Not being one of the fabled few who are blessed with a lack of nerve endings, my early years were spent mostly with .22's, progressing to 9mm's, then the bigger fellas. Mr. Rat and Mr. Rogan both echo my sentiments. I still shoot lots of .22, they're cheap, mild, and a great training round, or "reminder" of what you are and are not supposed to be doing. I've taught several people to shoot using my Ruger .22, which conveniently have the same basic operating controls layout as my Colt GM (thumb safety, bolt/slide release, and single action trigger), and they all progressed quickly and rather well from the rimfire to the centerfire after dryfire practice, then .22's, to the thumpers.