The term 'barrel diameter' refers to the size of a cylindrical object, such as a barrel. The crossword solutions for this term are BORE (4 letters), HALF (4 letters), RADII (5 letters), and mil (3 letters). BORE is the diameter of the inside of the barrel, while HALF is the radius of the barrel, which is HALF of the diameter. RADII is the plural form of radius, and mil is a unit of measurement equal to one thousandth of an inch. Together, these solutions provide a complete description of the size of a barrel.