G'Day Steve01 and LBG Dreamer,
What a great find, I bet the detector let you know you were onto something big.
For your information it's NOT a Colt revolver, the give away being your find has a full frame and Colt did not hold any patents for full framed revolvers till later in their history.
What you have is a Remington Model 1858 revolver, the clue here is the cone shaped hole in front of the cylinder at the bottom of the frame.
If you like to Google the name and model you should find plenty of images, I tried to up-load one but can't seem to get it to work.
Cheers, SinHof.