What's the difference between a gunshot and a shell?

not exactly, unless you consider everything that fires a projectile a "gun" , like would you consider a tank barrel a gun. Shelling is usually from field artillery, pretty much a cannon, I would not consider this a gun. I would consider a gun as a hand held device that fires a projectile, the people that are getting "shelled" are getting shot at from artillery that is much much larger, unless you talking about mortar tubes.