During a traffic stop, if a motorcycle gang member has a firearm with a scratched-off serial number, what crime is he committing?

The possession of a firearm with a scratched-off serial number falls under the category of misconduct involving weapons. This is primarily because altering or removing a serial number from a firearm is illegal and denotes an intent to conceal the firearm's identity, often associated with illegal activities or unregistered weapons. This action implicates the individual in serious criminal conduct, as it obstructs law enforcement's ability to trace the firearm to its original owner or manufacturer.

The other alternatives do not accurately describe the situation. Possession of a controlled weapon typically pertains to owning certain types of restricted weapons, whereas illegal possession of firearms usually relates to possession without a proper license or by someone prohibited from owning firearms. Grand theft auto specifically concerns the theft of motor vehicles and is not relevant in this case. The specifics of the firearm's serial number make "misconduct involving weapons" the most precise choice in this scenario.