Today's competing operating systems for semi-automatic shotguns are inertia and gas. One relies on recoil the other on the tapped pressure of the propellant gasses.
In the beginning, 1903 to be exact, John Moses Browning's Auto-5 operated on a long-recoil system. Hidden by age and the steam roller of Browning's invention was the Normal shotgun, launched in the same year 1903, that operated on an inertia system developed by the Danish gunsmith Christer SjÖrgren.
SjÖrgren's shotgun operated by means of the breechbolt being held in in battery by a spring, and when the spring was overcome by the rearward inertia of the gun, it released the bolt to travel rearward ejecting the fired shell, and then picking up a fresh round from the magazine.
Over half a century later Benelli adopted this system to their very successful line of semi autos. Today, in 2015, Franchi, Weatherby and Browning have all adopted this system as well.
Gas operation came about as a result of World War Two. There, dubbed "The greatest battle rifle," by General George Patton, the M1 Garand was what our troops relied on to get them through the war.
The Garand operated by bleeding off a small amount of the propellant gasses that then impinged on a long rod that drove the breechbolt to the rear ejecting the fired cartridge and on the rebound loading a fresh round. In the 1950s, High Standard came out with their Flight King that I saw as a boy in the local Sears, Roebuck store. Remington followed, as did Winchester, Browning and a host of others.
Gas operated shotguns bleed off gas much nearer to the action because of the considerable difference in gas pressure between rifle and shotgun ammunition. The latest wrinkle is Remington's VersaMax, now superseded by their V3, where the gas ports are located in the chamber.
The V3 has eight ports that are brought into play by the length of the shell: A 3 ½-inch magnum uses only one port, a 3-inch several more and a 2 ¾-inch load uses all eight. Gas ports, regardless of their orientation all involve a piston linked to some kind of bar/s that forcefully travels rearward under the pressure of the gas. This, in turn, causes the action to open, push the breechbolt rearward and complete the cycle.
Pros and Cons
Each style of operating system has its pros and cons.
Gas-operated systems have gained a wide following because in the time taken up as the various parts work, the recoil is also spread over that period, softening felt recoil.
Newton's laws of physics indicate that recoil is recoil, but spread out, it feels less, hence gas-operated shotguns from Remington, Beretta, Browning/Winchester and others have been embraced by competition clay-target shooters; but don't be fooled, the lesser felt recoil also translates to the blind.
On the negative side, it's inescapable that gas-operated semi autos must be cleaned much more frequently to be reliable. This is partly due to the carbon and unburned powder that are blown into the action by the bled propellant gasses.
Nevertheless, a gas gun kept clean and devoid of excess lubrication is as reliable as the sun rising in the east.
Inertia systems are very clean in operation. By the time the bolt opens, the ejecta has cleared the muzzle, and there is little residue that remains in the action.
Too, the inertia action is very simple with few moving parts, or as one friend once said, "There's nothing in there," and maintenance is quick and easy. However, recoil is the problem, since there is no spreading of the rearward forces as with the gas system, and due to the lack of gas pistons and action bars that add weight, felt recoil is increased.
Inertia-system manufacturers have added collapsing stocks, high tech recoil reducers such as in the Benelli Ethos, soft combs, high-tech recoil pads and other standard and optional features in order to tame the forces of recoil.
The conclusion is that both systems are reliable, and perhaps the decision of one over the other depends on how it feels to you and me, how it swings and most important how well we shoot it. Good hunting and be safe!