Rifling was a well-designed poly-groove 'hook section' type. It consisted of 38 grooves with a uniform twist of one turn in 38 calibres. Calibre was 3-inch (76.2 mm).

A feature usually associated with World War 2 guns was its 'squeeze bore' which Armstrong called a 'grip'. The last six inches of the bore at the muzzle was slightly reduced in diameter, the purpose being to ensure precise centring of the projectile before it left the gun. At the same time it swaged down the lead sheathing slightly reducing the diameter and thus improving its ballistic shape.