I.e., .50 caliber is not a cannon shell and 20mm is a cannon shell.
Do we still call them “cannons?” I thought they had specific names like Howitzer, Recoilless Rifle, Anti-Tank gun, etc. If a Colonel in battle shouted, bring up the cannon, would people look at him funny?
Cannon isn’t a term. “Guns” are based on caliber, a howitzer is a gun while an M16 is not (it’s a rifle or weapon).
This is called a cannon So’sthis. Commonly when mounted on aircraft.
Similarly the 20mm OerlikonAA piece of WW2 fame and still kicking around. And sometimes this thing as well.
At one time, cannons were for direct fire and howitzers were for indirect fire. I seem to recall reading somewhere that if the tube can be elevated more than 35°, it’s a howitzer. And nowadays, the Colonel would radio the artillery battery and request a fire mission.
Is it for fighting or is it for fun?
I will rephrase: Is anything over 15mm considered to be either a cannon or artillery shell by the U.S. military? 
I’m unaware if there is an official ruling by the US military on it, but yes, the general rule is anything greater than 15mm is a cannon shell; 12.7mm-14.5mm are heavy machine guns firing bullets, and anything below 12.7mm is a general purpose, medium or light machine gun or small arm again firing bullets. 15mm isn’t a widely used caliber; things usually jump from 12.7mm (the US M2HB HMG) or 14.5mm (the former Soviet KPV HMG) straight to 20mm or greater.
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The really factual answer to your question is that there is no precise definition of what is a “cannon,” and something that is a “cannon” can fire things that are sometimes called “shells” and sometimes not. Is the 120mm weapon on an M1 tank a cannon or a gun? There’s no real definition. Does it fire shells? Well, sometimes. But sometimes it fires sabot rounds, and it can also fire the LAHAT missile although I don’t know if the US Army has purchased any yet.
Common military terms often don’t have clearly defined lines. What’s the difference between a frigate, a destroyer, and a cruiser? There isn’t a clear difference.
Could you make a rough discrimination on the basis of explosive shells vs. inert shells? Pistols, rifles, and machine-guns fire inert rounds. (Slugs, so to speak.) The big stuff sends a package of explosive goodness to the enemy.
(Oops, except for the very basic solid-round anti-tank shell… Do we still use those at all?)
I was informed that a “gun” is a crew served weapon. So, “gun” would be a not quite all inclusive term describing machine guns, cannons, mortars, howitzers, recoiless rifles, etc.
Yes. They’re often made of depleted uranium, are fin guided, and have a discarding sabot.
Meet the M829-series “Silver Bullet” APFSDS-DU. It’s a tank killer.
I find it remarkable that even now in this technologically based society, we still have such imprecise terms for objects. There is no universal standard for what is a cannon shell vs. bullet, or a mountain vs. a hill or a ship vs. a boat. Some see this as a bug and others as a feature.
I wonder if other languages have similar characteristics of vagueness.
Cool to know! I remember when HEAT rounds were new (not to mention HESH!) so I had wondered if the old style was still used. (Adding fins and a drop sabot makes it all new again!) Thank you!
(Why, yes, I do use too many exclamation points!)