The purpose of the Second Amendment is exactly what it states, that men have the right to be conscripted into service as militia, and they have to supply their own guns to do so. Back then it really was a volunteer army, whether anybody wanted to be in it or not.
This was necessary for fledgling government that couldn't even afford to make its own money, let alone pay a standing army. Legend has it that Martha Washington had some of the family silverware melted to mint a handful of coins. The Mexican peso was commonly used in the US until 1857. In fact it's why we have dollars instead of pounds.