.243 would be a great option for some reason.270 kicks like a mule mine has more felt recoil than the parent case 30-06. If the he has settled on AR platform 350 legend is really easy just need a barrel and the mags to switch a 5.56 to 350 legend
.270 is loaded to a higher pressure than the 30-06, so the muzzle blast is probably just a bit more prominent. Felt recoil should be similar, I forget the equation, but stock design and weight can make a difference from rifle to rifle.
I'm going to have to put together a 350 Legend too, but I think I want to use .358+" bullets and the factory barrels are supposed to be 9mm/.356 so I need to think on that for while.