BTW, stopping rifles are nothing new.
Here is a video of Ian from Forgotten Weapons firing an old 4 bore stopping rifle:
Here is a longer video on 4 bore rifles:
There have been larger and more powerful rifles than these, but 4 bore was the largest and most powerful rifles that you could get that were designed to be mass produced. Larger rifles were one-offs designed to set records or very limited production runs designed for people who just wanted bragging rights to have the biggest and most powerful rifle around.
This 2 bore rifle for example is a one-off exhibition piece:
If you want bigger and more recoil, there’s always the punt gun. This isn’t a hand held rifle though. It’s designed to be mounted onto the front of a punt boat (hence the name) and the point of such a huge gun is to take out an entire flock of geese all at once.
The german Tankgewehr (literally, “tank rifle”, it’s an anti-armor weapon) and similar anti-tank rifles also have a fair amount of recoil. They were originally designed to kill tanks, but then they just made tanks with thicker armor, rendering the rifles obsolete. The anti-tank rifles still saw a lot of use against troop carriers and other vehicles that didn’t have armor quite as thick as that of a main battle tank.
Ian firing a Tankgewehr at a steel plate to simulate tank armor:
A Soviet PTRD-41 anti-tank rifle: