Every kill you get in mw3 = 50 weapon xp no mater how many points per kill for normal xp. Say you get a kill in kill confirmed you get 50 xp and 50 weapon xp, if your playing search and destroy you get a kill (500xp) the kill was a head shot (+500xp) the kill was payback (+50xp) the kill was a comeback (+100xp) and you got the kill in final stand (+500xp) (final stand kills, normal xp kills, and head shot xp are different by game mode) this adds up to 1650xp for the kill but you still only get 50 weapon xp. challenges are different the amount earned from a gun challenge results in the same amount of weapon xp a mastery gives 10,000xp for player and gun.
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