Good morning
Had to replace the nipple on my badly neglected 1859 Sharps when I got it. The nipple was badly hammered "dry fired" till a musket cap would not fit.
Fortunately the breach block is removable. Several days of soaking in penetrating oil along with propane torch sessions directed on the surrounding block area convinced the rusted threads to kindly let loose. I did use the backwards 1/4 inch socket routine which was a big help.
An old shooter once said " For every year that old gun was neglected multiply your patience by that factor and you probably will get it working again".
Mike in Peru