Does anyone have reliable info on the origins of the mid-length gas system? It truly seems like a very logical improvement on the carbine setup and I was just wondering who first came up with the idea of putting a 9-inch gas system in place of a 7-inch setup, and when. 
Armalite/Mark Westrom claim it was them.
That’s what I can remember. Their are many Mid-length builders and still no one standard or TDP that is available. Same with Piston uppers and kits.
I believe they did it in the mid to late 1990s. No doubt more info is on ArmaLite’s site in their library section…