I assume that the confusion is from the " twist steel" part. Damascus and " twist steel" refer to the way the barrel metal was made, not pertaining to any rifling inside. The Damascus barrels are considerably weaker barrels that a modern barrel and will probably blow up with the loads we use nowadays. The 3" part is the length of the chamber, it should say what length your chamber is on the barrel. Use nothing longer than what is on your barrel. I' m sorry I don' t know why the " use only in fully rifled barrels or rifled choke tubes" . That parts seems like an accuracy only issue but I don' t know.