For some reason I googled "draw down" and found this, it's been around since 1973!
draw down (on someone) v. phr. to reveal or display a firearm and point it at another person; to draw or get a bead on someone.
Citations: 1973 Steve Emmons L.A. Times (Sept. 9) “On Patrol: Weekend of Wrecks That Wasn’t” p. OC6: I thought I was gaining on the guy. I drew down on him, then I thought I don’t have a good enough reason to kill him and stopped. Then I saw it wasn’t the man I was after.
1977 Chicago Tribune (Nov. 8) “10 wounded in one-hour gun spree” (in New Orleans, La.) p. 1: They drew down on him and shouted “Freeze!” and he decided to give up.
1981 Don Hayden Daily Oklahoman (Dec. 23) “Cross-Examination Fails to Alter Skiatook Murder Suspect’s Story”: He still had the gun and could have drawn down on me at any time.
1982 N.Y. Times (Feb. 18) “Miami Judge Trades Shots With a Suspect” (in Miami, Fla.): I drew down on him, but I couldn’t shoot because the victim was in the way.
1983 Kevin Boland @ Minneapolis, Minn. Globe and Mail (Toronto, Can.) (July 9) “The good and bad of being there” p. S3: I guess if you live the life of a gunslinger, well, you always watch your back. You never know who might be drawin’ down on you.
1990 [Bob Kirkpatrick] Usenet: misc.legal (Dec. 15) “Let’s arm Joe and Jane!”: This doof locked and loaded with no prior discussion. My response was to draw down on him. There we stood in a dumb-ass Mexican standoff.
1996 Bill Morris Greensboro News & Record (N.C.) (Jan. 4) “Face-Off With Hege Draws Ruling” p. A1: He jumped out and drew down on me, pointed his gun at me. It looked like a silver .357 Magnum revolver.
1998 [TX Pyro] Usenet: tx.guns (Sept. 26) “Laser Pointers and Kids”: She asked what I meant when I said “draw down on him.” I responded that some people get scared if they see a laser dot on them. They may, respond with a gun and take action if they believe that their life is threatened.
2005 Xeni Jardin @ New Orleans Boing Boing (Sept. 13) “Katrina: a cameraman’s journal in NOLA”: Over and over you hear the same thing….”They just drawin’ down on people”, meaning they are pulling their guns.