Forward motion is the quickest move, so close the distance, and try to lock out his right arm with my left arm. If successful, at contact range, drawing weapon and firing. If lockout is unsuccessful, BG's one handed grip now has to turn sideways to shoot, which takes time and is awkward. Hopefully, he doesn't want to discharge his weapon in his own face, so he lowers it to shoot the side, which takes more time. In the time it takes for BG to discharge his weapon, draw weapon, at contact range hit with 100% of my shots, and quickly.
We run this drill often, and only the sharpest assailants can beat it.