`HighVoltage` only really just adds the Rails built in `caches_action` with a check to see if `HighVoltage.action_caching` is turned on or not. However your point about the private method, it isn't really a "private" method per say but the prefix of `_` certainly looks to imply it.

Perhaps a better alternative is to mock the `ActionCacheFilter` which is internal to `ActionController::Caching::Actions`.