An autonomous robot is actually already achievable. All you would have to do is give it a camera, a gun, and a simple computer brain. The camera scans for targets (which can be identified in many different ways such as thermal image, motion+size detection, etc), the gun is pointed at the target, fired. Very simple. The unit could be stationary to guard specific spots, like a sentry. Or it could be a roving mobile unit. You could make an automated machine gun that is small enough to be moved by a single person. Set it up so that when activated it automatically points at targets and shoots them. So a soldier could set it up then when enemies approached get behind deep cover and activate it. Seems like it would be really hard to disable a tiny gun with no person behind it while its shooting at you.

Of course whoever set up such a system would have to be totally ruthless and willing to destroy human life with wanton abandon. Thank goodness no such people exist with the technical expertise to set up such systems, right?

You could make an automated machine gun on a little tripod that is small enough to be moved by a single person. Set it up so that when activated it automatically points at targets and shoots them using its camera and computer and a few small motors. So a soldier could set it up then when enemies approached get behind deep cover and activate it. Seems like it would be really hard to disable a tiny gun with no person behind it while its shooting at you.