The Rocket Gun was a bit of an odd choice as the dedicated AT weapon in BF1 since they had to take a lot of creative liberties with it.
IRL, it was called the Vickers 1.59-inch breechloader and wasn't much more than a prototype. It had a much longer yoke and much larger mount than depicted in BF1. The mount was also attached to the middle of the gun instead of on the front like in the game. Granted, it was technically possible to modify and lighten the gun in such a way as to carry it around like in BF1, but as far as historical documents go, that was never done.
All that being said, it's pretty darn far-fetched as a weapon in PF. If they were to add a big handheld cannon, they might go with the Japanese "o-zutsu," a huge matchlock.
I strongly agree on the ZIP22's Rat shot. Seeing how difficult is to get a kill with the Zip (unless you got them trigger fingers) I feel like 10,000 will take if not like a year depending on your skill.