They are usually fairly slow, likely between 1:24" to 1:28" (which is what most modern inline muzzleloaders are).

Remington now has a bonded Accu-tip slug... and winchester offers an XP3 with a tungsten core.... if you shoot it with than and don't stop it than you won't stop it with much of anything less than the Great Wall of China.

Those Brennekke slugs and lightfields are bad to the bone as well.

If you want an ABERS (anti-bears.... get it!) round, then copper plated 00 buckshot is a good bet. Dixie slugs also make a Tri-Ball round, which is I think three .68cal lead balls that will all three shoot onto a 3x5 card at 20 yards.