I'm not sure if this was a movie, book, or TV show. The basic scene was a standoff where the protagonist (or possibly their mentor) vibrates the metal in the chambered bullets to jam the enemy's guns and make a quick escape. All of the details are hazy except for the bullets being ever so slightly expanded to jam the guns.
In Doctor Who: The Sontaran Strategem, the Sontarans use a "cordolane signal" to cause the bullets to jam in the guns of their enemies.
The figure reveals that the room is contained within a cordolane signal that excites the copper surface of the bullet and causes expansion in the barrel, rendering their guns useless, as well as blocking their radio signals (during which time Gray tries using his pistol but finds that is also useless).