To answer you second question, You are NOT going to see much difference in effect on game shot with a .75 caliber RB vs. a .62 caliber RB. Both are HUGE balls, with massive weight. Both, in pure lead, expand quickly even at low velocities.( Where a high speed hollow point bullet fired at 3,000 fps will not not expand much below 1500 fps,Pure Lead RBs have been known to expand well at velocities of 600-800 fps.)
For your information, High speed bullets kill by producing both a small primary wound channel, and a much larger secondary wound channel cause by the vacuums created by the high speed bullet traveling through flesh. The secondary would channel cause so much shock to the internal organs if a good chest shot is made, that blood pressure drops quickly, and/or the nervous system is shocked enough to cause the animal to become unconscious, and remain so while he bleeds to death.
By contrast, a Patched Round Ball( PRB) kills by making a huge Primary Wound Channel( PWC) in the animal. That is why ball placement is so much more important than it is with high speed bullets.
There is NO secondary wound channel created by a PRB striking the flesh, so there is NO reason to try to push a RB out the barrel fast enough to create the trailing vacuums to cause it.
The RB has terrible ballistics, which means is affected by DRAG more than any other shaped of projectile. Its fine if used under 100 yds, and the MV is kept under the speed of sound. However, if you exceed the sound barrier, the ball sheds velocity even faster than a stripper sheds her panties, and when the ball comes through the Trans-sonic zone, all kinds of bad things happen to move the ball in some direction different from your POA( point of aim).
You can expect 25 % of your MV to be lost in the first 50 yards. At 100 yds, 40-45% of your MV is lost. Once the ball drops down below the speed of sound, the drop in velocity slows, but the affect of gravity on the ball makes for a steep downward curve, so that accurate range estimation is critical after about 120 yards.
A 20 gauge ball has a Ballistics coefficient of .084, while your 75 caliber ball has a BC of approx. .102. If you look up the BCs for any modern bullet you will find all but the wadcutter pistol bullets greatly exceed these numbers.
The best conclusion you can make if choosing to hunt with a RB from a musket or rifle is that it is a 100 yard, and lesser range gun, largely due to the open sights and your ability to shoot open sights accurately. But compounding the problems of the open sights in hunting condition is the terrible BC of a RB. It just won't retain that velocity down range, no matter how fast it leaves the barrel.
There is a law of aerodynamic motion that controls all that, too, but I have forgotten which one it is. Basically, whatever goes faster in air, will slow faster. Since accuracy is the true name of the game for shooting MLers at game, learn to get close, pick your shots, and use a load that will assure you that you can place the ball where you aim.
Monster loads in any mler just are the wrong way to go. They generate too much recoil, and recoil causes bad flinches that spoil well aimed shots. Wounded game results, that usually gets away to suffer and die someplace else.
Do some penetration testing with your chosen RB gun, comparing target loads to that monster "hunting " load you think is necessary. You will be very shocked to learn that the two balls will penetrate about the same distance. A 4 dram load of powder in a .75 caliber gun is still a lot of powder( 110 grains), and more than enough to push a RB out fast enough to kill any game in N. America. a 3 dram load makes more sense, IMHO( 83 grains) with such a large diameter ball.
