Replica Firearms
Replicas are classed as "replica firearms" and "replica firearms" are classed as "prohibited devices."
A "replica firearm" that is designed to look like an "antique firearm" is not classed as a "replica firearm."
Unfortunately, many "antique firearms" are duplicated by modern-production firearms -- so a "replica firearm" that looks like a firearm that was once made and is an "antique firearm" also looks like a controlled firearm that was made yesterday. There is no way of resolving this problem outside a courtroom.
All flintlock rifles and shotguns -- but not handguns - are "antique firearms" even if made yesterday.
There is no penalty for being in possession of a "replica firearm." There is a penalty for manufacturing or transferring or advertising or offering to sell a "replica firearm."
It doesn't HAVE to make sense. It's GOVERNMENT POLICY