His real name is James J. Barney, and his rank is that of sergeant E-5. Grand Slam was born in Chippewa Falls, Wisconsin.
His primary military specialty is artillery and his secondary military specialty is an electronics engineer. He received initial training in conventional artillery and served with a 155mm battery. He graduated at the top of his class from Special Weapons School. Grand Slam also graduated from Artillery School & Advanced Tech School as well as being a qualified expert with the M-16, M-1911A1, the H.A.L. (Heavy Artillery Laser) and JUMP (Jet Mobile Propulsion Unit). He has an uncanny ability to estimate distances and plot trajectories with amazing accuracy. He is also noted for being an introvert, losing himself in escapist fantasy such as science fiction novels and comic books.