There's many variables which effect the worth of precious metals, depending upon the timeline of the SHTF event. During the initial period, the traditional bullets, beans and band-aids will have a high trading value. Once civilization becomes re-established, then precious metals would have an increased value. Personally, I would never trade firearms or ammunition because they could be potentially used against me. Transferable knowledge is probably one of the best investments, because there's no storage overhead. So learn a trade or skill which could have a definite value after a SHTF event, i.e., trapping, blacksmithing, or farming would be in great demand during a long term, widespread event.