Looks like Katsu's packing a QBZ-95 there. Looks a bit wonky, though, and her grip on it looks pretty awkward. Dried bloodstains are a nice touoch, though - suggests she's been in action for a while.
Also, I checked the Art Guidelines (https://www.newgrounds.com/wiki/help-information/terms-of-use/art-guidelines) - I'd say this definitely falls into T territory.