John Nelis, 38, took the party drug GHB before he shot 39-year-old Christopher Jacobs in a caravan in Geelong in July 2020.
Nelis saw his good friend was wearing a bulletproof vest, but it had not been properly fitted out with anti-ballistic panels.
He pointed the shotgun at Mr Jacobs' chest and abdomen, and fired without checking to see if it was loaded.
A witness saw him standing before Mr Jacobs holding the smoking gun.
"I didn't know it was full - it wasn't meant to be loaded," Nelis said.
Nelis then left the scene and disposed of his weapon before taking more GHB and heroin.
He managed to hide from police in regional Victoria for 20 days before his arrest.
Just over a year later, facing a Supreme Court trial, he pleaded guilty to negligent manslaughter.
Justice Lex Lasry said Nelis' failure to help his friend, knowing he was in real trouble, did him no credit.
He told Nelis that drugs were ruining his life, and any hopes for rehabilitation were directly linked to whether he could break his drug addiction.
Nelis will serve a non-parole period of eight years.
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