why not cast the silver into a rod and machine the silver for a press fit?
i'd worry about the copper distorting from the heat.
I wouldn't get water near the copper cause if the molten silver hits it you'll get a shower of molten silver...
you should heat the copper up if you are going to cast the silver into it ... enough at least to get rid of any moisture...
also the silver is going to probably shink after you pour it.
just my 2 cents