Iraq nearly constructed the world's largest gun this way.

The US spent a few decades pursuing enormous cannons as a space launch platform. As in, rocket equation be damned, give something high speed and high altitude by sticking it in a tube atop a dump truck's worth of gun powder. The main guy was really just into cannons. So when that project ended, he went into private industry as the Space Research Corporation, which unsurprisingly wound up making guns for normal gun-related activities. Ironically those too were slowly made obsolete by rockets. Unable to give up on big guns, he sold arms to apartheid South Africa, and when he got out of jail for that, he sold arms to Saddam Hussein.

And when I say arms I mean 150m long cannons permanently pointed at Israel. You would say he died under mysterious circumstances, if you are the sort of person who thinks Columbo is a whodunnit.

Anyway, the gun segments were manufactured in Yorkshire by a respectable major forge provided with final blueprints. British authorities became suspicious of the "plumbing equipment" because it was rifled.