I've taken down a few rhino with normal assault rifles. From what I've seen they tend to graze near water, so I find one, shoot it to get it to chase me, then dive into the water (they don't follow) and either nip out of the water when it's a safe distance away and rinse and repeat or find a boat and use that as a platform and shoot the rhino from there.
EDITED: 29 Nov 2018 18:00 by GRAPHITONE