RE: 5th Caliber opinion: The .22 rimfires.
I have far more 22's than any other caliber, other than my military collection. A 22 was probably the first rifle I ever shot and unless I die young, it will probably be the last caliber I can handle in old age. I was looking through my 22's the other day, 40X's, 52 Winchester, Anschutzs, Sakos, Kimbers, CZ's, and on and on. I love them all!