I may be wrong since I'm relying on my memory. It's my understanding that the Hornady XTP has a shorter seating depth and more of the bullet is in the case, thereby reducing the case capacity. This would be confirmed if what your looking at has a lower charge weight for the XTP. The velocity may be listed near each other even though the XTP has a lighter charge. I not sure what a 180gr bracketed bullet is. To be extra safe as your learning, and if you are using different components than what a manual lists, work up your load using data that lists the shorter overall length. You can set you seated length to whatever you like as long as it feeds and chambers fine. If you go shorter than what is listed be mindful that the pressure will rise.
Also make sure your resized brass has good neck tension. For auto loading pistol rounds the crimp (as it's called) does nothing to hold the bullet in place. In fact too much crimp can decrease neck tension causing the bullet to set back (increasing pressure).
