Second move to Machamp?

Rockslide's my gym clearing second move of choice, its much wider in coverage than heavy slam and hits common fighting resistance counters like dragonite and gyarados, only thing it hits for less effectiveness than HS is gardevoir, who seem's to have gone out of use with togekiss out now.

Avalanche? Do you mean Rock Slide?

I mean, if you've already splurged your stardust on all the meta-relevant Pokemon that would benefit from a second move then go ahead. Rock Slide does hit its counters hard, but then again, why not just have Rampardos/Tyranitar/Metagross (The last two have a god tier immunity to Psychic as well) as a backup option for your Fighting type party?

I understand that some people either didn't attend the community days or have enough resources to invest in Rampardos, so then in that case, go right ahead. (Just keep in mind that its counters can still be lethal even with the counter charge move)

I did. But I've got tons of stardust to spare and thousands of Machop candy. I use it occasionally and it saves some time, but if you are short on stardust then you can still just switch your attacker.

Rock Slide is the best PVE rock charge move, and rock provides a lot of type coverage for Machamp.

I have a 100% Karate Chop/Submission Machamp (/brag).
I have Heavy Slam as a second charged move.

When I originally unlocked the second CM, I got Close Combat (ugh), which I CTM'd into Heavy Slam. At that time, Rock Slide was not an option -- I don't have enough CTMs to justify trying to get Rock Slide, so I just make do with Heavy Slam.