fictitious war...pick your arms

[stanc]

We're in the 2nd decade of the 21st Century. Why would we be limiting ourselves to the AR platform?

I can't speak for the OP, but one possibility comes to mind: AR variants are in service with a number of the world's armed forces. The MDR is not.

There is also the question of whether or not the MDR has the requisite characteristics (reliability, durability, etc) to be acceptable for military use.

Of course, the OP isn't clear if he means pick a gun and ammo for use by an army, or a private individual.