Quoted:
"You can still make an SBR on new firearms, and if it is a centerfire rifle then it must be over 30" total length."
If it's a registered SBR, it can be less than 30".
If it is semi-auto and a new Centerfire Rifle (purchased/received post 4/4/13) in CT then it cannot be less than 30" or it is considered an assault weapon. Public Act 13-3 states that you must have been in lawful possession of the Assault weapon
PRIOR TO 4/4/13.
Example: You cannot go out and buy an M1 Carbine today, fill out a Form 1, and after approval cut 6" off the barrel which brings the overall length to 29.5" because you would have created a new Assault Weapon.
You could, however, cut 5" off the barrel, bringing the overall length to 30.5" ....
(iii) A semiautomatic, centerfire rifle that has an overall length of less than thirty inches;