the act in which one person provides sexual stimulation for another person by making a gun with their fingers, before thrusting the 'gun' in and out of another persons holes while making a shooting noise with their mouth.
John: "I was out last night, and this dirty hoe asked if i would finger gun her, what the fuck!?"
Person 1: "I was finger gunning your mum the other day and.."
Person 2: "Wait, what!?" *Beats the shit out of him*