Howdy
Trigger guards whacking your knuckle is pretty much related to how close your knuckle is to the trigger guard. If you grip the gun so that you leave a little bit of space between the trigger guard and your knuckle, you can probably avoid getting whacked. If you cram your hand up against the rear of the trigger guard, you can pretty much count on getting whacked with heavy loads.
Look at a 1911 grip sometime. There is no space behind the trigger guard and it is impossible to get a finger in there. So the trigger guard can never whack your knuckle.
A lot of shooters buy the Bisley grip because they don't like the plowhandle grip, they like to get their entire hand on the grip and the Bisley grip is longer, so you can get your entire hand on there easily. But if you cram your hand up against the trigger guard, you are still going to get whacked with heavy loads.
Try shifting your grip so that you leave about 1/4" of space between your knuckle and the trigger guard. The discomfort will probably stop.