pyro is dirtier but easier to buy. But if your only use for this gun is going to be this then you could probably use nitro powder (everyone throws their hands up in horror) but only because there is no projectile to create breach pressure. Nitro would go off easier, the big shock is that it would be in a gun proofed only for BP.
We are cosidering specialist loading here, half a grain of nitro powder would be enough if your looking at the range of the rider in your example.
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane