What's your budget for this range?
You'll need to consider also noise abatement (reduction) too but of course if you shoot silencers that's not such a big detail (only for those that have it and are allowed by Federal Law).
Here is a portable "cube" bullet stop. You basically build up like bricks to create your backstop. There are a few like these you might want to consider.
http://www.atsusa.biz/bulletstop.shtml
Good luck, have fun and check out your local zoning laws!