I've been trying to figure out stringing up rifles too. Problem for me is that our backroom where I take pics has a drop ceiling and I'm afraid to damage it.

For your lighting issue, try lighting the backdrop more with it's own light to blow it out.
(Not to sound like I REALLY know what I am doing, just trying to help. I was having the same white outline issue you're having until I blew out the backrgound with it's own light.)