And the Montana native looked right at home on Melbourne's Kingston Heath, playing the testing sandbelt course like a local to challenge for the Australian Open title.
Unlike the hyper-intense Martin Riggs, played by the famous Australian actor, a laid-back Johnston cruised through the third round on Saturday, adding a second round of 68 to his opening seven-under-par 65.
"My dad's side of the family has a bunch of R names, so I think you kind of run out after a while and they were watching it (Lethal Weapon) one night, and they were like, 'hey, that's a pretty cool name'," Â the 24-year-old said of his unusual moniker.
A recent college graduate, the world No.954 reeled off five birdies in six holes on Saturday, with a double-bogey on the par-5 14th his only blip in the testing conditions.
Ryggs Johnston had his problems on the 14th hole, racking up a double-bogey. (Con Chronis/AAP PHOTOS)
His co-leader, Victorian Lucas Herbert, spoke of the burning desire of locals to clinch the national crown, and the pressure that accompanies that.
"It looks so easy for Ryggs, who doesn't probably care about the Australian Open as much as I do - with no disrespect to him," Herbert said after his round.
"It just means so much being my home national Open, I'd love to put one of these on my resume."
Johnston acknowledged that he didn't carry the same expectation, but after recently graduating from the DP World Tour Qualifying School said he was used to the stress.
"For a while now I've been in pretty high-pressure situations, trying to get my card through Q-school - that final round is one of the most pressured and I did pretty well," he said.
"I just try to take that with me and use it as something to look back on and kind of tell myself that, hey, there's a lot of people watching, a lot of pressure, but you can still do this.
"Maybe you put a little less pressure on yourself (not being Australian), but I still want to win bad - it would be huge for my career to win."
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