To lessen ear damage, and for other reasons, I'm looking at using my to-be-built 9mm AR carbine as my home defense weapon of choice.  I am also planning to SBR the carbine to the "ideal" length...whatever that may be.  So, my questions are thus:

1.  What is the best length for a 9mm carbine, as far as velocity of the round is concerned?
2.  What rounds will be the best performers our of this barrel? Will WWB 147gr JHPs be a good choice?

Quoted: To lessen ear damage, and for other reasons, I'm looking at using my to-be-built 9mm AR carbine as my home defense weapon of choice.  I am also planning to SBR the carbine to the "ideal" length...whatever that may be.  So, my questions are thus: 1.  What is the best length for a 9mm carbine, as far as velocity of the round is concerned? 2.  What rounds will be the best performers our of this barrel? Will WWB 147gr JHPs be a good choice?

If you are going the SBR route, I would say 10.5/11" max. The 9mm is a pistol bullet and is designed to fully burn in short barrels. Anything longer is overkill and works against the bullet.

For ammo, I would suggest Winchester Ranger or Speer Gold Dot for defense. WWB does fine for plinking.

It's still going to be loud though.

Hollowpoints do not always feed in the Colt pattern guns.