Tyler Forsyth of Heatherton stormed a Mulgrave McDonalds armed with a sawn-off bolt-action military grade rifle.
The 20-year-old demanded money from counter staff and then made two customers kneel before threatening them and taking their wallets and cash.
But after the admitted drug user turned his back on the pair to stuff a bag with more than $700 in cash from the restaurant's cash register, the terrified customers struck back, tackling Forsyth to the ground and holding him until police arrived.
Forsyth pleaded guilty at the Victorian County Court to possessing the unregistered firearm and three counts of armed robbery.
Forsyth told the police he was forced to rob the store by three men who he owed a $700 drug debt.
He declined to reveal their identify to police but said he was remorseful soon after his arrest.
"I feel like a dumbass," Forsyth said during his police interview.
"I put other people in a situation I would not like to be put in. I made them fear for their lives," he said Forsyth was remanded in custody to be sentenced at a later date.
