How much more velocity does a 20inch barrel produce compared to a 14.5 inch barrel?
Any other info on the subject would be helpful. Thanks

This should give you an idea:
www.ammo-oracle.com/body.htm#ballmil

After you are done with that section, read the whole thing...

Only about 100 fps?  Ive heard that because of the short barrel, velocities are not high enough to cause hydrolic shock as in the 20inch barrels.

The difference in yards for tumble velocity is 75-80 vs 175-200 +/-

Wa????  Explain please

The longer the bbl, up to a point, the greater the muzzle velocity.  A 20'' has greater muzzle velocity and velocity over 2700fps or so is necessary for fragmentation. A shorter bbl rifle will have a  lesser muzzle velocity and as the speed of the bullet drops off its ability to fragment diminishes resulting in a diminished "effective range". It is fragmentation that accounts for more severe wounding capability.
As a result of this, the round drops more for a given distance. Acurracy is not effected by a shorter bbl, drop off is.

The ammo oracle is a must read.

What he said.

For M193, Bushmaster?s figures indicate a 195fps difference. Here is their chart:

www.bushmaster.com/images/faqchart.gif

This is a very significant difference with M193, and especially M855. This is because, even though a 20? bbl will only increase velocity by about 6% over a 14.5? bbl, these rounds need to be traveling at least ~2,700 fps to fragment on unarmored targets, so the maximum range of fragmentation can be doubled or tripled due to this difference. 5.56mm FMJ bullets do relatively minimal damage beyond their fragmentation range.

Refer to this section of the Ammo FAQ:

www.ammo-oracle.com/body.htm#fragrange

Expanding bullet types, as well as some heavy Open Tip Match bullets, are less sensitive to velocity in terms of terminal performance.

The moral of the story is: if you really have to use a 14.5" for defense, avoid M855 like tha plague, use M193 only if you have to, and REALLY use 64gr soft points, or the 69-77 OTM rounds.

Use 40 grain Hornady VMax rounds....

What about a 16in barrel!  I always thought the mid-length AR carbine was grossly underrated.