Very nicely done. I'm a big fan of Art Work done by other people.
I also like your style and your interpretation of Rainbow Dash.
Here are some points I'd like to point out.
1. Your idea of pony anatomy is quite bland, as you made the body a bit too long, but that's just me.
2. Some areas of her body could be fixed, but other than that, it's really good.
3. The shading (though not much) gives it a nice touch.
4. You probably seen this already, but her teeth isn't white.
5. No matter how much time you have put into this, realize that you did a great job, and keep that in mind if you are going to make another great picture.
Overall rating 4.5/5.
P.S: That shotgun looks cool man.