Basically the only way to properly master the DMR, the Magnum and the Battle Rifle is to play SWAT.
The Designated Marksman Rifle-Good for Medium to long range encounters due to it's accuracy but a decent rate of fire.
The Battle Rifle-Good for close tomedium range combat. High rate of fire and is still accurate at long distances...
The Magnum-Good for short to medium. The range depends of how good you are with it. High rate of fire and a decently sized magazine.