The M1 is a combination of two "one-size-fits-all" helmets—an outer metal shell, sometimes called the "steel pot", and a hard hat–type liner nestled inside it featuring an adjustable suspension system.
Helmet covers and netting would be applied by covering the steel shell with the extra material tucked inside the shell and secured by inserting the liner.

The outer shell should not be worn by itself.
The liner can be worn by itself, providing protection similar to a hard hat, and was often worn in such fashion by military policemen, Assistant Drill Instructors (known as AIs), and rifle/machine gun/pistol range staff, although they were supposed to wear steel at the range.
The liner is sometimes worn in U.S. military ceremonies and parades, painted white or chromed.
The depth of the helmet is 7 inches (180 mm), the width is 9.5 inches (240 mm), and length is 11 inches (280 mm), the thickness is 1/8" (3 mm), The weight of a World War II–era M1 is approximately 2.85 pounds (1.29 kg), including the liner and chinstrap.
The M1 is a combination of two "one-size-fits-all" helmets—an outer metal shell, sometimes called the "steel pot", and a hard hat–type liner nestled inside it featuring an adjustable suspension system. Helmet covers and netting would be applied by covering the steel shell with the extra material tucked inside the shell and secured by inserting the liner.
The outer shell should not be worn by itself. The liner can be worn by itself, providing protection similar to a hard hat, and was often worn in such fashion by military policemen, Assistant Drill Instructors (known as AIs), and rifle/machine gun/pistol range staff, although they were supposed to wear steel at the range. The liner is sometimes worn in U.S. military ceremonies and parades, painted white or chromed. The depth of the helmet is 7 inches (180 mm), the width is 9.5 inches (240 mm), and length is 11 inches (280 mm), the thickness is 1/8" (3 mm), The weight of a World War II–era M1 is approximately 2.85 pounds (1.29 kg), including the liner and chinstrap.
M1 helmet