Understanding the Benelli “Inertia” Action
Benelli's semi-auto shotgun action has been represented and misrepresented in many ways. Here is a quick look at what it really is (and really is not). Its description is memorialized in the August 12, 1986 US patent no. 4,604,942: “Bolt assembly with a rotating locking bolt head and a floating bolt element for automatic firearms.” Referenced are several previous patents, including the Williams patent no. 2476232 of July 1949.
Fundamentally, the Benelli action is a recoil loader with fixed barrel. See the Bruno Clarus U.S. patent of March 25, 1913. It is also related to Delayed blowback actions that go back to the Mauser Broomhandle pistol of 1896, while this particular embodiment is the Bruno Civolani platform from the 1960's. We have a floating bolt body in the action with a spring on both sides. The rearward spring is the standard recoil spring that closes the bolt on most autoloading shotguns.
In the well-known Browning A-5, rearward thrust from the shell being fired moves the barrel and the breechblock together, as a unit, to the rear. With the A-5, movement starts immediately, which is why it is one of the fastest cycling actions there is and faster than the Benelli. A properly set-up A-5 is also fairly soft shooting, as the shock absorber array dampens the moving mass considerably. A fair appraisal of the Browning system shows why it is not viable to manufacture: it requires a lot of machining, lots of parts and cannot dynamically compensate for different loads. Extremely heavy loads through the Browning with the friction piece and bevel washer at the “light” setting will punish the shooter and the gun, though after all these years we know the A-5 itself does not seem to care.
With the Benelli, the rotating bolt head is locked into the barrel. What is allowed to move is the bolt body itself that resides between springs. When the shotgun fires, all of the abrupt reverse thrust brings the entire shotgun straight back, not dramatically different from a fixed breech gun, as that what it is at that point except for the floating bolt body. The floating bolt, however, tends to stay where it was, as it is not locked to the rest of the gun. The gun flies back and the inertia of the bolt body causes it to stay where it was, smashing against and compressing the bolt head spring, billed as the “inertia” spring. Now that the primary recoil pulse is over with, the spring decompresses, sending the bolt body to the rear of the action dragging the bolt head along behind it. The rear recoil spring returns the bolt into battery with the barrel, just as it does on an A-5 or a gas-operated auto.
It is simple and extremely cheap to manufacture, far cheaper than the A-5 system and most gas operating systems. It has another benefit, as well. The faster an A-5 barrel comes back, the more shock to the system. Same deal with gas autos, the faster the bolt moves, the more shock there is to everything connected to it: links, struts and the like.
Not so with the Benelli. The bolt head spring gets full compression regardless of the load fired (as long as the recoil force is heavy enough to fully load the inertia spring). As a result, bolt speed is not varied by the load; it is driven only by the spring. This means that the floating bolt body is propelled backwards at a constant speed controlled by the spring. Hotter loads will not punish the floating bolt body or the back of the action. No struts or links to stress and no possibility of peening the back of the receiver with heavy loads.
Is the Benelli system reliable? The answer is yes and no. Like the A-5, it requires a firm shoulder mount to recoil against, something that gas systems do not need. Like limp-wristing an automatic pistol, the lack of a firm mount can cause problems. A good comparison is to the mechanical versus recoil-set (or inertia) single triggers in a double-barreled shotgun. One can be as good as the other in a practical sense, until the application turns to the 28 gauge and .410 bore. In those cases, there is often insufficient jolt to cycle the triggers from barrel to barrel. Mechanical triggers remain the preferred approach in small gauge guns for that very reason.
The bolt (bold body) of a Benelli does not slam home with the authority of an A-5 or most gas guns. It is a fairly soft return, for a reason. The recoil spring (return spring) has to be weak in comparison to the little bolt-head spring that shoots the bolt back in the first place. If it were not, the bolt would not reliably come back with enough vigor to eject the empty hull and recock the gun. With an A-5, adding weight to the barrel can impair function--sluggish barrel movement--and you may have reliability problems. In the case of the Benelli, adding weight to the gun can do the same thing. Soft, sluggish rearward movement during firing and the bolt body may not be able to fully compress the bolt head, or inertia spring. Light loads may also result in unreliable function for the same reason.
Whether a Benelli delayed blowback action or the classic A-5, if there was no recoil the gun wouldn’t work. In the case of the A-5, the weight of the receiver and the buttstock has no effect on operation, but the mass of the barrel does. With the Benelli, function is not contingent on barrel weight, but on the weight of the entire gun and the power of the shell. Add substantial weight to the gun and the cycling of the gun may be hampered. As to the question of whether a Benelli kicks, of course it does. If it didn’t, it wouldn’t work.
The advantage of the Benelli is few parts, making the action super-cheap to make. Not only can they easily be made light in weight, to a certain extent, making them light is necessary for function.
Like all blowback or recoil-operated guns, there is no gas porting or valving to clean or maintain. One bolt-head spring prevents the wide load versatility afforded by a compensating action, particularly on the low end. The Benelli action is cheap to mass produce, durable, low-maintenance and lacks the action bars, struts and linkages found in gas guns. You can appreciate the lack of stressed components.
Copyright 2009 by Randy Wakeman. All rights reserved.