"a well regulated militia being necessary to the security of a free state..."

This is an opinion and it is there to explain, not to set, the policy.

"the right of the people to keep and bear arms shall not be infringed"

This is the policy statement and stands with/without the previous phrase.

Besides, all of the Founding Fathers were gun owners and they explain the right to private ownership of guns in various documents too numerous to count.