Rifling

Brittle Fracture Origin of a Gun. Watertown Arsenal Laboratories.
Department of the Army. US Army Materiel Command.
Office of the Chief of Ordnance. Watertown Arsenal.
National Archives and Records Administration.
National Archives at Boston; Waltham, Massachusetts.

The guns made during this period were smooth-bore weapons. However, there was a period of brief experimentation with rifling in about 1500. The grooves were curiously straight. Rifling would not be tried again until about the 1800s.