The best way I can describe it is a larger version of a Dead Mule. It's not a Dead Mule product but works like one in that it has a spring-loaded weight that moves back when the gun bottoms out against your shoulder and is then propelled forward, "pulling" some of the felt recoil with it. It's roughly twice the length and diameter of a Dead Mule, which adds to its effectiveness as the weight being moved is heavier. I honestly did not weigh it before I installed it but it is not real heavy - six to eight ounces, I suppose.
Obviously, installing one in a gun stock will require a lot of room, especially length-wise. Beretta stocks are roomy as they have what appears to be two stock bolt holes drilled in them, one above the other. The top hole is where the bolt is located and I don't know what purpose of the lower one is. Weight reduction, perhaps? Together, they result in a large oval opening and all I had to do was make the lower hole an inch deeper and cut an extension from the device that I assume helps position it in the gun for which it is intended. I secured it in the stock with a pan-head wood screw with a flat washer under its head into the floor of the stock and bearing against the end of the device.
I've always thought that Dead Mules were the most effective recoil reduction device out there. I have one mounted on the magazine cap of my 870 and obviously can remove it quickly for comparison testing. Even weighted magazine caps that weigh a lot more don't dampen recoil as much as the Dead Mule. For me, anyway, they work better than weight alone or mercury reducers. In fact, I tried a 16-ounce (yes, one pound!) mercury reducer in my gun at one time and felt very little improvement. This Beretta gadget really works.
The left-handed Beretta shooter who shot my gun wants one for his 682 in the worst way but Beretta does not sell them yet. I'd imagine that as soon as the gun for which it is intended is released, the device will be available as a replacement part.
Ed