I’ve heard different things regarding the penetration ability of 12 gauge slugs against walls, car doors, and other types of barriers. Is overpenetration a real concern or is it overblown?
What is your target, what is your backstop, and what slug are you using?
^^^THIS^^^
There is no such thing as a generic “shotgun slug.” There are soft lead, hollow point slugs like used in the Federal LE 127LRS load that will flatten out into a donut after penetrating 14 inches of ballistic gel, then there are hard-lead slugs like the Brenneke Maximum Barrier Penetration load that will sail through an engine block.