The M1 CATTB aka the Component Advanced Technology Test Bed is an M1 Abrams fitted with the latest tech made that was available during the 1980s. The main gun is either a 120 or 140 ETC(Electro Thermochemical) gun which makes it faster and has more penetration than a regular tank cannon. The M1 CATTB wasn't adopted into the US army as it's meant to be a tech demonstrator like how the newest phone concept is at display but not ready for production as hence the use, it's a technology demonstrator