Granted, my eyes are still quite young (thank goodness for contact lenses), but with my Marlin 1895GS with the Marbles tang site (.45-70), I've knocked down deer at 150yrds. I can ring an 10" gong at 300yrds with ease with this set up (white 10" gong on a black buffalo silhouette), but I usually reserve that long of shots for a scoped bottlenecked cartridge.

I have three peep apertures for my Marbles, the finest pin-hole is only good for HIGH LIGHT conditions, but it really lets me reach out there. I bought the extended range model (longer elevation adjustment) since I planned on using it on my Marlin 1895 Cowboy, but I ended up using a Ladder style tang peep sight for it instead, and left the Marbles on the guide gun.