I agree--guns don't float.
Grossness alert--stop reading if you're easily offended...
I would think a body would float before the gun floated. Isn't that how drowned persons are often found? When the body begins to decompose and fills with gasses, I think it sometimes floats. Considering the floods they had in the reserve, perhaps that's what happened, Brian's body floated away from the area in which he actually shot himself.
Alternately, I can see someone who wasn't on the right side of the law picking up a gun and never reporting it.
I had considered his body floated away from where he killed him self, but it seems his backpack, notebook and dry bag were all nearby. So either they all floated to the same place and the gun stayed elsewhere or the gun was in the same area they found everything else.