>>1531 (OP)>Would it be hypothetically possible to have smaller types of shells that can be shot from a gun? RealisticallyYes, there is a element that has the physical properties that gets you down to about the size of a marble for spicy fun with fission.
The element isn't naturally occurring, but it is not that hard to bread it artificially.
The downside is that this stuff emits a lot of heat, constantly, because it's got a relatively short half life and you have to store your spicy bullets in a powerful refrigeration unit, or you'll have an extremely bad time.
Remember nukes make plasma, they're not actually explosives, the enormous shock-waves from the big boy nukes are from thermal expansion of heated air. The stuff i'm talking about mainly emits ionizing radiation, powerful enough to melt a tank, but it would not make a big boom. So if you want a radio-active puddle of liquid metal, this is it.
If you are wondering why i'm being a little bit coy about some of the details, that's because there is a somewhat cost effective path to mass-production. There's already places on earth that are contaminated because some people thought depleted uranium munitions were a good idea. This would be a thousand times worse.
If we are talking scifi, you can make a fusion bomb the size of a rice corn, assuming you have anti-mater as the "ignition source". Hardly any contamination, even compared to conventional chemical based weapons. 30 soldiers could raze an entire city. In the long run we'll have to get better at peace or we'll extinct our selves.