In airsoft, really anything that "greatly" surpasses the local demographic of guns in range and accuracy, is a DMR. To be more realistic, the title is only available to certain realsteel counterparts, like the M14, L86, SVD, etc.
The same rule applies for bolt action rifles, in that a stock rifle is NOT necessarily a DMR, since some of them shoot like crap.
I fully agree with everyone who's changed their energy limits to 400+ or 420+ being bolt action ONLY (for safety reasons). And it also gives slightly more advantage to running a DMR, as you're able to very effectively use .36-.43g ammo or heavier.
To clarify the roles, because 9/10 people have no fucking clue what a sniper is.
A SNIPER is an almost pure recon role. Your job is to use your incredible stealth ability to sit there and WATCH people. Occasionally you're tasked with taking out designated targets. Due to the fact it takes you an hour to take out a specific target and that target respawns in 5 minutes, the role is all but useless.
A DESIGNATED MARKSMAN is a member of a squad, either with a bolt action or semi-auto precision rifle, who acts as a marksmen when the need arises. EX: "Hey, I need you to shoot that guy in between those pallets!" or "Hey, I need you to shoot that dude in that window". The DM is otherwise an integral squad member and does everything a normal squad member does, he's just a better shot with a better gun.
Adding a bipod to your M16 does NOT make it a DMR or a sniper rifle.
Emptying a whole mag of semi auto fire into a window to shoot a guy does NOT make you a designated marksmen.