The SA-T1-Atomic Tank aka T1Atom comes with a twin ammo chamber barrel one chamber for a new T1-Atom round the other for standard rounds.
It also comes with a flame thrower, machinegun and an 8x8 equipment grid. It is an old heavy tank with increased explosive resistance, as a result it is less economic and moves slower.
You will need to research the Tank and Atomic Bomb before you can craft the T1 Atom and since this is a new mod the crafting materials are subject to change.