Hello Mark,
The gun you described would be an ideal all around gun. I would go with 2 3/4" shells for upland with lead 5 shot being a good all around for pheasant. I would suggest either 2's, 1's or BB's for ducks. I have a Browning BPS 3 1/2" 12 gage that I use for ducks and geese. I use a 3" shell although the 2 3/4" shells are loaded the same with steel. I like #2 steel for early season ducks such as teal. I like #1 and BB's for bigger ducks like Mallards. Get the fastest steel load you can get. 1 1/8 oz. loads at 1500 fps. or faster will work fine. Later in the season I go to 1 1/4 oz loads. I like Federal HHV loads. For geese, I would say go with a 3" shell loaded with some Federal HHV BBB in either 1 1/4 or 1 3/8 oz. loads. You will pay in the neighbor hood of 10 to 15 dollars per box for these. I buy my shells in the off season ( right now). They are usually on sale at some point. The local large outdoor chain here has the shells I just described for $9.99 per box of 25. I like my 3 1/2" 12 with 1 1/2 oz of BBB for large Canada geese. I shoot a more open choke than most also. I use improved cylinder for all of my shooting except late season, when I switch to modified. Remember.... with steel, the choke needs to be opened up by one. Meaning, If you would normally shoot full choke with lead shot, you need to put in a modified for steel. Steel shoots tighter than lead. Most chokes are marked for both lead and steel now. Mine say, Improved lead, modified steel. Modified lead, full steel. And Full, lead only. I have a Browning Citori Lightening that I shoot pheasants with and sometimes I use it for ducks as well. I have the bottom barrel improved and the top barrel modified. I keep them that way all year long. In fact, the only time I ever put in a full choke, is when I go Turkey hunting. I hope this helps you out. Go ahead and buy that Mossberg, they are a good gun and will serve you very well.