Taken from the patchnotes, some rifles that don't have an short barrel option were omitted, which of course is fine.

-- Most assault rifles will now be able to equip short barrel options (some ASR models have been omitted as their real-life counterparts do not have short barrel options).

But the ARX200 seems to have the option for a 12" barrel. If possible, it would be nice to have this option added to that rifle too sometime

Reference:
https://www.berettadefensetechnologi...00_-_cbq12.pdf