You have to sweep your missions which you didn't finish with an S-rank.
When you finish certain missions with S ranking, you unlock rare weapons & gadgets to aid you with the tougher missions
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I really hate that with games,at least with games like these.Because if i replay missions i can end up restarting a missions too many times because i can't get the perfect score.
And if i screw up towards the end of the mission that i for example have spent 10 minutes with allready,it gets really frustrating.And before i know it,it could take me an hour each mission to get it perfect.Many of them can be an easy S rank but i mean the harder ones.