It is like 9mm, but for rifles!

Since Nutnfancy ended the debate about the best caliber for a handgun, we can now move on to rifles:

"The .223 Rem. is a great cartridge. If you understand its limitations and versatility it is indeed a great one-gun choice. I know it's what I would choose. If the situation arose where I needed a bigger gun, I'd just borrow one."

So to summarize, don't get the largest round and don't get the smallest either. Something in the middle is the best in most situations.