12.7mm Round

12.7mm rounds are a rare type of ammunition manufactured by Hauer Premium

Gameplay
The Hauer Premium Show Stopper 12.7mm civic defense round is a large pistol round. While they have an identical bullet diameter to the .50 caliber rifle round, the much smaller case means it holds less powder and can be safely used in man-portable autoloading weapons.

Production
Standard and JHP 12.7mm rounds can be crafted at a reloading bench. A single cartridge requires the following ingredients:

Breakdown
Breaking down a standard 12.7mm round yields:

Weapons using this ammunition

 * 12.7mm pistol
 * Li'l Devil
 * 12.7mm submachine gun
 * 12.7mm submachine gun (GRA)
 * Survivalist's Rifle

12.7mm round, hollow point
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 (junk)
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.

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. This may be similar to 12.7mm ammunition's initial absence from most merchant inventories and the lack of a crafting recipe for the ammunition.

12.7mm round, JHP hand load
A new type of ammunition added with the add-on Gun Runners' Arsenal, JHP, or jacketed hollow point, use a metal-clad bullet that retains most of the damage bonus of ordinary hollow point rounds, but with a less severe armor penalty.

Locations

 * Vendortron, sold when the Courier is at lower levels.
 * Knight Torres in the Hidden Valley Bunker.
 * The Arms Merchant at the 188 Trading Post semi-regularly sells 12.7mm ammunition.
 * In the Bloodborne cave, 10-30 rounds - Two twelve round boxes, one by each 12.7mm weapon in the cave.
 * Daniel Contreras, Camp McCarran at fairly low levels. Sells in extreme amounts (2,000-3,000 at a time).
 * Cliff Briscoe in Novac will sell them when the player character is at higher levels.
 * Great Khans, Legionaries, and Fiends will occasionally carry some rounds. Especially Legionaries encountered with 12.7mm weapon.
 * Quartermaster Bardon at Hoover Dam will sell these in extreme amounts (1,000-3,000 at a time).
 * The Great Khan armorer will sell these in extreme amounts, from 500 to 3,000 rounds at a time.

Behind the scenes
Joshua Sawyer commented on the choice of including the 12.7mm round.