I don’t believe FN makes a pistol version of either the SCAR 17 or SCAR 16. Therefore, putting a barrel less than 16” on it, regardless if it has a brace or not, would make it an SBR and would have to be registered with the ATF.
This would apply to AR platform rifles as well. Factory receivers that leave as rifles have to stay as rifles unless registered. Simply putting a brace on it doesn’t make it a pistol
The FN America SCAR 17/17S Barrel Assembly features a 13" barrel. No other dimensions are provided by the manufacturer, I'd recommend reaching out to FN directly for more information.
No sence it's not sold as sbr it's not pinned.
The twist is 1/7 for the 13 inch barrel assembly.