Final answer:
The .243 Winchester is the smallest caliber rifle allowed for big game hunting.
Step-by-step explanation:
The smallest caliber rifle allowed for big game hunting is the .243 Winchester.
This caliber is commonly used for hunting deer and other medium-sized game. While the .223 Remington can also be used, it is typically not recommended for big game hunting due to its smaller size and lower stopping power. On the other hand, the .30-06 Springfield and .270 Winchester are both larger calibers that are commonly used for big game hunting.