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In a bore this big I would use something at least 72 and probably 80 to 100". 72 to 80 will work for bore much smaller than 4.
I would not use a 48 in calibers larger than 50 cal. Though it works very well in the 50 even with heavy charges.
I use the same powder charge in a rifle for everything. This way I become accustomed to its trajectory and know where it shoots and don't have to remember if i am shooting a "target load" or a hunting load.
Conical bullets were proven to be a poor choice in "bore" rifles back in the day so fast twists are really useless.

The British makers almost always used twists that were far too fast in RB rifles, why I have never been able to learn, British military tests circa 1790-1800 proved they were not needed.
Still they generally used twists of one turn in the length of the barrel or faster, falsely believing that larger balls needed faster twists. Thus they created rifles that were useless for their intended purpose since they would only shoot light powder charges and the trajectory and penetration was so poor that hunters like Selous gave up and used smoothbores or rifles with "belted" balls that would not strip.
Samuel Baker details the failings of conicals for dangerous game and James Forsythe details the problems with rifles having too fast a twist or using conicals of equal weight to a round ball. He found that a .69 caliber (about 14 bore) rifle worked very well with a twist of 8"6" and would shoot well enough for hunting with considerably less.
I do not know at what point the larger bores will begin to strip with loads making 1350-1500 fps. About what was needed for shooting heavy game. Smaller bores like the 20-16-14 can be used at velocities much higher 1650-1750, without excess recoil giving a nice flat trajectory to 120-130 yards. But the twist needs to be 72" or slower.
The modern use of conical in ML guns is largely the result of companies making conicals for people who cannot be bothered with the PRB and since they make it and sell it it MUST be better than a PRB. In reality for hunting it is not and in fact has some deficiencies.
Dan

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