Headshots are OP against bots.
I tend to prefer the liberator, I set it to burst or even single fire, calmly ADS, and then one shot all normal bots, one by one.
You can take down the piloted walkers in one shot, too, if you can hit the guy riding it.
Bigger bots I tend to spam bullets at a bit more, but for them too, headshots are extremely effective.