According to reddit, it's fanart based off a game called "Girl's Frontline" where the weapons take on the appearance of girls (or something). Hence why the one character is named "M16"
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https://www.google.com/amp/s/amp.reddit.com/r/anime_irl/comments/aqf06m/anime_irl/
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Edited because my damn phone changed fanart to canary again -_-
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https://www.google.com/amp/s/amp.reddit.com/r/anime_irl/comments/aqf06m/anime_irl/
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Edited because my damn phone changed fanart to canary again -_-