Forgot to mention size of Foster Slugs & I noticed you are referring to slugs 'pulled' from 12 bore ammo.
IIRC, Remington Foster Slugs are .690", which is actually 3 thousandths smaller than a normal14 bore - That .690" is .040" smaller than a 12 bore - 40 thou! Most slugs, due to being almost or actually pure lead, obturate to fill the bore (.730") before hitting the choke which in full choke is nominally .690" - that is a big of stress, but the hollow design along with the 'wings' many mistakenly call rifling, take this 40 thou compression in stride - no damage, which is another reason for being soft as well. They penetrate poorly due to very poor density.(sectional density is low)
Winch. and Federal are are largest at about .715" to .710", which is still .015" to .020" smaller than a 12 bore's smoothbore's inside dimension.