I have thought about having a DMR version but a lot of sites here have a 30m MED, so its too far. If it were 15 then I think I would do it, because I will get to use my pistol more.
My advice to you is to stick with normal assault, but I would advise against a longer inner barrel. It will make it a lot harder to lower your FPS to a legal level, especially on heavier BBs. The NPAS works, but only to a point. After a while, it becomes unstable and the FPS drops through the floor.
To lower my FPS I have tried NPAS, Low Flow Disks, and G&P adjustable low flow valve. I'm still not happy that I am below the 1.13J I need to be, but I am getting there with tweaking.