Generally, you can use a 3:4 ratio in loading Shotgun loads to get better patterns. That is, for every 3 parts of powder, by weight, use 4 parts of steel shot by weight.
It really does depend a lot on the individual gun, whether you are using plastic shotcups, is the gun choked, if choked, what degree of choke, is the barrel designed to use with steel shot, etc.
We ARE ALL waiting to see and test some of the new Non-toxic shot that is being advertised as being suitably soft to use in old barrels made to shoot steel. So far, its been mostly talk, and the testing done has show that the new shot types are still much harder than lead. For that reason, you might expect to damage a barrel made to shoot lead shot by using any of the substitutes.
Most modern Steel Shot users understand that steel shot does not perform well much beyond 40 yards, and that most shooters need to keep their choice of shots at 35 yards and under to insure a kill. I have shot large Canada Geese with steel shot and seen the stuff bounce off the thick feathers. Only a hit in the head or neck, or breaking a wing bone brings them down. This with commercial Steel Shot ammo in modern guns, at ranges under 40 yards.
If I were using steel shot in a MLers, I would restrict my shots to 25 yards, and less. With that restriction, there really is no need to put lots of powder behind the steel shot load. The pellet energy of even steel pellets at those short ranges is quite adequate to bring game down. I was using #1, BB, and T shot when hunting Canada geese back in the day. I was unimpressed with the #1 shot, and now only use steel BB and T shot, when I can find it to shoot geese. #3, and #2 Steel shot should do for ducks, out to about 30 yds.
If you are shooting a 12 gauge, start with a 2 3/4 dram load of FFg powder, and an 1 1/4 oz. volume measure of steel shot. See how it patterns. Work up your powder charges by 5 grain increments to a 3 1/4 dram load of FFg powder. More powder than that just blows the patterns, in my personal experience. I also recommend using an OP wad between the powder and that plastic shotcup, so the base of the cup is not burned, leaving plastic deposits to foul the bore of your gun for the next shots, which will destroy the patterns you get with subsequent shots, until its cleaned out of the barrel.
Here is a list of powder and shot loads from the same " dram " setting, by volume:
2 drams-= 55 grains---=3/4 oz shot.
2 1/4 dr = 62 grains---=7/8 oz shot.
2 1/2 dr = 68 grains---=1 oz shot.
2 3/4 dr = 75 grains---=1 1/8 oz shot.
3------dr = 82 grains---=1 1/4 oz shot.
3 1/4 dr = 89 grains---=1 3/8 oz shot.
3 1/2 dr = 96 grains---=1 1/2 oz shot.
3 3/4 dr = 102 grains--=1 5/8 oz shot.
4------dr = 109 grains--=1 3/4 oz shot.
In working up any shotgun load, you should be conscious of the Sound Barrier, because going through the sound barrier puts all kinds of pressure on round projectiles, including shot. In a shotgun with NO CHOKE, you will get better patterns if you keep the velocity of the load below the sound barrier( nominally 1135 fps.) In some guns that have choked barrels, you also get better patterns when the load is leaving the barrel at under the speed of sound. This does not hold true with all chokes or all choked guns. That is where individual testing HAS to be done, as every barrel, and choke is a creature unto itself.
The best load I found for my cylinder bore 12 ga. DB shotgun is a 2 3/4 dram load of powder behind 1 1/4 oz. of lead shot- normally #5 shot for upland game. This is not a 3:4 ratio. But it happens to work best in my shotgun. The load does leave the muzzles at under the speed of sound, however. I have not yet tried to use my ML shotgun for hunting waterfowl, so I have not tried to pattern steel shot loads through it. I did do extensive patterning with several different size choke tubes I bought for a modern shotgun I have used in hunting Geese.
I personally think using #6 steel for anything other than small ducks and upland game, where steel shot is required is the wrong choice of shot. even then, you have to keep within tht 25 yard range. Because the steel shot is so hard, it does not deform like lead shot, so you can expect it to stay in the pattern in greater numbers than if you used comparable lead shot. At close range, #6 shot is likely to completely penetrate a bird. But, at over 25 yards it begins to loose so much velocity and pellet energy that it becomes unreliable as a killer. I think Roundball's penetration test- use a tuna can and see how much penetration you get in these steel cans at a maximum distance, is a good test that is easy to do. Put the can at 25, then 30 yards, then 35 yards, etc. to see when the shot stops penetrating the can. Then pattern the load to see at what distance holes in the pattern large enough to let a bird pass through begin to appear regularly with that load, and back the range off until the hole is too small to allow that to happen. When a load passes both the pattern test, and the penetration test, you are ready to go hunting.
Best wishes.
