Anytime I hear the words “milspec AR-10” my teeth get set on edge. There’s no such thing. I’m gonna repeat that.
There is no such thing as a “milspec AR-10”.
Got it? It should be pretty clear. No. Such. Thing.
Let’s start with what exactly is “milspec.” You may have guessed it’s short for “military specification,” and you’d be correct. For the US Army (and some others), they like to take ownership of certain products. Sometimes there are competitive bids. For the M4, there’s a specification of what exactly an M4 is, and then the US Army can have companies bid on how much it will cost to make a certain number of rifles to that spec. The milspec is owned by the military (in general).
So there’s no “milspec AR-10” because no major military adopted the AR-10 and cemented the design that way. Emphasis on the and. Merely adopting the rifle is insufficient. The AR-10 was used by the Sudanese, Cubans, and the Portugese, but none of those made a spec. All of those arms were built by Artillerie Inrichtigen, which wasn’t able to get more contracts and stopped making AR-10s ages ago. Still no spec.
Things got confusing in the 1990s. By that time, Eugene Stoner was working for Knights Armament, and they made the SR-25, a modern take on the AR-10 design for use as a semiautomatic marksman’s rifle. This used the gas system improvements worked out on the AR-15 and had a number of parts shared with the then-issue M16A2. The magazine design is also used in DPMS’ AR-10-pattern rifles, among others, and you can find these magazines as “DPMS/SR-25” pattern.
There’s another pattern of magazines currently in use for AR-10-pattern rifles. This one is made, confusingly enough, by Armalite. The original Armalite company ceased operations in the early 1980s. The rights to the name were bought, and Armalite was relaunched in 1996, with a new AR-10 pattern rifle (among other things). At that time the 1994 Assault Weapons Ban was in effect in the United States. This bill prohibited the sale of new magazines with a capacity of more than 10 rounds, but older magazines that held more than 10 rounds could still be bought and sold. So (new) Armalite made magazines that had a common design with M-14 magazines, so buyers of their rifle could use existing, common M-14 magazines with a couple easy modifications. These are “Armalite” pattern magazines, and they’re not compatible with DPMS/SR-25 pattern magazines.
And no, neither DPMS nor Knights nor Armalite are responsible for any current milspec “AR-10-pattern” rifle.