What parts might be in it for the extreme use are already out there - it's called milspec, a minimum standard to protect the taxpayer and soldier, which provides a floor of reliability that will endure 5-10,000 rounds. You will never shoot that much, and likely won't survive carrying 500.
Bluntly, no one person will be guaranteed any life expectancy after 30 days. If it's really really SHTF, then there will be little food by then. Water, good luck, after three days you will need to purify every drop, you cannot carry enough of it.
So many of the gun threads totally ignore the logistics of survival - where are you going to get the majority of your basic needs? Water, food, shelter? You can't run and gun enough from one source to another without confronting numerous other people equally concerned about maintaining possession of their meager supply.
What gun you have or use will only take disposable income away from being able to maintain a potable source of water. Frankly, you'd be better off with a water collection system that can hold enough to ride through a dry summer, and a purification method that needs no arcane or hard to find chemical. Rainwater collection and a drum of bleach will be a better investment than ion treated parts. A loading press and brass catcher will do more than expecting to pick up ammo off the dead. No guarantee it won't be .30-30 - America's most popular cartridge.
Just leave it at iron sights, or a simple 2x, if it needs batteries, they will all eventually be used up. Remember, it's SHTF, the factories are all shut down, no transport, no stores, nada. Give that more than a few years, no socks or underwear, either. Your last battery might be used up as you listen to the final radio station sign off in a hail of gunfire.
Complete political collapse would be a picnic, just stand in an area clear cut by an EF5 and see nothing usable as far as the eye can see, then think SHTF. If the infrastructure really goes down, there won't be firefighting, entire suburbs and cities will burn to the ground - set to the torch by looters, opportunists, or those who lash out and want everyone to suffer equally. Natural fires from storms will sweep uncontrolled county wide, just like they used to do.
A working .30-30 lever will be the SHTF gun, it's the one cartridge that can be found nearly everywhere. 5.56 will be hoarded, only .Gov issued after the stores close, and resistance to the Authority Exercising Jurisdiction will be put down severely. Open carry of any military weapon could be met with Shoot on Sight.
That wonderful fantasy gun now the pride and joy of the local District Chief. Stray dogs being fattened for slaughter cracking your bones.
SHTF indeed. Childrens game. Lets start talking Apocalypse, now that's a disaster.
Entirely why these threads are entirely worthless. A gun will be the least useful answer. Clean water, some food, a place to stay warm, and a simple knife are what our ancestors prized in the day. Not having the skills to obtain the first three will kill more people in thirty days than not having a gun.
Focusing on firearms as the solution will create more zombies than survivors.