As far as I know, the only U.S. coins codified to be illegal to be destroyed are the penny and nickel (2006) because of the Cu content in the current nickel is 80% and the pre-1982 pennies 95%. You could melt 90% and 40% silver all day long but why would you as some as said the old coins are easily recognizable for what they are and if the shtf there will probably be a lot more things to trade than just silver or partially silver coins. My experience is that the 40% Kennedy halves have pretty much gone the route of the 90% (from my little experience of buying a $500 box from a bank this winter after Christmas and finding 0.0 out of 1,000 coins). If you're going to have to buy a silver coin from someone, might as well buy a 90%. Better yet, because the new full 1 oz. US Silver Eagles don't have the collectors value of being old, just pony up with those and not have to worry about refining the other metals out of them. If shtf, cut those up into quarters for easier trading. That's how "pieces of eight" came to be from back in the day...