The model's good, it seems to be rather accurate, I would just advise possibly changing the back of the gun a bit, to make it look a little less like the Halo 1 pistol, try scaling down where it begin to stretch out. What I mean is to make the sides of the gun flat from front to back.
Then I would've probably taken one of the screws and turned it into the iron-sight like things on it, since they aren't needed.
This is what I based all of that on:
http://www.freewebs.com/halo2dictator2/magnum.jpg
I like it though, it's good, it's close to the actual model. Keep it up.
8/10 for the model.