What kind of gun?
If it's a shitty Canadian Tire/Walmart gun, just use shitty .12's you get at Walmart.
If it's a real airsoft gun, don't use anything lighter than .20's, and get good ones like BB Bastards.
"Someone in a Prius tried to race me at a stop sign the other day. I couldn't believe it. I had him for the first 100 feet or so but I can only walk so fast."