Largest Gun Evar!!!
Pfft. I'll do you better:
the V-3 cannon
I can't even fathom how big the V-3 would have been if it'd been finished.
More than that, they'd probably have deafened the Allies to death with it!
Legend has it that something in the Gustav gun range was used to bombard Sevastapol with radiological shells, causing the Russians to threaten the Germans with poison gas.
The 'V3' high pressure cannon still has a much smaller calibur than the Gustav though
I didn't know about that legend. I thought they used Schwerer gustav to bombard Sevastapol with conventional explosives.
The US military experimented with sabot-shot canons (project HARP), a legacy of Gerald Bull in an attempt to loft small satellites into orbit. Unfortunately, there is no way any of these guns can get even close to a projectile velocity to 17,000 mph (7.6 KM/Sec, needed to achieve Earth orbit). They must augment the flight with rockets.