The obvious answer is the AR-15, or something like an HK416, MP5SD or, VSS Vintorez. All guns used by groups like SEAL's Delta and Spetsnaz. Hardcore dudes who actually do escape, evasion, and reconnaissance.
I did not bother reading the scout rifle thread yet as I'm crunched for time lately, but a while ago I designed a rifle similar to what you are thinking possibly.
It's basically a bolt action AR, but I'd like it to have a quick change barrel system so when moving you could just pull the barrel and fold the stock to have 2 pieces that will fit in almost any backpack. But the main reason the gun has a barrel change system is 2 things, 1: variating lengths. So if you were in CQC scenarios you could run a 10-14 inch barrel with heavy .300 blackout subsonics, then move to a sniping position and swap the barrel to a 20-24" .556. Which leads into the second reason which is to quickly change calibers. You may wonder why I chose bolt action, well it's mainly a necessity when wanting a barrel or caliber change system. Simply fiddling with gas impingement would be too hard. But there's also the huge benefit in accuracy, repeatability, and sound. A bolt action is the absolute most quiet you can get usually so it's a perfect fit for a scout scenario. I never designed this with a end of days scenario in mind but more of a SOCOM style weapon, but I just thought I'd mention it as I think it fits what you're looking for.
I did not bother reading the scout rifle thread yet as I'm crunched for time lately, but a while ago I designed a rifle similar to what you are thinking possibly.
It's basically a bolt action AR, but I'd like it to have a quick change barrel system so when moving you could just pull the barrel and fold the stock to have 2 pieces that will fit in almost any backpack. But the main reason the gun has a barrel change system is 2 things, 1: variating lengths. So if you were in CQC scenarios you could run a 10-14 inch barrel with heavy .300 blackout subsonics, then move to a sniping position and swap the barrel to a 20-24" .556. Which leads into the second reason which is to quickly change calibers. You may wonder why I chose bolt action, well it's mainly a necessity when wanting a barrel or caliber change system. Simply fiddling with gas impingement would be too hard. But there's also the huge benefit in accuracy, repeatability, and sound. A bolt action is the absolute most quiet you can get usually so it's a perfect fit for a scout scenario. I never designed this with a end of days scenario in mind but more of a SOCOM style weapon, but I just thought I'd mention it as I think it fits what you're looking for.