So to answer the question this thread submits, and as extension to the arguments about private property, the reason why one needs an assault rifle, is to protect property.
Property is not liberty. Liberty is liberty. Property is property.
The state exists to protect private property. Privatize the state, or allow land barons to control private armies and police forces, and you have "voluntaryism". This is a fancy word for feudalism. This is a system where two classes exist, (possibly a third, in the form of a clergy) one being owners, the other being laborers. The laborers are voluntary employees of the land owning elite who have hereditary power. They are not chattel, they may leave the estate upon which they are employed in order to find employment elsewhere. They are paid for their labor, payment which they then use to purchase goods they produced themselves, from the owning class, who takes a portion of the product of the labor and sells it back to them.