According to the Fidelity study, the average investment return of the people who ended up with $1 million in their 401(k)s from the middle of 2000 to the middle of 2012 was 4.8%. That might sound doable. But remember that between 2000 through 2012 there were two market crashes. Over the same period, the average return of the stock market was 1.3%. So, to become a 401(k) millionaire, you had to do three times better than the stock market over the same time period
Good luck storming the castle!