Of course. AR barrels can be had in just about any length you might want. But on an AR rifle build you are into NFA rules with a SBR. Not impossible but extra work and $200.
But if you take that 9" AR upper and put it into a pistol configuration, you don't have to worry about NFA.
Believe it or not, this shows a pistol NOT an SBR. Nobody said NFA laws make sense. They just are... But put a stock on it, and it magically becomes an SBR in the view of the BATF and the law.