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https://en.wikipedia.org/wiki/Composite_armour#History
The earliest known composite armour for armoured vehicles was developed as part of the US Army's T95 experimental series from the mid-1950s.
The T95 featured "siliceous-cored armor" which contained a plate of fused silica glass between rolled steel plates.

The first widespread use of a composite armour appears to have been on the Soviet T-64.
It used an armour known as Combination K, which apparently is glass-reinforced plastic sandwiched between inner and outer steel layers.

https://en.wikipedia.org/wiki/Armour-piercing_fin-stabilized_discarding_sabot
APFSDS projectiles are generally fired from smoothbore guns, a practice that has been taken up for tank guns by China, India, Israel, Italy, Japan, France, Germany,
Turkey, Russia, and the United States. Nevertheless, in the early development of APFSDS ammunition, existing rifled barrel cannons were used, (and are still in use), such as the M68-105mm cannon mounted on the M60A3 main battle tank.