I killed one deer at just about 100 yards with my flinter one year using 100 gr of Goex FFFg and a 300 gr Shockwave. My friend shot one the same year at 116 yards with the same charge and a 250 gr REAL bullet. If you know your projectile's trajectory I don't think it would be too difficult to take a deer with your sidelock, even with a round ball, out to 100 or 125 yards.
Two years ago in SC I shot a big old doe at 136 yards using 100 gr of Black MZ and 295 gr Barnes.But that was with my scoped Accura V2.