Every few hundred rounds is about the right interval for a thorough cleaning of most pumps and semi-autos. Guns definitely need cleaning after a downpour, sandstorm, or mud bath. I relearned this lesson after letting one of our high-school trap team kids shoot my Beretta 391 in a heavy rain this spring. I wiped the gun down, inside and out, adequately, I thought. It wouldn’t work next time I tried to shoot it, and just about every part inside that could rust, had rusted. Luckily, it all came off with some elbow grease and the gun worked fine.