It's a great piece, really stylish. There's nothing that really detracts from this piece that I can think of, but there is a couple of minors things I could point out if I may.
The character seems to be inside some sort of box frame, and the feet touch the very edges of the box. However her bum, at the very most, seems to only be touching the edge at a tangent. I think maybe her bum should be squashed a bit, like they do when anyone sits down. It's a smooth curve and makes it seem as if there is no sense of pressure, making her look as if she's hovering uncomfortably instead of sitting down.
The rifle is also slightly curved but that's part of the style, I get that, it's a nice touch.
The lighting is subtle but it's definately there. Overall it's a great piece, and there's not much else you can really do to improve on it. Nice work.

First of all, I really like the way her hair flows so gracefully and forms part of the frame--kind of softens up the box if you know what I mean. The rifle is great! I wouldn't be afraid to give it more highlights so it 'ops' more--especially down where the rifle projects into the black and gets a little lost. One other thing I noticed (about the rifle) was the butt, where it's yellow and black diagonal stripes. It shares the same lines as her hair so it almost looks like she's got a funky kind of ponytail clip. I think when you do that it's called a tangent line?

I also liked the slightly sketchy line you've used but I'm kind of a sucker for that style! I don't think a big bold black line would have looked as nice.

Now at first her right leg kind of bugged me cause I though it looked really uncomfortable, but I saw you used the frame as support so I calmed my crit rage down Overall, nice design and lovely color choices!