No commercially produced AR15 type rifle is "mil spec."
Configuration is a part of the military specification, but since many of us don't want/can't have an M16 coming off the assembly line, this is of little interest to most. (Collectors & machine gunners, forgive me!)
More importantly, none have gone through the government mandated test and inspection, let alone compliance and certifications. These procedures & requirements, hated by all manufacturers, are the best cost effective attempt at ensuring a reasonable level of quality in parts and finish, as well as assembly, from rifle to rifle. Remember, the lowest bidder gets the contact, normally. Rest assured, NONE of the commercial guns meet these kinds of requirements.
Many parts in AR15 type rifles simply could not be mil spec, since they are different parts, and not even used by the military. And not inconsequencial parts either: bolt carrier, hammer, trigger, disconnector, safety selector, etc. In fact, these are some of the most critical parts in the rifle. Even the steel called for in the bolt mil drawings (Carpenter 151, I think?) is virtually never used in commercial bolts.
And then there's Colt, who uses, from time to time, non standard hammer/trigger pin holes and take down pin holes, as well as includes a "block" in the lower for reasons bizzarre.
Manufacturers/dealers/etc. insist that their rifles/parts are "mil spec". What does this mean?
In the best case, it means that all the parts in the rifle's assembly are either actual G.I. parts (modified as required by BATF for semi-auto where necessary), or "virtually" the same, and, that they were assembled in a manner consistant with those levels of quality equal to, or better, than that found in "contract" rifles. By parts that are "virtually" the same, I mean made by the same contractor, the same way, as the mil part.
In the worst case, it's pure hype.
Normally, it's in between these two.
In virtually every case, a part made by a G.I. contractor is vastly superior to a part made by a non-contractor. Many part suppliers/rifle builders/manufacturers actively seek out "cheaper" parts supliers in lieu of the current, higher priced contractors.
We see lots of crapola parts in rifles sent to the shop for repair. Lots. However, current Armalite and Bushmaster rifles are typically just fine, though in serious need of lubrication--but that's another story!
Hope this helps!
A clarification on anodizing by Brad Fuller:
There are two types of anodizing, Chromic Anodize (MIL-A-8625E, Type 1 & 1B, Class 1 & 2) and Hard Anodize (MIL-A-8625E, Type III, Class 1 & 2). Chromic anodize is not the same as the Hard Coat anodize. The key as to what finish that is applied to aluminum AR15/M16 type receivers is the TYPE (I, IB, or III). A manufacturer may state that it meets MIL-A-8625E, but if no type is specified you don't know what you're getting. It must be type III for Hard Anodize to meet the spec for the M16. Types I & IB, class 2 can be dyed to the black or dark grey; the dye can rub off.
Type III, class 2 can be dyed and is specified on the contract drawings. Hard anodizing color will vary from light tan to black depending on alloy (this should be 7075 T-6 for AR15/M16 uppers and lowers) and can be dyed in darker colors depending on the thickness of the anodizing. Hard anodize coating penetrates the base metal as much as it builds up on the surface. This provides a VERY hard coating. If a thickness is unspecified on the contract drawings then the thickness shall be nominally 0.002" thick. The term THICKNESS includes both the buildup and the penetration. The upper receiver should then be coated on the interior with Solid Film Lubricant (MIL-L-8937D). I know Colt and Bushmaster apply this film and are HARD Anodized. Parts that have a purplish tone to them probably are not Hard Anodized. Older SGW parts were bead blasted and then chromic anodized.
Phosphate coatings are applied to medium and low alloy Steels. There are two types, M & Z, with class 1 thru 4, where M=Manganese and Z=Zinc. These are grey to black in color. Type M is more resistant than type Z to alkaline environments. Type M is used on AR15/M16 steel parts. The thickness of Phosphate coatings ranges between 0.00002"-0.00004".
For comparative purposes on the thicknesses, a human hair averages 0.015 thick.
Hope this provides everyone with some good information.