This Ghost Recon Breakpoint weapon guide will show you the location of the SC-20K blueprint in game.  Every weapon has its own unique blueprint which you must find in order to be able to buy the weapon at anytime in the game store.

Weapon blueprints can be found by searching all the in game locations and once discovered, a location will tell you on the map its unique loot

You can gather intel, the exclamation points you come across in game which reveal the locations of areas, items and weapon blueprints.

The easiest way and quickest way is to follow this guide.