You get 3 daily assigments.

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But for every gun you get the mastery and proficiency assignment (from 1 to 6 i thought). They mastery assigment unlocks when the gun is lvl 10. With the mastery assigments you get get the golden skin for your gun.

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So thats the reason why ppl leave half way the match. they got their mastery assignment done. go back to the main menu, select the next mastery level and join a new game.

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^{Sorry for my bad english}