Calibar is best determined by the game your'e after and the terrain you are hunting . Out west a 30/06 or 270 are very common. Back east a 30/30 is pretty common.I killed most of my elk and deer with an old Belgium made 30/06. I use a .308 Mannlicher model 77 Ruger now, with a 20" bbl. But in and out of the swamps I still go to my Marlin .44 mag with the carrier modified to use Garret Hammerhead cartridges. Again a 20" bbl and no scope, just Lyman peep sights. It kicks a bit but balances like a dream on your shoulder or in your hand. My Guide Gun or .35 Remington are winners but...................smaller/ lighter is better for a 60 year old guy with arthritus.