Melbourne Rebels sergeant-at-arms Matthew Bruce tried to sell the SKS assault rifle to an underworld associate and had his partner, Cursty Shields, film him firing it for promotional purposes.

"Do you want to play with daddy's gun?" Shields had jokingly asked the couple's one-year-old daughter in January 2019, according to extensive police surveillance.

Bruce went on to send his twin sister, Sarah, a photograph of himself holding the rifle in his bathroom, with his infant in the background.

The 36-year-old trafficked ice, codenamed "cold", and cannabis, called "green" while driving a Holden Commodore HSV with the number plate "FEARD".

Speaking about the unsophisticated codenames in the County Court of Victoria on Monday, Judge Bill Stuart quipped: "I don't know why they bother using a code."

"I would have thought they'd been broken years ago," the judge added.

Bruce had also been trying to sell the military assault rifle, at one point telling an associate: "I need to pick up my guitar bro."

The bikie enforcer pleaded guilty to offences including drug trafficking, possessing a trafficable quantity of unregistered firearms, negligently dealing with the proceeds of crime, arson, attempting to obtain property be deception and discharging a firearm in the three months before his arrest last February.

Judge Stuart labelled the video of Bruce firing the military assault rifle "clueless".

"He has no apparent understanding or recoil, he did not lean into the gun. Such a deadly weapon shouldn't be in the hands of what appears to be a novice," he said.

The fact it was to be sold to another criminal associate made the whole thing even more dangerous, the judge added,.

Bruce's crimes included torching two cars with his twin on behalf of an associate at Harkness on Valentine's Day last year.

Bruce and Shields helped commit a drive-by shooting in the same suburb days later.

That month, Bruce, his sister and his partner also attempted to dishonestly obtain more than $11,000 from RACV through insurance fraud at Melton South.

Shields has pleaded guilty to offences including drug trafficking, possessing a trafficable quantity of unregistered firearms, discharging a firearm and attempting to obtain property by deception.

Bruce's sibling has also admitted to trafficking drugs, attempting to obtain property by deception, arson and cultivating what Judge Stuart described as "a lonely, ring-fence, immature" cannabis plant found at her home.

"It's pretty humble," he said upon being handed a picture.

Bruce remains behind bars and is expected back in court on Thursday. His sister, who is on bail, is also due to reappear.

Shields will face court again on Tuesday.

Australian Associated Press