I used my G3SG1 in CQB with great effect, and our Canadian Forces use the C7A2 in CQB. It doesn't matter how long your rifle is, it's how you handle it that counts
As far as LMG's being DMR's, you could probably get a G&P M249 to shoot as well as a fully upgraded M16 type DMR at 400fps, but not at 470fps.
At 470fps, you don't have full auto, but the heavier ammo offers much greater accuracy over distance. Only issue is ammo gets really expensive above .30g....
I prefer just using my M4 to pick off snipers, it's got the range and accuracy, and it's only 380fps so I can keep my full auto