Incorrect, by your photo it appears 2" wide at the base...not the widest place.
However, comparing your first picture vs. the first picture in my first link and your diameter of the base it would appear to belong to a 3" 50 caliber.
Edit to add: NavWeaps 3" /50 What they are listing as VT ammo means Variable Time. That was the name given to the fusing / round to hide the fact it was Radio Controlled Proximity.