It is true that there's a much wider variety of ammo available for a 12 Ga.... If you insist on using a slug instead of a rifle bullet (would not be my choice!!), and want to use a 20, it''ll do fine if you use a slug OTHER than the Foster type, which is essentially a thin lead shuttlecock. Use as heavy a 20-ga. slug as you can find, shoot it out of a rifled bore, and choose a dense projectile like the Winchestyer Partition Gold, or one that is just as tough! In a 20 Ga. slug you have a bullet that already has a large diameter, so pentration is a lot more important than expansion!