Bullets have short range. So now you have to carpet the land with AA guns.

It'd be interesting to see how short that range really is.

A lot of assumptions about range were based on the idea of a soldier shooting at another soldier, more-or-less at a horizontal level. You had to design a bullet to accurately hit a target and disperse kinetic energy into biological tissue.

Now, you're aiming at something made of non-biological materials of varying size, but they're usually lightweight and have little in the way of redundant flight systems. There's a real chance that if you send up enough small arms fire, you could hit a drone at up to a mile in the sky and cause it enough damage to be unable to complete its mission.

Helicopters are known to be vulnerable to small arms fire. I don't see why an even smaller drone would be any different.