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Evaluation of Ar-15 Lower Receivers

Building your own AR-15 rifle begins with the lower receiver. The lower receiver is the one piece of your rifle that is classified as a "firearm" by the Bureau of Alcohol, Tobacco and Firearms (ATF). You must purchase your lower receiver from a licensed firearms dealer. There are numerous manufacturers of lower receivers, which are built to military specifications.

Types
- There are three main types of AR-15 receivers: milled, polymer and forged. Milled receivers are machined from a solid piece of aluminum. These receivers are made to military specs and treated with a Teflon coating. Polymer receivers are constructed of tough polymer plastic and are manufactured by plastic injection (the injection of molten plastic into a preformed mold). Polymer receivers are available in multiple colors and are made to military specs. Forged, or cast, receivers are basic aluminum receivers. The process of forging consists of pouring melted aluminum into a cast, which later separates when the metal cools (releasing the receiver).
  
  Milled receivers are more expensive, but they are ready for use right out of the box.
Ease of Use
- Milled receivers are complete and come ready to use straight out of the box. Polymer receivers also come ready to use. Forged receivers require work to make them usable. They must then be milled to military specs and the aluminum coated.
Cost
- Because they require further work, forged lower receivers are the least expensive. Conversely, since milled receivers come ready to use straight out of the box, they are more expensive.

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