1 ) The Lower Receiver
The stripped Lower Receiver IS the firearm. This is the part that has the serial number, everything else attached to the lower receiver are just parts. Lower receivers are available in a variety of types.
A "Forged" lower receiver refers to the process of forming a block of aluminum by pressing raw metal into its almost final shape. Much of the detail is outside detail is pressed onto the forgings, which are then machined to the specification with cut outs and holes etc… Forgings are the most common lower receivers, and can be considered the standard, on the market because they tend to be the strongest and nearly the cheapest to manufacture.
A "Cast" lower receiver is the process of pouring liquid metal into a shape. Though casting is possible there are not many manufacturers using this process to make lower receivers. After casting, the receivers are then machined. Cast receivers are considered to be weaker than the standard forged receiver. Cast lower receivers can typically be identified by the raised lettering and a rougher appearance within the mag well.
"Extruded" Lower receivers are not very common. Extrusion is the process of forcing heated aluminum through a shape . The Extruded lower receiver is then cut and machined to specification. It is then cut to length and final machined to final configuration. Extruded Lower Receivers tend to have less detail and shape than other receiver types.
"Billet" Lower Receivers are the 2nd most common fabrication method used for fabricating lower receivers today. A billet lower receiver starts as a block of aluminum which is then completely machined and shaped to the final shape and specification. This type of receiver is generally referred to as a "billet" receiver because the block of metal is what is known as a billet. Billet lower receivers tend to require the most investment because of the extensive machine time necessary in fabrication of the part. Billet Lower receivers typically are of the highest tolerance due to the total machining process but are not always completely "mil spec" because of some size variations.
A newer lower receiver to enter the market in recent years is the polymer lower receiver. Polymer lower receivers are the cheapest to make because plastic is cheaper than aluminum and the molding process is quick and fairly accurate allowing the manufacturer to form almost all of the detail into the raw part. However polymer lower receivers tend to be the weakest of all the lower receivers on the market. Some manufacturers are attempting to overcome the weakness of the material by utilizing metal inserts in the critical high-stress areas. Polymer is also more prone to size fluctuations due to temperature differences than aluminum. This may cause some "fit" issues.
Which is a better lower receiver? The general consensus is that a forged 7075 aluminum lower receiver is certainly the most common and perhaps the best on the market when made to meet or exceed military specification (mil spec). 7075 Aluminum is much tougher then 6061 aluminum. 6061 Aluminum is lighter, and often referred to as air craft grade aluminum.
Billet, though technically weaker than forging, are very popular because they tend to be the most pleasing in appearance and any weakness deficiencies can be designed out by adding more material where necessary. Cast is generally not preferred due to the rough appearance, and the weakness incurred during the manufacturing process. Polymer lowers are not yet time proven and are still an experiment as the bugs are worked out of the designs.
