Deadlands characters can make attacks of opportunity at range. The reach of a firearm and its threat area is anything within its first range increment. Any actions that would normally draw an attack of opportunity within a weapon’s first range increment are eligible for getting’ a cowpoke lit up like a Christmas tree. Penalties for cover apply normally and you have to see your opponent to shoot ‘em (DC 10 Spot check, modified by cover, Hide, etc. Note that characters who are involved in combat are distracted for the purpose of the spot check and receive a –5 to their roll.) which is why so many folks get to hidin’ when the lead gets to flyin’.
Example: Johnny Gunn shoots Black Bart, wounding him in the leg. About 30 feet down the road, Ed Evil decides to come out from behind a wagon and dart into the saloon, to get a better shot at Johnny. Johnny gets a Spot check against Ed to see if he notices the movement. It’s a DC 10 check, with –8 to the roll because of the distance and because Johnny is distracted by combat. He rolls a 19, meaning he can take an attack of opportunity at Ed as the outlaw moves. Johnny waits until Ed gets into his second square of movement, in the middle of the open street and out of the half cover of the wagon and the nine-tenths cover of the saloon and shoots.