I consider anything around 80gr. of 2F in a .62 as a weaker, 20 bore smoothbore load. Yes- it will kill a bear, but since the huge benefit of a rifle over a smoothbore is for flatter trajectory and longer range, I use more powder to make the rifle work.
I've never experienced a loss in accuracy with heavier loads - possibly due to using a thicker patch. The heavier loads, necessary for flatter trajectories, produce higher pressure and thus. Every time I've increased the loads over those close range, what I call squib loads, my accuracy has increased/improved, not opened up.
For me, a good big game hunting load in a .62 would be 130gr. to 140gr. 2f GOEX - certainly not less than 130.
If using 3F GOEX and a VERY thick patch, a load around 120gr. or 125gr. will duplicate those heavier 2F loads. Swiss might do it with 110gr. to 115gr.
Most people run into troubles with patches letting go, building fouling in the bores and reducing accuracy due to using a ball too small for the bore size.
In a true .620" rifle, .615" pure lead round balls are a good size to be used with a .022" patch (ie: 10 ounce denim). A .600" ball will require an much thick patch to get doen into the grooves and compress in the bottom of the grooves. I am not sure it can be done with a rounded bottom grooved barrel.