http://www.esrb.org/ratings/synopsis.jsp?Certificate=26728

Halo 3: ODST

Platform: Xbox 360

Rating: Mature

Content descriptors: Blood, Language, Violence

Rating summary: "This is a first-person shooter in which players engage in futuristic battles against invading aliens. Players use pistols, sniper rifles, machine guns, and rocket launchers to shoot and kill enemies during the constant and fast-paced battle. Firefights are highlighted by camera effects (e.g., blurring and screen shakes) and realistic sound effects, including screams of pain, gunfire, and explosions. Puffs of blood are emitted from injured aliens/humans, and blood is sometimes smeared on walls and on the ground. Some profanity (e.g., "bullsh*t," "a*shole," and "bastard") can be heard in the dialogue and radio chatter. "

Is this the first M rated Halo? I remember at least 2 being 16+

No, this is not the first M rated Halo. All preceding Halo games were rated M by the ESRB.

Oh, I guess M = 16 in this country then

Yes and M = 17+ in the US.

in joystiq interview one of developers(i think) said it will be kind of sandbox game is it true or what?

If anybody is interested. Halo 3 is the only game of the series which got an "18" label from the USK here in Germany.

The series has always been a soft M. Its borderline T (and should be since Uncharted got a T). So this is pretty much staying within series expectations.

Halo has got to be the least violent M rated game ever.