Pack Rat (Fallout: New Vegas)

Pack Rat is a perk

Effects
This perk halves the carry weight of items that have a carry weight of 2 pounds or less. All affected items in the game world will display their new lowered weight whether or not they are in one's inventory.

In Hardcore mode, this applies to ammunition weight, which could cause this perk to be especially useful, allowing the player character to carry twice as much ammunition at the same weight.