Before the War, the powerful 10mm Automatic round was the standard ammunition for the Bureau of Alcohol, Drugs, Tobacco, Firearms and Lasers and other federal agencies, while the military was more reliant on experimental energy weapons (although it remained the round of choice for conventional pistols and submachine guns). The G brand of ammunition was one of the most popular, offering excellent centerfire cartridges for a variety of applications.
The basic pistol ammunition, 10mm is commonly available and an excellent choice for close range combat with the 10mm submachine gun. Due to a bug in the executable, ammunition modifiers in Fallout do not work. As such, the choice between AP and JHP is purely cosmetic (although AP ammo trades for more).