I live on an island and so far bringing just one rifle or just a shotgun with a variety of ammo has been enough...all single shots as well.
If I ever got a chance to hunt off island I would probably bring an extra rifle.

Although I have tried others, I still stick with the one I am most comfortable with and I find most reliable: 1944 Lee Enfield .303.
I don't normally carry a back-up at all. If I head to the deer camp, we carry a few rifles/shotguns, but that's in case we fill tags we can switch out to coyotes, pheasant, or other small game. Currently saving up for an x-bolt .270 or 30-06 as have to give a 700 .308 back to my buddy that I've been borrowing for a few years. Then will probably pick up a marlin 336 30-30 for back-up/fun.
My X-Bolt is my primary rifle. My Axis is my backup. Both are 30-06 and using the same ammo so I don't have to lug around 2 sets.
First time I tried bringing two after my buddy talked me into it, I accidentally dropped the X-Bolt (climbing a steep hillside to gain the high ground) and broke the objective lens on my scope(Thank god for Vortex warranty). The Axis was able to get the job done and I will never go without a second rifle again.




























