That's a nice hunting rifle, bravo!!

Many of the gun makers of that period made fast rates of twist, some even as fast as 36" as-was a 13 bore, owned by Forsyh at one time. They were for round ball & due to the rapid twist would likely strip if tried with heavier charges needed for larger game in Africa or India.

That said, it will likely be fine with up to perhaps 120gr. or so of 2F powder - IF using a VERY tight ball and patch combination. The pressures will be low - it's the recoil that will, as in Forsyth's words "SHAKE THE STOCK". I would not attempt to shoot slugs in it, but stay with round ball. Slugs will increase the recoil and shock to that old gun, much more than will round balls.

You may find that 3 to 4 drams is quite enough powder for shooting deer, boars and bears. You would need these powder charges with a slug as well, in order to obtain any worthwhile killing power and trajectory with them. Round balls will be of more benefit for this purpose, I believe, flying flatter as well as delivering a greater "blow" due to their hemispherical shape.