cannot remember if its the martini or snider that as baderley screws on the bands as a slight compression first then unscrew works best.....
patience is the best medicine here....i have a quite rare EIC fusil carbine which only 5000 were ever made and as always the nipple was seized in badly and also broken....if you go at them hammer and fist you will strip the threads and its a wall hanger etc....took 2 weeks of heat and oil every night then a good fitting pair of small mole grips loosened it with no damage to the threads