From what I've seen, killtimes are:
LR Scoped, Carbine, BR/DMR, LR Unscoped
However, that assumes perfect accuracy, something I find much easier with the BR/DMR in most engagements.
At very long ranges, the LR seems flatly superior, but that's only true on a few maps, and it's considerably worse up close.
The Carbine also works well only when there is a clear LoS to your target for most of your engagements - it is great in those situations though, it's sort of the 'automatic' weapon of the precision guns, allowing you to spam the hell out of shots and down targets quickly even if your aim is off for a shot or two.
I'm still pretty fuzzy on the distinction between the DMR and BR. In theory the BR has some spread that makes for easier hits closer and more misses longer, but frankly I haven't noticed misses at anything but extreme ranges, and I can't tell the difference between the BR and the DMR at close medium to close range.
The Carbine works especially well as a cleanup weapon if you're playing with friends, as it is both accurate and has the highest rate of fire of the semi-autos, letting you take down a damaged target very quickly.
Conversely, the Carbine is much worse on maps with lots of cover, as you can't get the number of shots needed on target to score kills. The LR is actually better in those situations at range, as it can strip shields in three shots while scoped, making each of those jumping long range between cover shots count more, but it pays for that strength up close.
Long story short: The LR and the Carbine are worth using for certain mode/map combinations, but otherwise, it still feels like the DMR/BR will be the most popular and most usable in the widest variety of engagements.
I'm still experimenting with the weapons, and efficiency aside, I find that I enjoy using the Carbine and find the LR frustrating. I'm also tired of hearing endless DMR/BR shots because it feels like thats 90% of the killcams I see :V