The main difference between them is seen in the finish and the sights. The Beretta 92FS' standard finish is Black Bruniton, with a black synthetic grip. On the other hand, the M9's standard finish is blued with a synthetic grip.
The M9 is actually a military feature 92FS. It is used by the US military.
Stay safe & have a great day!

by James G - Lifetime Points: 81180
Hi and welcome. A Beretta M9 is essentially the military designation for the Beretta 92FS pistol, meaning they are largely the same gun with the key difference being that the "M9" signifies the model specifically adopted by the US military, while "92FS" is the civilian market name for the same design; both guns share most features and parts are interchangeable. Cheers and stay safe!