I would like to try a shiloh 50-90 someday. The 140 was never loaded by sharps. I read someplace there might have been an experimental 1885. I am with you too much of a good thing. Shiloh may have dropped that 140 along with the 45-120 which is another round that never was.
The 500NE is probably a must reload round due to ammo cost. That could be a plus for some buyers and a negative for others. I assume some will never buy ammo and just collect it. The single does not have the regulation issues of a double. The 500 takes standard .510 bullets. That leaves any owner with a lot of flexibility up or down the power scale. This is one I would buy for sure.
The S&W 500 X frame cannot be any cake walk either and they sell very well. And here too reloading changes the whole ball game. I see a lot of 500 SW ammo on the shelves. They use .500 bullets.
I think there is a market for guns that are a challenge to shoot or bragging. Or dreaming!
