Rather than use the old .32-40, try a .32-20. Mine will send a 110 grain bullet out at about 1300 fps, which is more than adequate within 50 yds. Because of the Cowboy Action shooting craze, you can find new lever action .32-20 rifles today.
The same 90-110 grain bullets cast for the .32-20 can be sized and shot accurately out of a .32 caliber muzzleloading rifle using black powder. Since the .32-20 was originally a black powder cartridge, you can load those cartridges even today with black powder for a nostalgic hunting experience, too. Use an OP wad behind these bullets in barrels intended for shooting the PRB, to seal the gases behind the bullet, for best accuracy. There just is not much body mass to stop any kind of conical bullet you use, regardless of caliber. A hit under the wing bones will put a turkey on your table. The nice part about using a slower bullet is it doesn't destroy much meat.