Looks good. For the poly count reduction use this as a guide.
You can clean up the model just by fixing these few things. Cylinders need to be lower when making low poly models. I always use 8-12 sided cylinders and use the texture and normal maps to give the smoothness of a higher poly cylinder.
The faces on the side of the lower receiver can be made into one quad instead of the five or six you have. Also delete any faces that are not going to be seen by the player when it is in use.
The point of low poly is to get as close to the object as possible without using alot of polys. Give me a few minutes and I will remake the model you did and show you the difference.
Remember to press "crtl+n" when you are done so it recalculates faces outside.