I use the 40 gr HP MaxiMag from CCI. Its easiest to get around here and shoots great out of my Marlin 25M. I took a small woodchuck cleanly at 110 yards this spring with a body shot that dropped him right in his hole. The other day I dropped a crow at 85. Also used it on a squirrel head shot last fall. Its pretty much my do everything gun around the farm for squirrell, racoon, and woodchuck. Hope to put it to use on fox and coyote in the future.
I've been pleasantly suprised by this rifle and the 22 WMR. I bought it to replace a Ruger 77/22 Hornet that I tinkered with forever with different loads and could never really get to shoot.
I bought the Marlin for $175 used, slapped a 4X Tasco and bought a box of CCI MaxiMag that was the cheapest ammo on the shelf. Right away it shoots 1 hole 5-shot groups from a sitting position at 25 yards and not much worse at 50. Something to be said for simplicity.