Well, if you are asking for reasonable loads for a 12 gauge, with cylinder bore barrels, you need to keep the velocity under 1150 fps MV, keep the charges reasonable, and not over load the gun. It can shoot good patterns if you don't put in too much powder, and try to get velocities up in the 1300 fps, as is common with modern shot loads. Remember, that BP guns are not normally loaded with Plastic shot cups, and the shot cups made for 12 gauge modern guns are actually 13 gauge, and don't seal effectively with most 12 gauge BP smoothbores. The shot cups work best with fast powders, higher Muzzle velocities, and choked barrels.
In my 12 gauge DB shotgun, which is cylinder bored, I use a light 2 3/4 dram trap load with 1-1/8 oz of #8 shot, for trap and skeet shooting. For hunting birds, I load a 2/34 dram load of FFg powder( 75 grains) with 1 1/4 oz. of #5 shot. Both loads are under the speed of sound, so they pattern is not affected by the sound barrier. Both loads pattern better, when I use OS cards in place of an OP wad,(1/8"), and 1/2" Cushion wad, the shot, and then One OS card( .010"). Instead, I use 4 OS cards, the shot, and two OS cards to hold the shot in the barrel. I run a greased patch down the barrel either while seating the OS cards on the shot, or after seating the OS cards on the shot. The cards all have an off-center hole punched into them, with an awl, and the holes are place so that no two holes are aligned. That way, as the cards hit the air outside the muzzle, the air separates the cards so that they quickly fall away from the line of fire, and have no effect on the shot. This also leaves me only one kind of card or wad to carry into the field, and eliminates the need to count wads, and use separate pockets to carry all of them.
I credit this fine idea to Iron Jim Rackham, a member here who comments all too infrequently. He came up with this system a couple of years ago, and used it in a new fowlder that Mike Brooks built for him. Last Fall, he was kind enough to publish some pictures of the gun and some of the grouse he had killed with it. From comments made on this forum since, I am not the only member who has tried Jim's system, and found that it works.
I have killed pheasants with my shotgun using the #5 shot out to 33 yards. If the barrels were choked, I would feel confident that the load would kill pheasants out to about 40 yards, with even better patterns.
Considering the short to medium range Grouse are normally shot within, your suggested load of 1 oz. of #7 or #7 1/2 shot, and 70 grains of powder sounds like a good bet. I would use #6 shot, because #7 shot is a bit more difficult to find here. # 7 1/2 shot is okay, as it puts a lot of shot on the birds out to 25-30 yards, but its the smallest shot I would use, unless I knew shots would be 25 yards or less. Then, I might use my trap loads. I do use the trap load for both quail and dove.
