Adding armor that blocks small arms fire is straightforward, although even this might add too much weight to a BD robot. Armor that blocks a .50 caliber rifle

Adding armor that blocks small arms fire is straightforward, although even this might add too much weight to a BD robot. Armor that blocks a .50 caliber rifle bullet is a completely different story; a Barrett 50 round will go through an engine block. If such a round hit the robot anywhere, it would almost certainly disable it. Detecting an approaching robot would be trivial with radar, IR (these things have to make heat), tripwires, etc. Not sure what you mean about "setting up" a 50 cal gun; a Barrett 50 is handheld. Setting it up takes about a second. Although in practice a perimeter would probably be established in advance with robotic guns using closed loop feedback. My point is that it would be a significant engineering challenge and at least an order of magnitude cost increase to design a BD-style robot to deal with even the most prosaic threats of a hostile battlefield. And the power supply would probably need to be a gasoline engine rather than batteries just for reasons of weight and endurance.