Since this is a gun thread, I figured I'd ask this since I know next to nothing about guns, but in F3, and hence I presume New Vegas, when you're wearing an assault rifle or a hunting rifle across your back, how are you able to wear it without hinderence in combat? (In a realistic situation I mean)
Without hindrance
in combat? If you're in combat, you've most likely got it in your hands anyway.

But if you mean needing to momentarily stow it because we need to free up our hands for something else, such as digging out and tossing a grenade, I would either just lay it on the ground if I didn't expect to move anytime soon, or sling it across one shoulder if I was going to move (such as clearing a room) if we're talking about a standard sling. They've recently built and issued special http://www.mfiap.com/smallarms/3-sling.shtml for especially this purpose, as the way it is attached to your body you can literally just let go of your weapon and then have it back up at the ready much faster than a standard sling. They gave me one while we were prepping to deploy to Iraq.