A factory Sheridan CO2 rifle I believe would put them out there around 590 to 600 ft./s. This is a little hotter at around 675. When that 20 caliber pellet hits them. It’s all over.
Out past 20 yards, the chipmunk looks so small through the peep

if you can find one of the old Sheridan CO2 rifles, I would highly recommend it.
mike