Some of the crack may be due to breaking the speed of sound (about 1100 fps), but I noticed someone said the CB ammo is much slower than that. However, be that as it may, an airgun that shoots faster than the speed of sound will make a very audible crack too, just like a whip will. This added velocity may not get you much in the practical sense except more racket.

My personal airgun hunting preference is a .20 cal Beeman R1. This is a top-quality accurate, extremely dependable and longlasting combination for plinking or filling a pot. Not only that, but there is an extensive manual on tweaking, repairing, and maintaining the R1 that was published some years ago by the operator of a very popular airgun website (The Beeman R1 - Supermagnum Air Rifle, by Tom Gaylord). It's a worthwhile addition to any R1 owner's survival library.