Armour will slow a robot down, and will often have weaker spots. The right kind of particle accelerator (think e.g. a miniature railgun) should be able to accelerate small pieces of metal to a high enough velocity that they could deflect AK rounds. The smaller the pieces are, the more can be carried. If each robot carries 500 pieces each, then a squad of 5 robots can deflect 2500 AK rounds, which corresponds to quite a few AK magazines.
If the robots are quick (which is easier with less armour) to disable identified targets, then not that many counter-projectiles need to be fired, anyway.
There will of course be downsides and upsides regardless of which design one chooses.
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