Jiom: I don't have any loading data on the 69 cal. rifles. If I do, I don't know how to put my hand on it quickly( I find all kinds of things months after I needed them!) But your comment did remind me that the Dixie GW catalogue has Proof loads for various gauges, and a 14 gauge Proof Load is shown as 262 Grains of powder behind a 432 Gr. Lead Ball. Using your suggestion that a Proof Load is twice the maximum charge, I would set my maximum charge at no more than 130 grains of powder. Those are Belgium proof loads.
On a subsequent page, they show 1887 Proof Loads for a .693 Bore, of 306 Grains of powder, and a 457 grain "bullet".( I am assuming that these are from the Birmingham Proof House, only because they follow the " Service Load below, in the Dixie Catalogue.)
The Service Loads from 1896 for the Birmingham Proof House, for a 14 gauge gun was: 3 drams(82 grs.) of powder, 492 Grain Ball, or 1 1/8 oz. of shot. Note the substantial difference in the reported weight of the ball for the two Proof figures, ie. 432 vs. 492. I think the heavier ball is the correct size, as 7000 divided by 14 = 500 grains.
About the only thing I am comfortable about this data is the recommended " Service Load " of 3 drams, or 82 grains of powder. I would start at 60 grains and work my way up to 80 or 85 grains, tops, to see what gives the gun the best accuracy. Your 44 inch barrel can burn 190 grains of powder efficiently, but the recoil of all that powder would be too much even for a large man to hold, and would not deliver any truly usable accuracy.
A friend of mine, who built his own slug guns, made a .69 caliber slug gun, that weighed 100 lbs, 8 inches across the flats, barrel about 4 feet long, underhammer action with his own make of scope on it. It fired a 1760 grain bullet, made in 2 pieces and paper patched together in the false muzzle as it was loaded, on top of 350 grains of FFFg powder. His velocity at 20 feet was 1020 fps. The recoil would dislocate your shoulder if you didn't really grab hold of that stock, even with the huge weight of the gun helping. He had a 10-shot group fired at 500 yds. that measured 5.26" center to center. That is half a pound of powder, and 2 1/2 pounds of lead fired.The group contained 7 shots in one ragged hole, about 3.5 inches across, and the remaining 3 shots hitting about an inch and a half off to 10:30 O'clock. He said that the bullet did not seem affected by cross winds, until the winds exceeded 25 MPH, so I am assuming that the reason for the 3 shots that hit wide was because his eye focused on the target, and not the crosshairs for some of the shots. Its a common mistake of habit for target shooters using large magnification scopes on rifles.
I onoly share that with you because your rifle weighs so much less than his slug gun, and there is no way I am going to volunteer to shoot many rounds out of that caliber gun with anything exceeding 120 grains. I fired a .50-140 Sharps rifle with black powder and 550 grain bullets. Twice. I know how to control recoil but that rifle was just not very pleasant to shoot. Other than the owner of the rifle, I was the only member of my BP club who was willing to shoot it more than once.
80 grains should be totally adequate for any round ball load, IMHO.
