Requiring a license prior to purchasing a firearm will maintain a citizen's Second Amendment right while creating more accountability for the firearm.
The licensing process must serve to demonstrate that the citizen has met the applicable state requirements to legally obtain a firearm without changing those requirements.
It can be modeled on many state's practice of a graduated licensing process to obtain a driver's license in which provisional permit holders are required to demonstrate proficiency with a firearm and good moral character in addition to previously established requirements before being allowed to take possession of the firearm.