I've never hunted with anyone needing a loaner rifle. However, I always have a back-up rifle in camp on all of my hunts. At them moment is either a Remington 700 in 243 with a 4-12x40 vxII or a Winchester 70 in 300 Win/3-9 VXII. I wouldn't hesitate to use either as a primary rifle on a hunt.
