[Low-Tech/High-Tech] Looting and Improvising arms and armour in Victorian London

[Green-Neck]

I would be very surprised if the proof loads did not have bullets, you really need that mass to get the pressure up.
To increase the pressure without increasing the powder charge, one can use more finely ground black powder. this would not be done for regular ammunition as there is a big pressure spike, followed by low pressure as the bullet fires down the barrel.