Metallica tried out quite a few bassists after Cliff Burton's death in 1986 before choosing Jason Newsted. Some of the candidates for the position included Greg Christian (ex-Testament), David Ellefson (Megadeth), Joey Vera (Fates Warning, Armored Saint), and of course Primus bassist Les Claypool.
In an interview with Kerrang!, Claypool said he's pretty sure he would've been fired after a month. So whatever the band would've sounded like with him, would've been short lived.
"Well, however it would have sounded, I would have only lasted for a month or two before they kicked me out. They've got enough chefs in the kitchen in that band — they don't need a guy like me. And the guy they have right now is unbelievable. Robert Trujillo is one of the nicest and sweetest guys I've ever met in the industry, and his playing is unbelievable."
Claypool also said he wasn't super into metal at the time when he auditioned, and only did it because he knew Kirk Hammett from high school.
"I just knew that it was my buddy Kirk's band and that they were doing really well," he said. "It wasn't until the audition that I got a sense of what was really going on. It was a bit of a heavy thing, the audition, and I didn't realize until I got there that it was kind of a big deal. Afterwards I fantasized about quitting my carpentry job and touring Japan with them, but I didn't get the gig."