I think a few people do not 1) realize you are using a climber, or 2) have not used a climber, because you cannot sling your gun over your shoulder and use a climber, especially one with a bar (like a Summit Viper). Even hand climbers require too much movement to sling your gun and climb either safely or efficiently.

I simply attach a haul line to the upper half of my climber. I then run the other end of the line through the trigger guard (the gun's empty---it cannot fire). This ensures that the gun comes up/goes down butt up/barrel down. I have a rubber cover (picked up at the hardward store in the loose bolt section) that I slip over the barrel.

I keep the haul line attached to the top half of the climber, so I don't forget to have a line with me.

And I'll second the concern of slinging a gun while climbing, although I myself will sling my gun when using a ladder stand.