I sure hope for you that it is an 1856 Flying Eagle Cent but I have to be honest, the chances of finding one while metal detecting, is pretty slim and I mean real slim! It is quite possible that it is an 1858 with part of the right side of the upper loop of the last "8" missing due to corrosion and it just looks as if it is a "6".
You should post pics of the coin and especially a closeup of the Date here to get opinions but you should also seek opinions of Coin Experts at Coneca and elsewhere. I believe that a Coin Expert can compare your' coin to the known Dies used (which have known Die Markers) in minting the 1856 Flying Eagle Cents and if your' coin has these, then you have a winner. If it is found that it is in fact a real 1856 Flying Eagle Cent, then I would contact PCGS, NGC, ICG and ANACS to see which one will conserve, certify and grade the coin, submit the coin to the appropriate Coin Grading/Conservation service, then go from there. Surely, if it comes back as "Genuine" with only a G-04 grade and notated as "Corroded", it will still be worth quite a bit of money and probably worth consigning it to a well known Coin Auction House to be auctioned off!
Frank
1856 
consult a coin expert , Good Luck
