Mod Master

Mod Machine is an achievement/trophy in Fallout: New Vegas which is awarded for applying 20 modifications to various weapons.

A full list of vendors that supply weapon modifications can be found on the main mod page.

Behind the scenes
The cut version of the achievement was named Mod Master, and required the player to attach all the mods in the game.

Bugs
Despite receiving the achievement, it may not show up in Steam.