#6 or #5 EcoTungsten (NiceShot)...latest direct lead non-toxic replacement.
Expensive, but the good news is that all patterning can be done with equivalent real lead shot, then just a couple shots with EcoTungsten to be certain and you're ready for the ducks.
http://www.ecotungsten.com/shots.html
For the possibility of getting a chance at some geese on a local farm, I needed some non-toxic shot for my .62cal smoothbore...tried steel but wasn't satisfied. So I bit the bullet and ordered some EcoTungsten/Nice-Shot...the latest non-toxic and one that is advertised to be a direct replacement for lead in terms of same loading recipes, same performance, but no special shot cups, won't harm your oldest, finest shotgun, etc, etc, etc.
I tested EcoTungsten #6's against my proven turkey load of lead magnum #6's as a very critical way of testing...looking for a very tight pattern to get killing head shots out to 35-40 yards, and found EcoTungsten to perform exactly as advertised.
Because my EcoTungsten shot charges cost about $3 each, I first check tested my smoothbore with a few 'lead' turkey loads, then several shots using 'nickel plated #6's as possible improved turkey loads for the future, then finally, I tested 5 shots of EcoTungsten.
The turkey target below shows the average results at 35 yards through a 'Full' Jug Choked barrel, 80 pellets on an 8.5"x11" target paper, no shot cup. (lead=54 pellets, nickel plated=58 pellets, EcoTungsten=80 pellets)
Should be excellent for geese head shots in this full choke barrel if I'm limited to that due to 3 fairly close houses, or should also be fine out of the Imp.Cyl.+ barrel for wing shooting if I find there's clearance and angles to do that...just need to figure out the schedule and flight path the geese take in and out of that farm pond.