Barrel length and twist rate are both contributing factors for velocity. The tighter that twist is, and the longer that barrel is, the longer and faster that bullet starts spinning.... For example, a .308 with 175gr bullets with an 11.25" twist 24" barrel will produce higher velocity and greater accuracy at distance, b/c it won't start tumbling as soon as say a 11.25" 20" barrel...
Yes, all different calibers have different FPS variations depending on twist, barrel length, load data, etc... All the different calibers that use the same diameter bullet all have differences. You have to get VERY specific when it comes to your gun. Every gun is individual. Every gun will shoot different, even if it is 2 identical guns made consecutively by the same gunsmith. It's the nature of the beast.