Carlos,
I can't recall a Cowbowy Action side match for Big Bore Repeaters that was over 300 yards. Most have been in the 100 yard to 200 yard range, with one or two out to 300 yards. I shoot a 45-70 in these side matches and it has always been just fine at these ranges. If you do get a 45-70 (or any other big bore for that matter) you need to verity, and record your sight setting for future reference, for the various ranges that you may be shooting. Good luck and good shooting.
HHW