I don't remember a robot, but I remember a cowboy that would do something similar to what you described, but he was located behind Splash Mountain where the Pooh store is now. You put your money in the machine and these saloon doors would open. I can't remember exactly what he said but if you missed he said something like "Hide the women and children cockeye's got a gun" and if you killed him he said something about pushing up daisy. Thats all I got for ya.
I also remember this cowboy in the starcade. He wasn't there very long, but he was there. My brother always wanted to play it, but never did because it was a lot of money for us little kids that only had our little allowances to spend!
Yes, there was a robot gunfighter in the Starcade and a cowboy gunfighter in Teddi Berra's arcade. Both were similar in nature and peformed the same way. I liked the one in Critter Country because of all the constant insults he threw at you. His name was Johnny Rio. Lots of guests would pull the gun from the holster even though they were not going to play and Johnny would say "Hey, hey, bushwacker, put the gun back in the holster, next time there better be money in the box." Put a coin in, the doors would open and Johnny Rio would speak, "I've been lookin' for your beady eyes! My name's Johnny Rio, the baddest, toughest, and if it weren't for you, the ugliest ombre around." He had other insults like "you better try that again ya cross-eyed prarie gopher" and "you're useless as lips on a chicken, come back when your eyes are open." If you cheated, Johnny would call you out, "hey, ya cheated me, now your'e gonna pay!" The robot in Starcade threw insults but the metallic voice dampered the effect. I read a thread once on a.d.d. where a CM felt the need to get even with Johnny after hours and he climbed in there and hit him a few times.
As far as Disneyland rifle games go, I miss the ones that used to be in Frontier Mercantile. There were about six games in there, three of each kind. The first was an Autumn scene with two tree trunks. Squirrels would "climb" up the trees and you would shoot at the red crystal target on their sides and they would fold back when hit and reset at the bottom of the tree. The second game was a bear chasing a dog and they would run around and around a small forest scene. You would shoot at the bear. When the bear was hit he would raise up on his two legs, exposing another target, turn, lower, and run in the opposite direction. There are times when I examine the wall where they used to be. I miss those homemade shootin' games at Pieces of Eight and Teddi Berra's. They were fun and had original soundtracks. Fortune red is about the only original coin-op in Pieces of Eight now.