You could use a different rifle, as long as it fits in the same category, for instance, changing military repeater for a different military repeater. But, for instance, a 12 pound Sharps buffalo rifle couldn't be substituted for a trapdoor carbine. One year I used a Swede Mauser for the long range instead of the Lebel I used for the rest of the match(didn't have enough brass for the Lebel), no problem, other than maybe some good natured "gamer" comment for using the Swede.......