One of the reasons for buying the TRESO nipples has not been mentioned. That is the size of the flash hole is much smaller on the TRESOs. It restricts the amount of blow back through the flash hole thus allowing the caps to remain in place after firing.
I find that no matter how closely the hammer/nipple relationship is adjusted, after extended firing the nipple will start to mushroom on the end from the hammer strike. When I see my caps (#10 Remingtons) begin to split as they are forced onto the nipple I pull them out and do the following: I chuck the nipple by the threads into a cordless drill (lower RPM). I turn the drill on to a low speed and touch the swelled portion of the nipple to a fine file for a FEW revolutions. Usually all it takes is enough metal removed to leave a shiny shoulder on the nose of the nipple. Do not allow the flat nose of the nipple to touch the file as this would alter the hammer/nipple engagement. This slight taper also makes it easier to start the caps onto the nipples.
I find that no matter how closely the hammer/nipple relationship is adjusted, after extended firing the nipple will start to mushroom on the end from the hammer strike. When I see my caps (#10 Remingtons) begin to split as they are forced onto the nipple I pull them out and do the following: I chuck the nipple by the threads into a cordless drill (lower RPM). I turn the drill on to a low speed and touch the swelled portion of the nipple to a fine file for a FEW revolutions. Usually all it takes is enough metal removed to leave a shiny shoulder on the nose of the nipple. Do not allow the flat nose of the nipple to touch the file as this would alter the hammer/nipple engagement. This slight taper also makes it easier to start the caps onto the nipples.