I have a Bushmaster Hbar M4gery which has to have 40,000-50,000 rounds on the receiver. Two FCG’s (just upgrading, but technically added service life via preventative replacement), replaced bolts, bcg’s, gas blocks, & gas tubes with barrels every 5-8k, replaced buffer spring every 5-8k. Haven’t had any failures in it. It’s a garbage disposal. Cheap rifle at the time. Relatively.
Good barrels, good bcg’s and bolts, staking/pinning/loctiting everything which needs staked/pinned/loctited, and replacing consumable parts as they come due seems to keep them happy. I think a lot of the spare parts we see guys keep around aren’t valuable. I have never broken an AR firing pin, and never had an issue with gas rings. Gas blocks come loose if not pinned, especially if not loctited, and bolt lugs break. I’m pretty persnickety about what handguards I use - for the exact reasons
@taliv mentioned. Never happened to mine, but I have seen guys knock a cheap freefloat tube off and bend their gas tubes (of course, 200-250lb dudes banging into barricades will stress a handguard).
One thing I do which I believe helps to ensure good bolt service life is lapping the bolt lugs to ensure high contact. I have seen rifles with only two lugs touching - and naturally, having such little load distribution can’t be good for the lugs.
My bro-in-law had a DPMS Sweet 16 he brought to me after it started failing to cycle sometime after a hundred rounds or so. When I got it out of the case and pointed the muzzle down to carry it into my shop, the gas block slid clear off the end of the barrel. I dimpled the barrel and ran red loctite on the set screw, never slipped again.
He also built an Aero/Odin/cobbled together thing, then couldn’t get anything to feed, and the bolt wouldn’t close on a lot of rounds and misfired on even more. The hammer was closing the bolt carrier into battery, which gave light strikes, and his extractor bevel was too short, so his bolt would dead stop against the case, instead of snapping over. Of course, this all happened on Day One, so once he brought it to my shop and I tuned it, it hasn’t had any issues since. Wasn’t really a cheap rifle, wanna say around $1,000 in parts?
Broken bolts happen. Part of the game. A dude on my squad at a match in TX last month had a lug break - he dropped in a new BCG and picked up the next stage. Also wasn’t a cheap rifle, precision built AR-10’s aren’t cheap. At the same match, another guy with a NEMO Omen had his forward assist come rocketing out of the upper - broke the retainer pin and broke the indexing pawl. Only way I figure that could happen was the pawl was a little too long from factory, or he got gunk in the FA bore which held the FA forward when it fired (or held his thumb against the FA when shooting free recoil), and it was snagging the BCG notches) just enough to eventually cause failure. That’s something like a $5k rifle if I recall. Not sure on the round count for the rifle. Ran fine before, during, and after, just had to replace the FA assembly.
Had a case rupture in a Bushmaster Varmint Special, blew the mag out, had to replace the mag catch, the mag, and my underwear, but everything else was groovy.
I did have a Colt gas key screw break on me once, gas key went loose, didn’t want to fully eject. High round count. New BCG, rather than trying to EZ out the broken stub.
The ONE thing I would say is a common problem for cheap AR’s or cheap AR parts kits is the grooves on the FCG pins are too shallow, so the pins walk out of the receiver at one end, and the FCG’s get sad. A couple minutes with a triangle file to clean the grooves a bit and it never happens again.