The Hauer Premium Show Stopper 12.7mm civic defense round is a consumer-grade pistol round. In essence, these rounds consist of a big flat bullet in a straight case, with a rebated rim. While they have an identical bullet diameter to the military-grade .50 caliber, the much smaller case means it holds less powder and can be safely used in man-portable auto-loading weapons.
Hollow point bullets 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 plate; drastically reducing the weapon's penetrating power.
12.7mm round (hand load) (GRA)
A new type of ammunition added with the add-on Gun Runners' Arsenal, JHP, or jacketed hollow point, uses a metal-clad bullet that retains most of the damage bonus of ordinary hollow point rounds, but with a less severe armor penalty.
12.7mm round (junk)
The following is based on the JSawyer mod and has not been confirmed by canon sources.
Junk rounds are makeshift bullets made from materials commonly found while scavenging, such as scrap metal and tin cans. Junk rounds are only good as a last resort ammo-type, as firing them causes extra wear on the firearm, and do all around less damage.
The Junk Rounds perk from the Dead Money add-on was at one point intended to create ammunition of a special subtype. Called "Junk" or "JNK", these rounds all provided the same modifiers regardless of caliber: 0.75x to Damage and 1.5x to Gun Condition. The rounds remain present in game code but are not placed in any locations, and if somehow obtained cannot be used in any weapons, even those chambered for the correct caliber.
Curiously, the presence of 12.7mm JNK rounds indicates that the perk was to provide a recipe for crafting 12.7mm rounds, though the final version of the perk does not include one.
JSawyer integrates these rounds into the Junk Rounds perk and allows them to be craftable in the game as originally intended.