For muzzleloaders, 300 or even 350 is not a "heavy" bullet. Many older conicals and even BP cartridge gun(military and hunting) loadings were for bullets weighing 400-600 gr. The velocities were not high, but these guns killed game very efficiently. Why? Full penetration along with some expansion of the soft lead slugs.

These days, muzzleloading velocities are approaching the range where you can start talking about "hydrostatic shock" like is seen with high power rifles.

I'm still in the big bullet, moderate velocity, full penetration camp, and probably will be until there is a sabotless bullet that I can shootto 2000 fps terminal velocity (say 100 yds).