1. Cast Aluminum AR Lower Receiver

One of the processes for manufacturing an AR-15 lower receiver is to ‘cast’ the lower. This means the lower receiver is formed using a mold. Molten aluminum is poured into the mold to form the overall shape of the lower. Once the aluminum solidifies the aluminum ‘casting’ is removed from the mold. The ‘cast’ lower receiver is then finished on a CNC machine.

Cast lower receivers offer buyers some of the most economical lowers of the three manufacturing processes. Furthermore, by casting a lower receiver from A380 aluminum the lower is given the best synthesis of mechanical properties offered by a casting process.

Cast receivers are also the blunt of unfair criticism. Due to a handful of manufacturers who became known for poor casting and inattention to detail, the cast lower has received a bad rap. Although, if properly cast and heat treated, a cast lower receiver can be suitable for most AR-15 applications.

Below is a stripped AR 15 lower, that was poured into a cast.
 

2. Billet Aluminum AR Lower Receiver

A billet lower receiver is formed from a solid block of aluminum often called ‘bar stock.’ This ‘bar stock’ is formed from extruded aluminum. “Extruded” simply means the aluminum was formed into a shape by rolling between two rollers. (A mental image might be to think of it as a piece of dough, which was extruded or ‘rolled’ and cut to various shapes to form different types of pasta.) From a piece of this extrusion or ‘bar stock’ a CNC machine will cut the billet into the shape of an AR-15 lower receiver.

Due to the CNC’s ability to machine billet aluminum into designated shapes, billet lower receivers are considered the most aesthetic. While one-piece trigger guards, fine lines and geometric designs do little for overall performance, some AR builders consider these to be bonus features when considering the overall look of their AR build.

Below is a Tactical Skeleton AR 15 lower made from machine billet aluminum.
 

** all credits go to original authors who are referenced below**

References:

• Ammoland. “AR Rifle Lower Receivers: Cast vs Billet vs Forged Aluminum.” AmmoLand.com, Ammoland Inc, 10 Jan. 2017, www.ammoland.com/2014/08/ar-rifle-lower-receivers-cast-vs-billet-vs-forged-aluminum/#axzz6KN5tbOTd.