I always thought it was loosely based off the SIG MCX Series which has only really been released recently but we have to remember Bohemia Interactive also has the division Bohemia Interactive Simulations which sells military grade simulators so of course they get to go to all the fun Military Expos around the world and can see some products before hand. The MX Series in the game is based on a Rifle that can do multiple roles without producing separate weapons. The SIG MCX also does such a job or is going to do such a job. This is my conclusion on why Arma 3 used the MX series. (Not just because MCX and MX are VERY closely named the same.)
