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What is the difference between an inertia trigger and a mechanical trigger?

Inertia triggers require the recoil from the first shot to set the single trigger system up to fire the second barrel. This is commonly called "selecting". This is the most common trigger arrangement for both selective and non-selective single trigger systems. Selective systems allow you to choose which barrel fires first. Non-selective systems are normally designed to fire bottom, or in the case of side by sides, the right hand barrel first.

Mechanical trigger systems do not require recoil to set up for the second shot. With the most common design, movement of the sear as it is disconnecting from the hammer disconnect the trigger connector from the firing sear thus allowing the same trigger connector to move to the second sear.

Both, inertia and mechanical systems have good and less desirable features. The inertia system is less prone to the phenomena; bounce firing (also called fan firing, doubling, etc.). But, most standard inertia systems will not function when recoil is reduced to lighter gages; i.e. 12 gauge down to 20 gauge. Another negative feature is there is no second shot if the first barrel fails to fire. The mechanical trigger doesn't depend on recoil to select to the second barrel if the first doesn't fire. But, they are more like to have bounce fire occurrences if the shooter does not employ correct and proper mounting, handling and/or trigger control.

Most inertia trigger double guns can be converted to mechanical systems utilizing the disconnecting connector method. A few guns, like the Beretta 680 series, are converted by altering the inertia system, making it so sensitive that the jar of the hammer striking the firing pin upsets the connector allowing it to connect to the second barrel sear.

At the present time, we do not know of any shotgun that we cannot convert from an inertia system to a mechanical system.

These pictures illustrate both top and bottom sear systems.

June June
Rizzini with over-mounted sear
SKB SxS with under-mounted sear

Average labor time for most mechanical trigger conversions: - Beretta model 680 style (simulated mechanical system): 2 hours - Most over (top) mounted sear systems: 3 hours - Most under (bottom) mounted sear systems: 4 hours - Hammer modification systems (GAMBA STYLE): 5.5 hours & up