I'm not 100% sure, but it might be happening because you don't have enough edge loops on your low poly to get a smooth bake. Try adding more loops to the curved parts of the low poly model.
The shape seems nice, the stock/handle area (the bit at the back) could do with a bit of work. Probably pulling down some of the bottom loops to make it more curved and less boxy.
Try to avoid solid white bg, its harsh on the eyes. You have a ton of loops that are doing nothing. You should work on learning sub-d and using turbosmooth As far as the gun, i would concentrate more on the shape before moving on like Stock should be more rounded and its a separate piece from the receiver The ejection port…