Speaking of antitank weapons, in the retreat down the Malay peninsula, the British used a .55 Cal. Boys rifle against the lightly armored Japanese tanks. It was a bolt action gun and the bullet was an armor piercing round adapted from a .50 cal. round.
The M2 Browning machine gun was originally conceived during World War I to serve dual anti-tank and anti-aircraft roles. General John Pershing requested a heavy machine gun that could pierce armor, leading designer John Browning to scale up his .30-caliber design to fire a powerful half-inch round inspired by captured German anti-tank rifles.
Anti-tank rifles in, up to 20mm, were fairly common prior to WWII in almost all armies. Some were even for mail order in the fifties and sixties.