I love reading about ballistics. It combines two of my favorite things in the world: firearms and science. (yeah, big nerd right here)

I just found some great websites. The first one is pretty much a bunch of tables showing how increased barrel length leads to higher velocity. Every inch matters It has some data for popular defense rounds.

I also noticed that they had a section on the left side of their page "other resources" and those links seem worth reading as well. Unfortunately, I can't go through all of these right now since I should be studying, but I hope these help!

Ballistics by the inch:
gives velocities of popular defense rounds based on barrel length and has muzzle energy graphs. Has data for all popular pistol calibers.
http://www.ballisticsbytheinch.com/calibers.html

Brass fetcher:
Videos of bullet ballistics. Great charts and data. And the oh so fun "9mm vs 45acp" comparison. I have not gotten too deep in this page yet, but looks promising.
http://www.brassfetcher.com/