Complex sentence: He decides that the perfect way to prove that he is a grown up is to purchase a gun so instead of going home he goes over to a local store to have a look at the guns in a Sears Roebuck catalog.
Split into simpler sentences: He decides that the perfect way to prove that he is a grown up is to purchase a gun. Instead of going home, he goes over to a local store to have a look at the guns in a Sears Roebuck catalog.