How does barrel length, seating depth, and powder charge effect velocity (Its complicated) part 1

Sit tight this is a long one. In today's video I test different 9mm bullet weights in a 3.2" and 4.7" barrel. I am using Hodgdon longshot powder, a shotgun powder but also a slow burning pistol powder to determine what effects barrel length and bullet weight have on projectile velocity. I briefly go over how pressure inside the cartridge plays a role in how fast a bullet goes. I plan on doing a part 2 with much faster pistol powders to see if the results stay the same.