The Ontos. It mounted 6 105mm recoiless rifles.
It was originally made for army paratroops but after they developed it, they turned it down. It's role was to be a tank destroyer.
The marines picked it up and ordered like 300. And it was to see some action in Vietnam.
Since the recoiless rifles didn't have any recoil, it could fire all six at one time. It was like calling in an artillery strike.
On the top of each recoiless rifle, you can see a rifle. These were targeting rifles, they would fire a tracer that match the trajectory of the recoiless rifle so that you'd know when you were on target.
The marines used it as infantry support.
The main problem with it was the armor was too thin, especially the bottom, which made it susceptible to mines.
