One of only a few known to exist, NOT classified by ATF as NFA! Of 22 made, all but those few were destroyed/confiscated in 1982. Open bolt fire, slam fire design, read more about this amazing shotgun...
The easiest way to understand open bolt design is to think of it as the opposite of the closed bolt you see all the time. Your semi auto pistol, rifle or shotgun is safe when the bolt is "locked back" and you can see into the (often empty) chamber, right? Then, you flip the slide release, or push the shell carrier, or insert a mag, etc., the bolt closes, and you're ready to rock.
When the OPEN BOLT gun is closed, it's usually safe! In closed bolt designs, the firing pin either floats or is held below or behind the primer contact point until the bolt is closed and the trigger pulled. In open bolt designs, the pin extends as you pull the trigger, or is already extended in the open bolt, and your "PULL" both closes the bolt and fires off the round.
By far, the main reason people love open bolt designs is that they make it relatively easy to convert a semi into a full auto. This has absolutely nothing to do with the original intent of the design! Open bolt designs keep guns cooler between firing sequences, and create very simple and easy trigger control group designs with many fewer levers, pins and springs than closed bolt TCGs. Neither design is really "better," as there are many guns that do well with both. Virtually all large belt-fed machine guns are open bolt due to the belt feed channel design, cooling between VERY hot firing sequences and simplicity of trigger design.
The SLAM FIRE shotgun you read about here is one of the few open bolt guns that are grandfathered ("pre-ban") and available without NFA paperwork. Virtually no other shotgun in the world used this design, until very recent (and partially classified) high tech new military shotguns resurrected the design for full auto shotguns, some which fire explosive shells with virtually no recoil due to shock absorbing spring designs. From the early trenches of WWI to MidEast terrorists today, many enemy combatants who have little fear of the .223 are scared shortless by shotguns. The Germans threatened to execute any American Marine in the one brigade that fought in WWI if he was caught with a trench gun, and complained that war conventions do not permit shotguns. The war ended before anyone was executed, and before any answer was given on the shotgun complaint.
Of historical interest: The British Secret Service put out an instruction sheet for captured friendly forces in rapidly occupied European countries during WWII. The sheet detailed how to replace the existing firing pin on several European semi auto pistols with a slightly larger version that would create an intentional open bolt slam fire. When a soldier was about to be taken prisoner or executed, he locked the semi open in a "surrender" gesture, pointed it at the enemy, and when the bolt was released, the action closed and slam fired, emptying the clip in full auto from the open bolt, potentially allowing escape or "taking a few with me." God bless the brave ones who tried it!