Anybody that would shoot steel or Hevi-Shot (other than Classic Doubles) through a damascus gun needs to have their head examined. Pay no attention to Bruce, he's not a duck hunter and knows pretty much nothing about what shells to use for them in old guns as is obvious by his above post.
The cream of the crop as far as non-toxic, double gun friendly, duck loads is either the NiceShot loads from RST or the lighter charge Kent Tungsten Matrix shells. I've shot a lot of the 1 1/4 ounce Kent stuff through 12 gauge Parkers and it's deadly though probably the lighter 1 1/8 ounce shells would be easier on the old guns.
I've used the RST 10 gauge NiceShot loads and they're absolutely terrific. I've not had a chance to try the 12 gauge ones but I'm sure they're equally good as that company doesn't put out a bad shell. The great thing about RST is that all their shells are made with old guns in mind so the pressures are low and the recoil light. I can't say that about the Kent ammo, but the shot won't damage old barrels for sure, even damascus if the loading is reasonable.
Your other choices are the Hevi-Shot Classic Doubles (not the regular Hevi-Shot) and the new bismuth shells from Pinnacle. I've never used either but have heard they're both decent. I've used the old brand bismuth quite a bit and it was fine so I'd say the newer stuff is fine as well.
The one drawback to all this stuff is price, for a standard 12 gauge shell with a 1 1/8 ounce charge you're looking at $2.75 to $3.50 per cartridge depending on which you go with. It's the price you pay for wanting to use an old double for waterfowling. But, I can assure you, the satisfaction you get with taking birds using your Grandfathers gun will far surpass any you'll get from shooting them with a Benelli or other modern fowling piece.
Here's a link to RST:
http://www.rstshells.com/
If you're buying anything short of a case you'll have to contact one of their dealers but that information is on the website as well.
The rest of the stuff can be ordered through the various online sources and some can probably be bought off the shelf at places like Cabela's and Bass Pro.
Good luck and good shooting, I've been killing about 90% of my ducks and geese with Parkers for the past 10 years and I wouldn't change a thing even if the ammo is high dollar.
Regards,
Destry