Of course there weren't.
But from 1794 to the 1820s silver dollars and British half-crowns were both current in the US and about the same size and weight and considered fungible. In other words, if someone in 1810 Virginia told you that he wanted two dollars for the bridle you were buying in his shop and you handed him a half-crown piece and a silver dollar, he wouldn't be likely to raise a fuss.
The term "silver dollar" is one that was used very loosely in the early 1800s.
I'm not saying to story is not apocryphal - I am merely pointing out that a athletic teenage sixfooter can quite feasibly throw a small object 250 feet.
How come I supply sources for my statements and you never do?
“Easily the most famous of the Hanoverian kings of Great Britain was George III (1760-1820). This long reign witnessed his descent into madness and Britain’s first great territorial loss, the 13 American colonies. The grandson of George II, he was the first of the Hanoverians to be born in Britain, and he thought of himself as English. During the early years of his reign, very little silver was coined, as this metal was undervalued and didn’t circulate. Aside from the four Maundy coins, only shillings and sixpence were struck, and these nearly all in 1787 alone. A semi-official, token coinage in silver was produced for the Bank of England from 1804 to 1816, and this resulted in some very peculiar denominations. The only substantial coinage of silver during this reign consisted of crowns, halfcrowns, shillings, sixpence and Maundy coins produced at a new lower weight from 1816 to 1820. A new reverse was used which featured the national arms, surrounded by the Garter and surmounted by a crown. The five-shillings or crown piece portrayed St. George slaying a dragon. “
http://www.coinsite.com/content/Articles/BritainSilver.asp
“Spanish silver dollars were so trusted and accepted by the American public that they remained official legal tender until 1857 and continued to be used out West into the 1870s. When you hold these coins, you will actually be holding American history in the palm of your hands! Could these coins have passed through the hands of George Washington, Thomas Jefferson and Betsy Ross...? Who knows! Each coin bears either the portrait of Charles III or Charles the IV, the Spanish kings that appeared on this coin from 1772 to 1808. This coin was the workhorse silver dollar of early America. Because of their high silver content, many of these coins were later melted down to help produce U.S. trade dollars, and some even ended up in Morgan Silver Dollars! What makes these coins even more magnificent is that they’ve all been graded in Very Fine condition - incredible considering their age and popularity as circulated coinage!”