The answer is not a definite answer because it depends also upon the ammunition. For example Military Ammunition is hotter than Commercial manufactured ammunition as a general rule. But I will give you a for instance on an example.
For a general example:
A 20 Inch barrel in this example we will figure the velocity at 3,000 Feet Per Second.
A 10 Inch barrel shooting the same ammunition would have a velocity of 2,400 Feet Per Second.
So you see that there can possibly be a considerable 600 Feet Per Second difference!
Also understand that the terminal ballistics recommended distance drops significantly also. The reason is most projectiles for the 223/5.56 generally require around 2,700 FPS for proper expansion. Moral of the story! If you go with an SBR as mentioned shoot soft tip tactical or something like the Hornady Tap rounds for self defense. And figure on anything close to terminal ballistics around a max distance of 70 yards and in. Not that I would want to be shot with any of them at any distance. Hope this helps explain the answer to your question.