People throw the term around all the time, especially with firearms. "I'm looking for a mil spec upper receiver." or "I run a milspec bolt carrier group on my AR."
Its time to stop.
Let's start with a definition for mil-spec.
Mil-Spec describes the essential technical requirements for purchased materiel that is military unique or substantially modified commercial items. MIL-STD-961 (reference (n)) covers the content and format for defense specifications.Defense Standardization Program (DSP) DoD 4120.24-M
There you have it. Mil-Spec is the technical requirement for a part made for the military, its Military Specification. MIL-STD-961 outlines how those parts should be labeled, measured, and identified.
What the term Mil-Spec is typically used for, however, is clever marketing. Here is an ad for a Spike's Tactical lower: "CNC Machined from a 7075 T6 MIL-H-6088 Forging. MIL-A-8625F Type III Class 2 Hardcoat Anodized Finish inside and out."
Lets put that in context. A Mil-Spec M16 lower requires a 7075 T6 MIL-H-6088 aluminum forging. Only there's a few problems with that. First, consider quality. A 7075 T6 billet aluminium receiver is more durable than a forged aluminum lower receiver. Second, consider context. The U.S. Army fields M16/M4 rifles. Not AR15 rifles. These Spike's Tactical lowers don't meet the Mil-Spec standard because they do not have the hole drilled in them for the auto sear.
Those are the first two major problems with the Mil-Spec buzzword. There are better quality items available, and sometimes the item being touted as Mil-Spec doesn't even meet the specification.
The third problem with using Mil-Spec, is that it gets thrown around in places it has no business.
Take Brownell's for example. They're selling a Texas Weapons System Dogleg rail system. Which is a pretty nice rail, the problem is, its not Mil-Spec because the U.S. Military does not field AK47 rifles, and does not carry this part in its inventory.
Some folks will make the argument that for a foreign rifle, mil-spec can be used to refer to meeting that military's standards. Not true, by definition Mil-Spec refers to parts used in the U.S. military. Further, using this example, foreign militaries aren't using a rail system made in Texas. Even if they were using the part in question, think back to quality. Do you really want the bare minimum quality part? Do you want something that's just good enough for the Iraqi Army? Or do you want a commercial part built with the best technology and materials available?
Thats it. Stop saying Mil-Spec. Instead, find yourself better quality parts and better suited parts.