One of the largest was the WWI "Paris Gun" developed by the Germans. It fired a 210lb shell about 81 miles. The gun weighed 256 tons. It was not very accurate but an effective psychological weapon.
Other than a toy gun or water gun, the next most powerful one a citizen can have without a permit is a "B.B. gun".
The most powerful bomb ever made was the tsar bomb.
577 T Rex
star cannon
The magnum .55 is the most powerful, but some Desert Eagle has the highest calibre bullet.
Fluorine is the most reactive.
10 gauge shotgun
Probably the siege gun.
It is the dragon destroyer.
It depends if the memory from the caster is happy enough.
Hydrogen bomb is the most powerful bomb
The most powerful water type is Suicune.