I use to use a 12 gauge, but the park police complained about the noise. Did not do my RV much good either. So, I switched to a fly swatter. Turned it into a "search and destroy" game. Personal best is 132 kills in one day. We had a fly problem when we first got here. Turned out there were wild burrows in the area, who would leave their "calling cards" as they cruised through the campground at night. Once I cleaned up all the wild life fecal matter around our RV, 200' in all directions, the flies moved on to what they perceived to be better surroundings. Not much of problem now. Flies are attracted to human waste, dead things, garbage, and animal feces. Get rid of and/or control those four items, and flies won't have much interest in your area. They will go "bug someone else".