#1 thing you need to know about building a DMR: barrel length means nothing.
#2 run flat or Rhop and use the heaviest ammo possible. BB weight + hopup equals range and accuracy
The DMR thing is just a look. Long range and excellent accuracy can be achieved on a 10.5" MK18
Optics are all preference. I use a 4x spectreDR for both CQB and field games, it works for me.