Deer 655
Its not quite that simple, think of it like this the pressure peak of 777 looks like a sharp peaked mountain the pressure peak of BH209 looks like a plateau its a combination of pressure and the length of the push that gives BH209 the added velocity this is partly due to the fact that it is a progressive burning powder while the other subs are a constant type powder Black is a low explosive which I am not going into. As for your question on velocity the best comparison is 777-FFF which is about the same up to 110 gr the BH209 walks away. [ we will ignore the fact that only rarely do you get good accuracy with heavy loads of FFF and good accuracy is normal with heavy loads of BH209 at least in my guns.
Lee