Complex sentence: The book holds that guns were uncommon during peacetime in the United States during the colonial, early national, and antebellum periods, when guns were little used and the average American's proficiency in use of firearms was poor.
Split into simpler sentences: The book holds that guns were uncommon during peacetime in the United States during the colonial, early national, and antebellum periods. At that time, guns were seldom used and the average American's proficiency in use of firearms was poor.