The .45 Auto round is a large caliber auto-pistol cartridge, also suitable for submachine guns. After the completion of Honest Hearts the ammunition type will also begin appearing in the main game, sold by most suppliers (i.e., Gun Runners, Cliff Briscoe, Mick & Ralph's, etc.).
.45 Auto, +P (Over-pressure) is an ammunition designed to fire at a higher internal pressure than standard ammunition to ensure higher velocity, stopping power and a slight armor piercing ability but at the cost of requiring more maintenance than usual. This ammunition can be bought at most vendors in small quantities. It can also be made at a reloading bench.
.45 Auto, hollow point
.45 Auto, hollow points is an ammunition manufactured by Federal Cartridge corporation and designed to mushroom and break up on impact, causing massive trauma on fleshy, unarmored targets. However, this also means they break up when they hit things like armor plating, drastically reducing the weapon's penetrating power.
.45 super (hand load)
.45 super is an ammunition load designed for much higher pressures than the gun was originally intended to fire. Muzzle velocity is 20% greater than even +P resulting in much better armor penetration and damage, but causing considerable wear to guns not specially designed to operate these "Wildcat" rounds. You must take the Hand Loader perk to craft this round at a reloading bench.