Magnetic Accelerator Cannons are weapons used by the UNSC. The above seems simple enough; 120,000kg at 30,000m/s. Feed that through the kinetic energy calculator and have you 7.200E+13 joules, or 17.2084130019 kilotons. Let's also get the speed of the shots too.
T = 30000m/1s
= 30,000/340.29...