Unless his interests change, he will probably be the guy who goes out and buys a box or two of factory ammo, checks zero and goes hunting.
This is critical information. If you decide not to ask him, then get something that's easy to feed off of the shelf in a big-box store. I'd say 30-06, but I chronographed a ton of 30-06 ammo in the few years before the pandemic and it's a lot closer to the 308 than the ammo makers would have you believe.
That said, 308, 30-06, and 270 would be my choices. There are still a few Remington 700 Package rifles available in those calibers and they're quite the bargain. Just replace the scope and mounts and you're good to go.
Okie John
