You can also shoot through a stack of old automobile tires.... But there goes your mobility. My favorite 22LR ammo is standard velocity, subsonic. It's more accurate and quieter than the high-velocity stuff, but still probably louder than acceptable for this particular situation. It can be silenced with a suppressor, and that may be your best option. But suppressors are more heavily regulated, even if legal in one's jurisdiction, than I am willing to deal with. That makes the CCI Quiet an attractive option.