No AR company makes their own raw forgings.  A foundry to make these forgings costs many millions of dollars, and all of the AR companies put together don't do enough volume to keep such a plant busy for 1 week a year.

Thus, when a company needs to make more ARs, they buy a bunch of raw forgings (solid chunks of aluminum in the general shape of an AR part) from one of the forging houses.  These companies have forging dies for these parts on hand, so they can make more when needed.  The marks on the uppers are the forge die markings from these aluminum companies.

[img]www.concealcarry.org/ar15/pix_files/forging_004.gif[/img]
Raw "0%" Lower Forging

The AR company then machines the forging (or sends it out to be machined per their specs) into final form, then applies some type of surface finish.  Mil-Spec finish is a hard-coat anodizing, with black dye added during this process.  Some companies use other finishes.

Anyway, the source of the raw forging (meaning: the forge die markings) really mean almost nothing, except for early Colts.  For the last 15 years or so, Colt does what everyone else* does, which is buy raw forgings from whomever has them available when they need them.

What separates one receiver from another is primarily the machining work (some vendors skip less noticable machining operations to save money), and secondarily, the finish.  Often, parts that may not look the prettiest are actually better than nicer looking parts, because the *machining* is done better.  Forgings have thick forge seams, and these seams can often look rough and pitted.  This may look unsightly, but functionally, means nothing.


*Note: Armalite is an exception.  They supply their own forging dies to the aluminum companies, and therefore are the only ones that have access to forgings from those dies.  They do this because they wanted to ensure the highest quality surface finish on their receivers, and accomplish this by using only their ultra-fine machined and polished dies.


-Troy

BiggerHammer is wrong about this, unfortunately, and their list is also way out of date.  I was among about 15 people, most from the AR15-L, who worked to gather this information back in '98 or so, but a lot more information has been made available since then.

About Colt & Diemaco: They have agreements between them as part of Diemaco's license from Colt to produce these guns.  One of those agreements is for Colt to be able to purchase items from Diemaco during spikes in demand, such as happened after 9/11, when the military stepped up their orders, along with all the police departments.

Still, we've seen the same forge markings (Alcoa, Anchor Harvey, and others) on parts from Colt, Bushmaster, RRA, Olympic, DPMS, and others.  And we know for a fact that more than one subcontractor was doing machining for these parts.

These days, no company makes everything in-house.  That isn't efficeint, and business has been SCREAMING "efficiency!" for the last 20 years.  So, work is out-sourced to companies that specialize in machining, finishing, and so on.  This lowers prices and increases availability overall (though you may still have a wait occasionally, due to a company's need to have minimum inventory).

-Troy