MD has a "list of regulated firearms" if the rifle is on that list then you need to take a safety course, apply for permission to own/transfer it, get "Non-dis-approved" by the PD & then do a waiting period.
Now this covers transfers It gets really iffy if there is no "transfer" within the state of MD.
Pre-owned rifles that come into MD as owned property aren't usually considered as being "imported" but again this is variable.