This is a bit of a guess on my part, but the reason the stallfighter .50's feel like they have more pop than the b&z .50 could be related to convergence, and its effects.
As I recall the .50 calls are more noticeably affected by convergence since they do not explode like a cannon. So when you have them all hitting at the same spot/time that collective energy gives them a big boost...you're getting a lot of focused damage done on one spot. With a stallfighter you tend to sit in that convergence sweet spot much longer during a fight, therefore it appears that when you shoot, you do more damage. With a b&z ride, alternatively, you can be through the sweet spot before you pull the trigger, or alternatively not even get to it, thus giving the impression that the guns are not hitting as hard.
My two bits anywhooo.