The 2 Bore falling block that Giles built is measured based on the British bore definition of "projectile weight to the pound of lead". If you can track down the video from Shooting Times you'll notice that they are shooting different weight projectiles and different loads. Just from memory the small bullet in the video is 2400grs and the big bullet is 3500grs. I suppose instead of guessing I could just email Giles and ask him, I haven't talked to him in a while but I'm sure he would be happy to provide more details.

Colin