They tried using colored gas in the original version. The problem was that they were filming in low light in most cases, so when they fired the gun you didn't see anything. You just heard a hiss and the actor would drop, they then switched to green powder for the visual effect. There were two versions of the gas gun. One where he preformed a weapons check, it had a removable magazine. For the firing version, they ran an air hose up Van Williams sleeve to the gun.
David.