Dude, I love "The Rifleman". I watch it anytime I come across it.
If I may make a suggestion. Go with a 308.
It's got plenty of oomph to get out to 500 yards (just gotta know your drop and adjust accordingly).
It will punch through any close cover easily (lots at long range too) i.e. walls, car doors, body armor, helmets (blue ones to be specific), etc. etc.
It's a short throw bolt so the rifle is fairly compact.
It's a super popular cartridge so in a SHTF or TEOTWAWKI situation it should be plentiful (i.e. abandoned gas station or walmart in rural America)
Their are multiple reliable platforms for it. M1A, FAL, CETME, AR-10 (DPMS, ARMA, etc. etc.) or savage if you got the dough.
It's just shy of what the 7.62 russian will do.
It's a nato round so you can buy surplus. Plus the surplus isn't corrosive (don't quote me)
In my opinion 308 reigns supreme as far as a battle rifle is concerned.
Here.
http://www.rguns.net/rifles/rifles-semiauto-ar15-complete-308.shtml
Of course, your in line behind me. I placed my order months ago.