You probably think of most shotgun slugs as slow chunks of soft lead, but these are not most slugs.
These are custom-made tungsten carbide perpetrator slugs. The secret to the armor-defeating rounds is the ultra-hard tungsten carbide core.
The hardness of the tungsten core more than makes up for comparatively low velocity of the projectile because it does not deform when striking hard surfaces. Overall these are pretty impressive rounds, especially considering they were homemade.