My hunting buddy has been a quadriplegic for over 20 years. He has a variety of rifles and he is always looking for something bigger and better. He had a 7mm mag in a Remington Sendero with a brake on it; did very well for him. Now, he has a 28 Nosler, custom build by Alex Wheeler, weighing in around 12 pounds bare rifle. With a brake, it recoils like a .223 and all it has is a brake on it. I wouldn't limit your dad to a short range caliber as he could really get into shooting and want to go longer. My friend has taken an antelope at around 585 yards in one shot in a 14 mph wind! This year, he shot his antelope around 368 yards from his van with the 28 Nosler. It was loud but with hearing protection it was fine. It didn't break any windows and the noise was harder on me than it was on him. He also uses a 308 and a .260 for whitetail. With a brake, I think anything will work.