The area in question has a texture that can be searched for stones, but not for missing battle rifles.
I know it may seem foolish to expect a gun to stay on the ground, especially when "G" was used to remove it from hands. That being said, I should be able to throw my gun at a tree trunk, and catch it on the bounce. I should be able to throw my gun into a creek, deceive someone into loaning me a canteen, and pick up the gun from the shallow water and shoot at them. Guns should not just disappear into the earth or water.