I am thankful that this is only an exercise of imagination! And also without asking the next question, (What would be the primary purpose of this one gun?), I will assume for the sake of discussion we are talking about rifles. And if I should also assume a " rifle for hunting" . In that case I would most probably select a Stainless/synthetic bolt, with controlled round feed, chambered for .338 Winchester Magnum. If price were no object....then maybe a custom Mauser 98 to meet the above criteria. If price were an object then possibly either a Ruger or Winchester to meet the above criteria.

If I were to assume only one gun...period!!! For everything....then I might be more inclined to choose an M1A1 in 7.62 NATO, (.308 Winchester). In a pinch...(in most places...PA excepted)....it could be used for taking big game. Or single loaded with handloads for small game. It would certainly excel for personal protection, (what it was purpose built for), but would be nearly impossible to carried concealed...Ha...ha!!!

Thankfully we live, (at least still for the moment), in a nation protected by a Constitution that recognizes man' s birth rights. (Not privileges!!!!) And this is only an exercise of the imagination!