Gross domestic product is an important tool to measure the health and strength of a country’s economy, but it doesn’t always give a full picture.
One key factor for America's success is that no other state was trying to prevent America's fast growing at 19th century. The civil war did not last too long and the country went back to united with not too much damage. This allows US to grow from thirteen colonies to a sleeping giant at WW1 and WW2.
Abundant historical evidence indicates that the Second Amendment was meant to leave citizens with the ability to defend themselves against unlawful violence. Such threats might come from usurpers of governmental power, but they might also come from criminals whom the government is unwilling or unable to control.