I was given the opportunity to spend a couple weeks developing cast bullet loads for an expensive custom Abercrombie & Finch double rifle chambered for .30-40 Krag. I found loads for each barrel that grouped tight at 100 yards. Unfortunately the two barrel's groups never overlapped!

Generally, sensible practical guns do not need to be custom ordered. Custom building is how you get odd balls that few people want. A custom double for Bambi? Why not? He'd bound off before you could fire a third shot.