Take a look at the V.M. Starr article on Shotgun loads at Bob Spenser's Website, "Black Powder Notebook". He gives advice on 10 gauge loads.
I checked my Hodgdon Data Manual for 10 ga. loads, and it starts with 102 grains of FFg( 3 3/4 dram) for a " Light Load", behind 1 1/4 oz. of shot. MV is listed at 1050 fps.
The "Heavy" Load is 4 1/2 drams( 124 grains) of FFg, and 1 1/2 oz of shot, for 1067 fps.MV.
You do want to keep the velocity UNDER the Speed of sound( 1135 fps.), especially in a cylinder bore gun. Otherwise, you will blow the patterns wider than necessary.
I don't think you are going to damage anything using 3Fg powder in a 10 gauge, but understand the powder will burn faster, and give you higher pressure, and, presumably, higher MV. More important it kicks the soft shot harder, causing more deformation ( obturation) of the soft shot, particularly at the back of the shot column. I am not convinced that this obturation is solved by using cushion wads- at least I didn't see any improvement in patterns. Perhaps in a choked shotgun, results will be different. Hit the patterning board.
If you check the Lyman Shotshell Reloading manual, they have a chart that shows various MVs, and then down range velocities, Pellet energy, Time in flight, and drop in flight, for 20, 40, & 60 yards. Of course, they are using smokeless powders, and the MVs are all mostly faster than anything you can get out of BP, except with light loads in small gauge guns. But, as the load goes down range, velocities drop and its not that hard to figure out what your ML, PB load performance will be.
The first thing you should notice is how Quickly shot pellets, no matter what size, lose velocity within the first 20 yards.
That data is what convinced me to stay away from FFFg powder in 24 and larger gauge shotguns.[ I do believe that FFFg powder is useful in .28, and .410 bore shotguns.]
If you wrap your shot in a paper " cartridge", closed and taped at both ends, you can shoot it across the screens of a chronograph to get your own reading on MV for a given load, without endangering your chronograph.
I would not trust the readings I would get if I dared to put my chronograph out at 20, 40, or further yards, simply because that solid "slug " of pellets is going to have less drag than the individual pellets that make up the load, at those distances. The readings would be HIGH compared to the actual velocities of shot at those same distances.
However, if you take the velocity you get at the Muzzle, using your chronograph, and then use that chart in the Lyman book, you should be able to find out how quickly the pellets will lose velocity based on your load, and how much pellet energy is correspondingly lost.
I consider BP shotguns without choke to be 25-30 yard guns, MAX. You can do a bit better than that, but the opening patterns makes it a function of luck as you get further from the muzzle. If you choke these guns, then I believe you can move the range out to 40 yards, with some patterns, and pellet sizes.
A lot of modern Shotgun shooters consider their full choke guns shooting smokeless powder to be quite capable of killing game out past 50 yards. That may or may not be true, based on choke, how long the choke is in their barrel, the size shot, the total volume of the shot, and powder charge used.
I do not believe you are going to make a BP shotgun into a 50 or 60 yards gun with any load of birdshot used, regardless of the choke. The Vast majority of game birds are killed inside 30 yards, regardless of species.
With waterfowl, you may have to pass up a shot that you could have taken with a choked modern shotgun, but there are plenty of shots passed even with modern guns, because the birds are still out of range. The fun of hunting waterfowl is seeing if you can turn that flight back around to come back over you set, but lower, or to actually call the flight back and down to your set of decoys.
