Gucci Mane -- Random Prisoner Tries to Free Rapper ... By Playing Lawyer

It's like the prison version of "My Cousin Vinny" -- a random inmate in South Carolina has just prepared legal docs to get Gucci Mane out of jail ... and they're hilarious.

51-year-old Jerry Lewis Dedrick -- who's currently locked up in a Williamsburg federal prison -- has taken it upon himself to get Gucci off gun charges (without Gucci asking) filing a scribbled motion.

To his credit, Dedrick has been spending a lot of time reading -- not like he has other stuff to do -- and claims he's discovered the key to Gucci's case.

Several problems -- first, his argument makes no sense. Dedrick rambles on about how possessing a gun in itself isn't a federal crime.

Dedrick isn't Gucci's lawyer, so he can't file docs on the rapper's behalf. But Dedrick wants the court to forward his legal work to Gucci's lawyer ... hoping it will crack the case.