATF for a SBR (short barreled rifle) permit. Many companies offer a 14.5" barrel with the required 1.5" muzzle device pinned and welded on, per the requirement to attain the min. 16". This would be the best choice for this role.
A compensating (controls barrel rise) flash hider would be the best choice, such as the Mil-spec A2 flash hider. This will help you stay on target during rapid fire and keep you from going blind in low light situations. Pencil or M4 barrel profile would be ideal to save weight. Keep in mind though that lighter barrels heat up quick.
Quad rail or modular handguards are needed to attach lasers, fore grips and flashlights. These will be referred to as drop-in handguards and use the delta ring to hold them under tension and in place. This is usually the cheapest type of handguard. To use a drop in handguard you WILL need an A2 style front sight. This is the classic M16 looking sight. This category is usually called tactical rifles.
For Target Shooting or Competition
You'll often need more powerful optics for longer range, as well as a few more specialized items. A free floated handguard improves accuracy by removing any contact points on the barrel. Often these are longer than the rails used for CQB to utilize the modern stances competitive shooters use (isosceles stance) and to place in rests for bench rest shooting. Free floated rails are typically smoother as the need for attachments is gone. A longer handguard will also allow the front iron sights (detachable/flip-up) to be placed further away from the rear sights resulting in greater accuracy. This length is referred to as Sight Radius. Longer is better. 16-20" M4 profile or mid-weight barrels are usually used for increased accuracy without gaining extra weight. Upgraded trigger groups are used to increase accuracy and improve rate of fire. Often these types of rifles will be referred to as race guns or 3-gun rifles.
For Varmint Control or Long Range Precision
You need the whole build to be focused on accuracy. Weight is not an issue for this class. Fixed stocks are often used instead of adjustable to take away any wobble or rattle that could affect accuracy. High powered optics are used (12-30x) for increased accuracy, with large objective lenses 40-50mm to aid in low light conditions and provide the best clarity. Upgraded triggers are a must, 3lb or less. Bolt and bolt carrier must be excellent to increase accuracy. Long, 20-24", heavy barrels are used to increase accuracy, often from top tier manufacturers. Free floated handguards are used to increase accuracy. Often muzzle brakes are used to reduce recoil since you will most likely be sitting at a bench or laying prone, both of which receive recoil more than standing or kneeling. To get the most out of this rifle, expensive match grade ammo is used, or hand-loaded ammo is tailored for this specific rifle. This class is called Varmint or sometimes SASR or SAPR (semi-auto sniper rifle or semi-auto precision rifle).
This will be the end of part 1