Far as making white smoke at BP shoots, I know for a fact that a 45-70 case will hold two 50gn pyrodex or 777 pellets plus some loose powder.
You cut the cartridge stop back, move the ejector back and grind enough clearence to the ejection port. That should get you started after you have the chamber lengthened.
At one time I contemplated enlarging the chamber on an 1895 to 45-100 or 120, give it a smooth bore, and custom choke tubes, load it to 20ga levels. Then they came out with that wimpy 2.5 .410 for a while, too bad they did not make it lighter.