This might help:
In Real Steel:
A DMR is a large caliber rifle (7.62 or larger) that bridges the gap between assault rifles (such as M16) and sniper rifles. They shoot in semi-auto only, which means they can have precise long-range shots yet a higher ROF than a bolt-action sniper rifle.
Since all airsoft guns use the same "caliber," theoretically any gun can be a DMR. But most airsofters with DMRs use large guns such as M14, SR25, M16, etc. Generally, a DMR will have a long (500mm+) tight (6.03-6.01) barrel and shoot above 450fps on semi-auto only.
A bipod and magnifying scope would help.
I hope this is right