What junk silver actually means
"Junk silver" is circulated silver coinage with no collector premium — worn, common dates, valued for its metal. The word junk refers to the numismatic value, not the silver, which is entirely real. In the United States that means dimes, quarters and half dollars struck in 1964 or earlier, at 90% silver.
The elegant part is that the US Mint struck those three denominations to a common weight standard per dollar of face value: 0.7234 troy ounces of silver in every dollar of face, whether that dollar is 10 dimes, 4 quarters or 2 halves. That is why the whole market quotes a single multiple of face value rather than a price per coin. Dealers generally figure 0.715 ozt per dollar instead, allowing for the roughly one percent of weight that circulation wears away.
The Coinage Act of 1965 removed silver from the dime and quarter entirely and cut the half dollar to 40%, which is why 1964 is the date everybody checks. Kennedy halves from 1965 to 1970 are 40% silver. Jefferson nickels from 1942 to 1945 — identifiable by the large mint mark above Monticello — are 35% silver; ordinary nickels contain none.
British and Canadian silver
British pre-decimal coinage was sterling at 925 until 1919 and then halved to 500 silver from 1920 to 1946, after which silver was removed altogether. The weights follow from the old standard of 66 shillings struck from one troy pound of sterling, so a shilling is 5.6552 grams and the florin, half crown and crown are simple multiples of it.
Canadian coinage ran at 800 silver from 1920 to 1966, dropping to 500 in 1967 and 1968. A Canadian half dollar of that era weighs 11.6638 grams — which is, by coincidence of two unrelated standards, exactly one tola.
Check the date before the scale
Coins are the one area where melt is most likely to be the wrong question. A common worn 1964 quarter is worth its silver. A key date, a mint error, or an uncirculated example can be worth many multiples of it, and melting is final. Look up the date and mint mark before a bag goes anywhere near a buyer.
Coin silver does usually trade closer to melt than scrap jewellery, because the content is known exactly and needs no assay. Expect the upper half of the published payout ranges rather than the lower.