I see no point in filling the bottom of a tank w/ a bunch of empty bullet shells unless your tryin' to be the 50 cent of fish keeping. Usually you'd want to try to replicate the natural environment a fish lives in in it's natural habitat as opposed to a G-Unit music video. But that's just IMO. I'm sure tons of other people that post pictures of their guns in their avatar, profile pics, and just about anywhere else they can, thought it was mad cool Though.
As far as the rubber tire I think it all really comes down to the effect of submerging whatever material it is and if it would let off any chemicals. I'm not too sure on this either but I'd definitely wait until you get a definitive answer from someone that knows there stuff before adding it.