The M811 is an automatic, externally powered, single- or dual-fed autocannon firing NATO standard 25mm ammunition, designed by Nexter Systems (previously GIAT Industries).

Its intended use is in an air defense or direct-fire role, mounted on either naval or land platforms, replacing existing 20 mm cannon where applicable.

The Model 811 has been installed on the Nexter Vextra 8 × 8 technology demonstrator for trial purposes.

Operation of the Model 811 is by an external electric motor that drives a camshaft lying inside the receiver.

According to Nexter Systems, the accuracy of the Model 811 25 mm is such that all rounds fired will hit a 1.5 m diameter target at a range of 1,500 m.