Ckm wz. 30 was the main heavy machine gun used by Polish Army in 1939. Design, strongly inspired by american Browning M1917 HMG, was modified and improved since 1931, including such differences as greater caliber and improved sights. Machine gun was also equipped with water-cooling system and advanced tripod mounting, allowing crew to use Ckm wz. 30 for anti-aircraft fire. Machine gun was crewed by 3-man squad - machine gunner, loader and spotter, acompanied by 4 additional soldiers, providing support and ammunition. This weapon was also used in many Polish vehicles, such as 7TP or Vickers tanks. Cavalry used Ckm. wz. 30 mounted on horse-drawn carriages, called “taczanka”, to improve mobility.
Polish infantry forces equiped with those weapons played vital role in defence efforts during September 1939, for example in the course of famous Battle of Wizna, known also as "Polish Termopylae". For over 3 days small Polish forces counting around 720 men and including machine gun formation, defended 9km long line of defence positions. Against them stood over 44 000 Germans, supported by 450 tanks and Luftwaffe. Poles hold their ground until the 10th of September, when became clear that further defence was impossible. Handful of Polish soldiers were taken prisoners, but Captain Raginis, the commanding officer of the Polish forces, took his own life to keep his oath to never surrender to the enemy.