I hunted birds and coyotes in NE MT and there wasn't much public land, some WMA's mostly for ducks. Eastern MT covers a lot of country. IN central MT we had lots of public land and a lot of private land open through the states hunter access program. Be sure and get the hunter access booklet from the state, it has maps of the ranches and all the restriction for the land.
The East Central and Southeast part of the state might have more public lands, I've camped near Glendive a couple times and it is pretty country also, NE part of the state is pretty plain.
Have a good time.