Hey Bitmap, thanks. I'm currently having a little trouble determining the overall width. I got it from two points.
I suggest to model the barrel as soon as possible, then define the "eight-sided looking" shape where the ammo is loaded. It gives the width of the general thinner flat part, you can guess the plastic width from many photo references you already posted.
The beefy part toward the tail is a bit trickier. I got it from one of the exploded views
in the manufacturer's gallery. You can see in the image a "butt pad" (the cushion where you place the shoulder) from top, so you get the height, which is the same in a side view, and its width. The rifle is slightly larger than that.
More to come soon. I hope this helps and many other join in.