I grew up in roundup...visited last weekend. The musselshell is hard to hunt consistently...
there are some ponds North of the highway that hold some birds - but as you are finding - not a lot. There are some public ground areas towards Forsyth. I believe that the FWP has some listings for them - some are walk in, but most you have to call to reserve - the good ones are right on or very near the Yellowstone.
You can find land ownership maps on the web at http://nris.mt.gov/gis/ownmaps.asp
this will help for calling and getting permission to hunt...a lot of the better areas are being leased out and you can't get on them.
There is a small group of canadians by the sewage ponds on #4 just outside of town - i think that the city leases that land to the guy that lives just across the bridge on #4...not positive though, you could stop and check with them. Those birds would be gimmies.
I went out west of great falls last year and there are a bunch of birds in that area..but it is a ways to travel.