Aim Lab - Practice your FPS skills
I found this on steam yesterday and its actually really cool. The "game" is free on the steam store, called Aim Lab. It uses a number of challenges and AI to make a profile on your FPS skills. It will then give your advice and tips on what you do good and bad with. Using the data it collects, it will recommend specific challenges to sharpen your skills.
Best part? It allows you to simulate game settings/sensitivity from a bunch of different FPS titles including Squad, Rising Storm 2, Post Scriptum, Escape from Tarkov and a bunch of others.