I dont know about hand fitting parts and processes followed during manufacturing, but I can tell you that there are some differences in the requirements. For example the M9 requires a steel guide rod vs the plastic guide rod of the 92fs. As per the M16 rifle they require higher tolerances on the barrel for pressure as the military buys ammo from the cheapest supplier. This is the reason you don't want to shoot 5.56rounds out of a .223 AR15. The military requires parts to be able to sustain greater barrel pressures. Beyond that I have never look too deeply into the subject. Hope this helps.

 

....As per the M16 rifle they require higher tolerances on the barrel for pressure as the military buys ammo from the cheapest supplier. This is the reason you don't want to shoot 5.56rounds out of a .223 AR15. The military requires parts to be able to sustain greater barrel pressures.....

Sir you do not have a clue what you are talking about.

 

When you hear the term "mil-spec" it can refer to alot.
As to your question, it is a list of tollerances and specifications for coatings.
As an example all FCG pin holes, sizes, locations have a range of tollerances. That range is the mil-spec.

The type and manor in which the corrosion resistant coatings that are put on a weapon, is defined in the Mil-spec.

 

For each variety of military rifle, there is a "pattern rifle", which in some cases may be an actual rifle or in others might be just a detailed description and set of blueprints. The pattern establishes the dimensions of each part, the manufacturing tolerances allowed, the materials to be used, the finish, etc. A rifle which matches the pattern in all respects is said to be "Mil-spec" or built in accordance with military specifications.

 

MILSPEC refers to the standard to which the military requires for the manufacture of anything. It covers toilet paper to rocket engines. It outlines for the manufacturer what is the acceptable rage of tolerance in all aspects of the production of the product. There is a group of eggheads that draw out the long documents to provide to the companies that make the goods the military uses. I needed an electronic part once to repair a multimillion dollar piece of equipment. It was a simple capacitor that was readily available from Radio Shack. But since it was not "MILSPEC'ED" I could not purchase the .25 cap. I had to order it through supply which had a 2-3 week wait and cost over $5. Why does it cost so much, well because the manufacturers have to comply with the milspec. It does not mean they are better, in fact the same part I had gone to Radio Shack to purchase from the same MFG was the part I received when the cap finally came in. I hope this clears things up a bit. If it did please explain it to me again. I tried to understand why anything had to be milspec'ed for years.

 

If you really wanted to find out what the milspec's are for the M16, ar15 is the civilian version, you might be able to go to the GAO and ask for the document. But you probably would have to ask for the translation because it refers to other documents that spell out what each code means. It takes a special division within a company to decipher the codes. Then another dept to ensure compliance. That is why parts cost so much. It adds 2-3 or more steps in the mfg process.