The robots are tiny
The enemy have powerful anti-robot countermeasures, (EMP weapons or what have you). So our heroes have found the best way of maintaining stealth is for the robots themselves to be very small. Maybe as small as a spider that can crawl about undetected, and fit through tiny gaps in the enemy's defenses. There's enough room for the electronics, cameras, etc. but not enough for weapons, let alone ammunition.
If you have to keep the small 9mm gun, then maybe make it a bit bigger, but still only capable of carrying a small number of bullets (like a single clip or even less) and the gun mechanism itself. This is considered a kind of 'last resort' or emergency measure as the robot would have great difficulty withstanding the recoil of the shot.
