Is there an interview or something with 343 on the reasoning behind including the DMR and BR?
343 wanted players to be able to choose starting weapons that suit their personal play style.
BR is a three round burst, good at short to mid range, not so accurate at long distance, 2x scope.
DMR is good at mid-long, not so good at short, very accurate but also has mild bloom, 3x scope.
Light Rifle is a three round burst while unscoped, single shot while scoped. Takes four shots to kill while scoped. Alternating between the firing modes takes skill, though, so it will have a higher learning curve.
And I have no idea how the Carbine handles.