how long does it take to transfer from police officer to forensic ballistics expert?

[untimelyfarewell]

Suppose someone decides to get a degree in criminal justice with the intention of becoming a forensic ballistics expert, it seems to be easier to do by becoming a police officer first. So, how long on average would they have to serve as a police officer before being eligible to transfer to forensic ballistics instead? The sooner they're out of harm's way the better, but I'm curious nonetheless. thanks