All my pellet guns are .22. Just happened that way, and not a conscious decision.
If you're stuck with a pellet gun, Walmart sells Gamo Rocket pellets which are simple lead pellets with what looks like an almost 177 BB in the nose.
Biggest I've killed were skunks with them, but I love the concept. They will make chipmunks spread their guts over a decent area if you hit them wrong.
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