Is it Safe to Shoot Slugs in a JC Higgins Model 60 Shotgun?

No, the JC Higgins Model 60 is not compatible with modern slug ammunition. The JC Higgins Model 60 was marketed from 1940 to 1946 as a budget, single-shot 20 gauge shotgun. It would be dangerous and most likely damage your firearm to shoot slugs through the Model 60. Modern slugs are typically longer and heavier than the traditional lead shot the Model 60 was designed to fire, and the Model 60's barrel is not able to withstand the increased pressures generated by firing slugs.