Pricedo:

Why not do a Goggle search for 45-70 Forager rounds. No conversions required. I've played with them and they are effective on rabbits and tree rats out to 10-15 yards. They would work farther but the rifling causes the shot to spread real fast. Civil war soldiers where actually supplied with a 45-70 Forager round - it was a hollowed out wooden "bullet" that was filled with shot.

Then you come back to the original point of why spend $1000 on a "double rifle" just to convert it (spend more money) to a combo gun that can be bought for half that price (Savage 24).