Okay here's some criticism, I know this is your first model but I'll critique it anyways since it will (If you learn from it) most likely make you better.
First of all the main body of the gun is what you would call a cardboard-cutout, meaning that it's basically like a... Well cardboard-cutout, being flat on each side without any real roundness or definition going on, people say that low poly models are made that way but really, they're not, you can achieve a very nice rounded gun without going over the "low poly limit". And since it usually scares off a lot of people on this forum (And anywhere for that matter) it's a important thing to fix.
Secondly, the cylinders that you used have way too many divisions, and by divisions I mean how many lines define how round they're supposed to be, lowering it will drastically decrease poly count and still if you put enough effort into smoothing the normals and such, and look just as good.
I won't go too in depth since it is your first model and it can really only get better from now on.
Just keep at it man, and you'll triumph.
EDIT: On second look at the screenshot, on the cylinders caps of your model (The faces at the end of the cylinders that cover up the "hole" if you know what I mean, are huge N-gons, if you don't know what N-Gons are it's a way of saying how many sides a face has, the series goes something like this:
Triangle - Three sided Face.
Quad - Four sided face
N-Gon - anything with more than 4 sides.
Why is it called N-Gon? Well, to put it simply, the N is a variable that represents the number of edges the face has, and it starts counting after four. And these are in general a No-No in here, although depending on the game engine they might be fine, as for example, the source engine handles N-Gons quite well and because of that they're just fine, but in general you would want to expect most engines to not support them and just model everything in quads, since it helps you animating (If it's a model that will deform in the animation) and in general is a good thing to practice.
Also if you get some job as a 3D Artist you will most likely work under a boss of some sorts, who give you assignments and different requirements and one of those requirements will mots likely be to have the model entirely in either quads or tri's when it's done.
Alucard94, the member of the future of the past.
