I haven’t really played much online halo 4 yet, but I should have it set up for good in the next few days. From the few times I’ve played online, my aim seems to be off with the dmr…In every halo, I’ve played at a higher sensitivity setting, either 5 or 7. I’ve been decent at using every scoped weapon,consistently outgunning plenty of people with the same weapons both online and offline. In halo 4, the ONLY weapon that I seem to have trouble with using is the DMR; I have no problems with using the other scoped weapons. I had no problems with using the dmr in halo reach, either; in fact I liked it a lot more than the battle rifle. Has the hit box been shrunk? Is the auto-aim for the dmr significantly different from the previous game? Is it maybe the slight change in rate of fire or the smaller bloom that’s affecting me? Is it maybe that I got too used to using the light rifle? Or did I maybe pick up some bad aiming habits in halo 4 somehow by being lazy with my shots? (elites are WAY easier to kill in h4, so you don’t have to make sure that every shot hits…). Any suggestions on what it might be and how I can overcome the difficulty would be appreciated. 
I’m not really sure what your “problem” is, but I’m gonna guess it’s from using the light rile…I tried to use the light rifle after using the DMR quite often and couldn’t hit the broad side of a barn with it. Gotta be something about the speed of the projectile and how much you have to lead your target.
Well I definitely feel like my aim is way off, leading is probably a part of it. Even in campaign, I’ve had trouble shooting jackals in the head after knocking away their shields, with the dmr. I am almost tempted to turn down the sensitivity, but on the other hand I like the quick turn time. 
I’ve never had a problem with precision aiming before…grrrr maybe it’s the autoaim…
I’m no expert at the game, but w/ the DMR, I don’t really lead my shots other than general expectations (compared to the BR in H3). Maybe thats ur problem? Also, in close combat, i’ve seen many players speed up their shots with the DMR even if it becomes a 6 shot kill, and at distances, they slow down the shots but get a 5sk. Hope this helps.
And yeah, try practicing in customs w/ a sensitivity 1 or 2 higher than what you would normally use (although your’s already seems pretty high) and then bring it down in matchmaking. Sometimes it makes it feel easier.
I don’t know about you guys, but I feel like the DMR is the easiest precision weapon to use in this game.
I have no problem with dmr. Bloom is no issue on it for me since I’ve never really paid attention to it. I’ve noticed my main problem is I am for the head all the time. Its a habit I can’t break . Aimming at the chest\chin then finish with a headshot is alot better but that’s my issue. Also I play through my cell phone connection so alot of my bullets don’t register. Or else I would be using BR. That thing is useless to me on my connection.
The DMR is easy to use, it has a huge aim assist. Just spam Rt and everyone will die.
Grrrr, it’s very frustrating. Earlier today, I went and played a game in halo 4, finished last place, using mostly the dmr. Then right after that I played a match of slayer in halo reach, and got first place easily, using the dmr, again. I know people say that the dmr has a larger hitbox in halo 4, but I keep missing that final head shot and dying whenever it comes down to 1v1 combat. 
When i started playing, i couldnt kill anyone in 1v1 situations
I turned my sensitivity down to a 1 and my k/d improved from a .8 to a 1.9 and still rising.
I suggest setting your sensitivity to a low number and once you get used to it, increase it gradually
or, keep it at 1 like i did
I’m not really sure if I would approve of that for my sanity…
But it has extreme magnetism therefore it’s overpowered plus the huge hitboxes so it’s a really easy weapon to use…