I think it would be cheaper to build a slab on grade structure with poured concrete walls. And then use typical roof tusses....or better yet attic roof trusses. The bottom chord of attic trusses is usually deeper, like a 2X8, so you can hang steel plate from so a bullet doesn't get accidentally shot through the roof.
The backstop can be a 45* angled piece of 1/2" AR500. I think there is actually a special rubber mat used at some indoor ranges, which catches all the bullet fragments once the bullet hits the steel backstop.
You can rent 9 foot tall Symons concrete forms.
Symons can also be stacked one on top of each other like this:
There is this neat interlocking wedge system which cinches all the forms to each other and to the form ties:
So once you are done with the forms....the concrete walls are cured....and you strip the forms off the walls, you put the forms back on the trailer, and the rental place comes out again and pulls the trailer away.
The wall thickness is up to you, based on how long of a wall tie you go with.
In the picture above, that is the rusty metal tab sticking out between the two forms.
If you go with a 9 foot Symons form, or stack a one foot wide form on top of an 8 footer, you can always use a laser level and grade nails to mark where the level pour should go. Versus just pouring to the top of the forms and striking it off with a trowel, and then dropping in anchor bolts.