Having run the Gunsmithy at Gunsite for 15+ years, I got a pretty good look at quite a large sample of guns and what goes wrong. So, here's a generic list of what is needed to keep guns running.
Springs. Springs are consumables just like ammo. You should have at least one each of every spring in the gun, personally, springs are cheap and I keep two. They break all the time, I replaced a LOT of springs.
Extractors. High wear commonly broken item, second only to springs. If it needs a pin for installation, get the pin as well.
Firing pin. Rare issue, but possible if the original has a flaw. I've probably replaced three or four over 20 years.
Type specific items:
AR-15 types. Gas rings. Keep several sets and replace them every 5k rounds or when they loosen to the point they will not hold the bolt carrier on when held vertically. Extractor pin and bolt cam pin, FP retainer. If the carrier key is properly staked from the outset, you should be good, but if it's not, get spare screws, replace the existing ones and stake it properly now. A spare charging handle is not a bad idea, but it's pretty rare to damage it. I just keep a complete bolt in a Magpul pistol grip compartment, simplifies field/match repairs.
Glocks. Spare slide stop. Double up on slide lock springs and trigger return springs. Replace the recoil spring every 5K rounds. If you have not already done so, install metal sights.
1911. Fit a spare extractor now before you need it. Get a slide stop and firing pin stop. If you are familiar with the plunger tube installation, that's not a bad item to keep on hand.
If anyone has a model specific question, feel free to ask, but this covers most everything actually. Absent battle damage, abuse or loss, springs and extractors will keep you running.
You don't need a gas tube, gas block, bolt carrier, trigger group parts, barrel, etc. If you are going to keep a complete parts kit, frankly it might just as well be a complete gun.
