Everyone knows my preferences for heavy caliber medium vel rounds.
For backup, go for something that is either easy to reload and has a lot of rounds, or something that packs one hell of a punch. You either need the ammo to fire a lot and keep heads down, or the punch to fire once and remove a head.
I like my Soviet Simonov SKS for emotional reasons. It's the only gun I've ever used, and I'd had it since I was boy. I trust it. I suppose Americans would call it a semi-automatic rifle, but to me it was always just my gun. In some countries, every boy was given a bicycle. Where I grew up, every boy had a gun.
Current research project: I am translating the Gospel According to Lazarus.