Linear algebra.

Chain shot was a thing. Two cannonballs attached by a chain (or by a bar making the projectile look like a barbell) were used in naval combat to cut the rigging on the sails. They were fired out of a regular cannon and were one of several attempts to make attacking the rigging on ships more impactful.

I have no idea why they’d try a double barrel field gun for a solved problem. The ammo did have an issue where it caused damage to the barrel and was ridiculously inaccurate compared to ball shot, but I don’t see how this design solved that. The chain would have had to be six feet long at least for this gun and the ball would certainly run into it on the way out of the barrel.

I bet they wasted less money on this than the fabled Sgt. York M247. It was an experimental intelligent anti-aircraft gun that did things like lock targeting onto the spectator stand during the demo, or insisted that the latrine fan was a slow moving rotary aircraft.

Yes. that was the argument put forward. Similar arguments have been put forward for almost every military and major terrorist action ever taken. People can subscribe to the justifications, or not, as they see fit. The real thing to be cautious about is if you accept such justifications but only when your country is the one making them.

I haven’t seen the film yet so I don’t know if they get into this, but a large number of the scientists involved with the Manhattan Project were working because they were terrified that the Nazis would build a bomb before the Allies. When, for several reasons, that failed to happen, they were relieved that the bomb wouldn’t have to be used. They felt betrayed when it was used against Japan, who were not developing a bomb and who could have been defeated using conventional means.