There is restricted xcr's just like ACR's the owners simply had a new short barrel made and sent in the paperwork to the cfc for the status change to restricted.
The caveat being you are instructed not to use the rifle (it remains in safe) until the paperwork has been processed by the cfc. Just because you have 30 days to report a status change does not make it legal to use in that period of time after the modifications were performed requiring the status change