Stock-Still is a patent protected concept.
Every firearm works with the same principle - "for every action, there is an equal and opposite reaction." That is Newton's third law of motion. When a bullet (or any projectile) comes out of a firearm, that firearm is going to move in the opposite direction of the bullet. This is a universal law, and cannot be changed or violated. This backward movement of a firearm is commonly called "recoil." Depending on the size of the bullet, and the velocity at which it moves out of the barrel, this recoil can throw a large person forcefully back. Recoil is an unwanted part of firing a weapon, but it cannot just be wished away.
One way to deal with recoil is to mount the gun to a tripod, and mount the tripod firmly to the ground. now, instead of throwing the firearm backward, Newton's law says that the "reaction" is trying to move the Earth - and no gun is big enough to move the Earth! But tripods are not practical for most shooting situations.
In the patent world we talk about prior art. Prior art is all the things that exist at the time a patent is applied for. In this case the prior art would be all of the pads and cushions and other things that people have done trying to mitigate recoil, but all of these devices and methods at best make recoil less violent but they simply cannot defeat a law of the universe that is immutable.
Stock-still is the very first device that makes no effort to defeat Newton's law of motion, it actually uses the law against itself not to reduce recoil but to completely eliminate it. We call this Recoil Nullification.
Ok, so how does it actually work? A "normal" rifle or shotgun, basically has a barrel (we will call this barrel 1), a stock, and a trigger mechanism. The bullet comes out the barrel - along with hot gases, and the gun moves backward, generally into the shoulder of the shooter. That is the recoil part.
A Stock-Still weapon is a bit more complex. It still has he barrel (this is barrel 1), the stock and the trigger. But a Stock-still weapon has the stock partly hollowed out. Inside this hollow is another barrel (we will call this one barrel 2), and instead of a bullet, it has a piston. At the same time as the trigger ignites the bullet, it also opens a gas canister that forces the piston to move inside barrel 2. As the bullet reaches the end of the barrel 1, the piston reaches the end of its' barrel (2). So the bullet causes the weapon to start moving backward and the piston causes the weapon to start moving forward.
By adjusting the bullet weight, the bullet exit velocity, the piston weight and the pist0n velocity to correct values, The gun movement forward and backward exactly cancels out, and the recoil is ZERO! This has been proven scientifically and mathematically, making Stock-Still the very first weapon that a very small, light person could fire and not get thrown backwards.