Board Thread:Unanswered Questions/@comment-24407862-20150525204353/@comment-2094030-20150526062254

Kill gauge only unlocks aircraft colors. To unlock SPW for purchase, you need to use the special weapon available before it several times. I'm not sure how many sorties or how many enemies killed. But the weapons would be unlocked if you use any weapon currently available. This is made to encourage usage of all aircraft. Unlike in previous games where you can easily get 100% completion by using just one aircraft. In this game, to get 100%, you need to use all aircraft which will also unlock its special weapons. Generally, the best weapons will be unlocked last, but it's not always the case.