For anyone who has never shot a machine gun outside of Call of Duty, it looks like it would be easy to shoot down small UAS. The vital components of the plane like the engine, battery, receiver, fuel tank, etc. are very small. The main body of the plane is pretty tough and can take numerous hits without affecting it.
This footage was filmed in Arizona at the Big Sandy Machine Gun shoot. These tiny RC planes planes zip back and forth along the 1/4 mile firing line and are an irresistible target for the shooters.
The pilot is very skilled and his defensive manoeuvres make it very difficult for the gunners to predict where to lead the plane.
Source: YouTube