The heavy gun armament of some small escorts can look like a provision for air defence. Some of their rapid-firing weapons were intended to deal with E-boats that came close enough to engage. Those guns could still be described as anti-aircraft weapons. Their designation therefore does not by itself explain the surface threat that helped determine an escort's armament.
Notable Point · 1:57-2:21
Why small escorts carried anti-aircraft guns against E-boats
Some rapid-firing weapons on small escorts were intended to counter nearby E-boats even when described as anti-aircraft guns.