What do you mean with "real coins". Do you mean collector or proof coins? If yes wouldn't the additional premium for those coins be lost if you use them for barter. Wouldn't make junk silver coins make more sense as they have the lowest premium?
There's also folks selling sacks of "silver quarters" and other old circulated coins with silver content... These are dross that's been picked over for value already. There's folks that will cheerfully sell them for 3x the face value of the coins tho.
here is a Google search I did regarding this idea:
>>> How many US silver coins make an ounce? There is 0.18084 troy ounce of pure silver in a standard pre-1965 90% silver quarter. So, it takes about 5.5 pre-1965 90% silver quarters to have a full troy ounce of silver.
Current spot price of silver ~22 , lets say $20...so 20/5.5 = $3.60 or approx 14.5 X Face. Still a better deal but that is not the prices the sellers are asking that would be more what a dealer may offer to buy at.