Some weapons have blueprints, while others do not. The most common weapons to have blueprints are the shotgun and revolver. Other than these two types of guns there is no way to get additional weapon mods through gameplay or in-game purchases. Weapon mod blueprints can only be obtained by completing challenge missions that need you to find specific items scattered around Manhattan with a certain type of collectible tag attached to them called an ECHO device, which act like chests and spawn enemies when opened