What exactly is the difference outside of the obvious increase in velocity. I was reading about the Sturmgeschütz III and the article was talking aobut how its' 75mm went from a low velocity to a high velocity weapon. Does that mean its barrel was lengthened? I read that the longer the barrel the greater the velocity of the shell. Is this correct? Excuse my ignorance as I have just started to read more in depth about weaponary.