What episode of Starsky and Hutch has them pose as gundealers and have a gunsmith reactivate a submachine gun?

The episode you're referring to is titled "Guns" from Season 3 of Starsky and Hutch. In this episode, Starsky and Hutch go undercover as gun dealers to investigate a series of violent crimes. They enlist the help of a gunsmith to reactivate a submachine gun, which leads to various twists and turns in their investigation.