Never hunted rats at the dump but would shoot them at my Granddad's grain elevator. Couldn't use .22s because of the danger of a spark/explosion. Did kill many a rat though with my trusty 5mm Sheridan Blue Streak and our colley dog "Duke". I'd blast 'em and Duke would retreive them--quite an assembly line it was. I also liked to go down across the creek on great- grandma's farm and shoot rats in an old dilapidated barn. I would use my "old reliable" Remington 550-1 .22 semiauto and Winchester Super-X LR ammo.
Nothing, however, beat hunting cottontail rabbits in the snow with the same rifle. Used up lots of ammo though blasting away at wild running, albeit, slow hopping snowbound rabbits. Great fun!