You can now craft ammunition at the reloading bench using alternate materials (scrap metal and tin cans). Specifically, a single round of Guns ammunition can be made with 1 scrap metal, 5 tin cans, and the appropriate casing or hull. Most Guns ammunition types are supported:
Unlike the Hand Loader perk, all recipes unlocked by this perk have the same skill requirement: a Repair skill of 45 or greater. This can allow players to craft rare or high-level ammunition, such as .45-70 Gov't or .50 MG, at a low level, albeit at a significantly higher cost in terms of resources used.
For most ammunition types it is more cost-effective to simply sell the required crafting components and then purchase the appropriate round from a merchant, even with a low Barter skill. Though this of course does not work for ammunition that is stocked rarely or in low quantities.
Although this perk would seem suited towards scrounging for ammunition within the Sierra Madre Casino and the nearby Villa, only 45 pieces of scrap metal are found placed in the area; another 122 may be found in various containers, though there is only an approximate 8% chance of finding scrap metal in a given container. Assuming the resources are not used for other crafting recipes, a player who takes this perk can only expect to craft a total of 54 "junk" rounds while exploring the Sierra Madre.
Bent tin cans will not work with this perk, only the undamaged variant will.
A new ammunition subtype, JNK, or junk rounds, was included in the Dead Money add-on, presumably for use with this perk. The new ammunition type existed for all Guns skill ammunition and provided the same effects for all calibers: 0.75x to Damage and 1.5x to Gun Condition. The new ammunition type covers two calibers which are not included in the existing version of the perk: .22LR rounds and 12.7mm rounds.
However, the recipes granted by taking the Junk Rounds perk simply produce one round of normal ammunition, rather than of the junk subtype. JNK-subtype ammunition is not used anywhere in the add-on, and cannot be obtained by normal means. If it is obtained it cannot be used in any weapons, as the ammunition is not added to the appropriate lists used to determine what ammunition types a weapon can load.