The future of warfare is drones and it's going to come down to numbers. Ten million drones swarming a city are going to be easy targets except that there are ten million of them, so it's going to come down to that boring truth of warfare, it's not glory or gallantry, it's logistics.
So a single rail
gun shooting 6,000 rounds per minute at 5,000 miles per hour would be able to basically build a pretty formidable wall against a drone swarm of any size. Might some get through? Yea, but again, it's logistics. A swarm of 100 million 5 cent metal rounds costing about as much as a medium sized pickup truck taking out fifty million dollars worth of drones is gonna be a rout.
So here's how the wall works. The stream of rounds paints a spiral pattern starting in the middle and twirling outwards to the extent of the blocked area before turning into the center again and repeating the cycle. This would effectively create an impenetrable wall of red hot metal as every square foot would be covered. If you were to look directly into the line of fire you'd see a round wall of metal.
As has been discussed in science fiction many times, and as we're seeing now, calculations about who would win any engagement might really reduce actual warfare as increasingly who would win any engagement would be known without having to actually do it.
So would this prevent drone attacks? Well, to areas so equipped, yes.
And here's a quandary: might there come a time when both sides push their respective AI war machine buttons and five seconds later a surrender is announced by one side? "We've calculated that our force of 300 million drones to their 100 million drones plus their "Iron Wall" defense system would result in our defeat." Might need an override button saying "Fight anyway despite our being projected to loose."