In The Siege of Boston (April 19, 1775 – March 17, 1776] BRITISH GENERAL Howe asked for all the citizens of Boston to surrender their guns, coz he knew EVERYONE was ARMED, even the police. Not everyone complied.
People in Boston had guns before a police force existed.
The people in Boston who had guns [everyone] established a police force with guns.
Ergo. precedent and practice plus 2nd Amendment rule.
You could even argue that the Second Amendment REQUIRES adult citizens to keep and bear arms so that in an emergency they can assemble as the militia in short order, fully equipped and ready to go.