A shell is a type of munitions that has a bullet a type of propellant and a fuse and primer. So I would say most people would understand either term used in a conversation
the why are shotgun shells called shells?
'cause they are hollow and filled with Propellant and Shot!
But we are talking about anti-Aircraft-Guns,a Horse of an entirely different color!
Anti-aircraft guns are weapons designed to attack aircraft. Such weapons commonly have a high rate of fire and are able to fire shells designed to damage aircraft. They also are capable of firing at high angles, but are also usually able to hit ground targets as well in a direct fire role.WIKI
I used to work for OERLIKON/CONTRAVES,and they would definitely look askance at you of you called those Shells,Bullets!
Also served in the Artillery,and definitely wouldn't call those 105mm-and-155mm Shells Bullets!
Bullet is SOLID,a Shell ISN't!
They were actually filled with some nasty High-Explosives,and produced Fragments out of this world!