My "DMR" is a 14.5" SR15 with a 4x scope lol
Doesn't matter what it's referred to as, the only thing that matters is that at 1.6j it's hitting people accurately at 250ft =p
As far as barrel length, 450 is the sweet spot for a full cylinder in terms of air volume. As for actual accuracy, the type and weight of BB you're using has a far greater effect on accuracy than the length of your barrel. My vector has a wee 5" barrel and it's almost as accurate as my SR15 at 250ft.
Likewise, my pistol has but a 3" barrel and it's deadly accurate at 160ft using .32s