First off, if you want to create game ready models, you have to understand a few very important steps..
When a model is Low Poly, it is not High poly...
8,601 Polys is a hell of alot for a game engine to handle, especially when this model has so little detail to it as yours... it should be around 1000-3000 poly max depending of the quality and its use, is it viewed in FPS, or on distance...
Each Polygon of your model is limited to contain 4 Vertices MAX, not 5, 6, 7, ETC.... i will not explain why in here.....
Smoothing is a very important part of modelling, your weapon is so boxed...
Your texture lacks alot of work! it looks like you just dropped a few metal images on it..
Your model description says its rigged? i bet that is not entirely correct....
Iam not saying your worthless, you've got potential, because you actually did create a weapon that looks like the M4A1..
I would not recommend you selling models before you got better, you're 10 years old i can see, that is actually pretty amazing boy, you keep practicing, and your good to go!....