Two archeologists find a ancient Roman coin on which is shown a chariot and driver. But behind the chariot rises a mushroom cloud. The two archeologists speculate that ancient peoples had such weapons as atomic bombs. Cut to: Two military scientists are designing a new weapon. A catapult or giant crossbow that can deliver artillery shells.
Note: This summary was told to me in the early 1970's by a class mate. I have always assumed it's a real story, somewhere. I believed it was an episode of the old TV series "Night Gallery". But checking episode guides shows nothing like it. Comicbooks? Movies? Part of novel?
It sounds a little bit like a story by William Golding (?) author of Lord of the Flies, from a fifties paperback that also had a Titus Groan story by Mervin Peake. That probably isn't it, but it is as close as I could come.