WWII the Nazis had a couple of these that ended up being destroyed before they were used.
They are also multi chambered. There is the main charge that starts the round up the barrel. As the round travels it passes other chambers that fire off pushing the round faster. I believe they had about 10 chambers.
HBO had an original movie on Gerald Bull and his super gun, it was a good flick but can't remember the title, it's likely available on video though if anyone can remember what it was called.
Also, "60 Minutes" or equivalent gave a good bit of time to Bull's efforts.
It is suspected by many that his murder was done by agents of Israel's Mossad. Given Israel's quite-intelligent policy of pre-emptive self defense, that suspicion seems reasonable to me...
I think his murder was CONFIRMED to have been carried out by the Israelis... It wouldnt have been the first time they struck "The First Blow" against Iraq, and the people working for them.
I remember watching a special on the Discovery channel. There was a hint of blame directed at the US government for not giving him the funding. That's sorta like blaming Chamberlin for Hitler. Chamberlin was damned if he did and damned if he didn't. Just turned out he was a wimp. Because the US didn't fund the guy's efforts and he turned to anybody else does not make us responsible. The guy just turned out to be a prostitute.