From what you've seen, what are the first parts (4 or so) to fail or degrade in a higher end AR rifle, from most to least?

Parts coming loose due to no locktite.
Mags
Springs
Cam pin or bolt

Detents and Detent springs. Though, those went wrong when I tried to "tear down" my rifle without knowing what I was doing. Good practice and invaluable knowledge however.

As I'm buying spare parts, relative to other pieces I'm good on magazines.  Do I go BCG's galore?

Extractor
extractor spring
Gas rings are considered consumable I think but I've only replaced them once and I'm not sure if it was needed
Too much copper fouling screws with accuracy, you gotta clean the bore

That's pretty much it but I only shoot 4k rds per year approx.

my trigger finger

on a non shrouded Colt bolt carrier, the firing pin got pretty beat up on the shoulder... only real part I've ever broken was an extractor spring, which had disintegrated...I had included a neoprene 1/8"  "O" ring and the carbine continued to function... over pressured white box ammo, flame cut around the firing pin hole pretty badly on the same carbine...I've replaced a couple of bolt cam pins when the wear patterns started to look funny

brand new gas rings are thinner mid circumference ...if you've never looked at a replacement pack...new rings taken off  look like they are worn

Extractor and gas rings are the only items I have ever had to replace.

My firing pin retaining pin I have had one of those break.

Parts you should have on hand IMO

Gas rings
FP retaining pin
Set of hammer and trigger springs
Spare Bolt
Extractor

Hell I have a spare BCG myself just in case I want to build and cannot find one or I need to do a quick swap.

Just buy a new bolt and a LPK and your all set.

This.

Maybe add a bolt carrier group service kit.  It well have a cam pin, firing pin retainer and all the bolt small parts.

Can't really disagree with this at all.

I have never had these parts break, but I replace them after 10K rounds or every other year to be safe (I don't pay for my ammo, you do.  Thanks).
I use Colt factory parts from work or Brownells if the Armorer doesn't have what I need.

- Action Spring
- Extractor
- Extractor Spring

These are wear items that cost me about $20 to replace (if not free).  I would rather replace them before they fail than have them cause my weapon to fail.

I inspect the bolt, firing pin, cam pin when I clean.  Replace as needed?  I never have had anything break, but I have seen lugs break on weapons with over 5k rounds (and mucho FA from a 10.5 barrel) on coworkers weapons.  The weapon still functioned and the broken parts were found during cleaning.  I have never seen a broken firing pin, cam, or lower parts fail on an issued weapon (all have been A1/A2, M4, and 10.5 Colts) or a "professional grade" personally owned weapon.  Having said that, I have never had an issued weapon for more than 5 years without having a new one reissued.

If I had a personally owned weapon that saw an average of 5-6K rounds a year then I would probably replace the entire upper receiver group, including the bolt/bolt carrier group and replace the trigger springs every 20K rounds or 5 years just to be safe.  If you think about it that is approximately $6,000 of ammo so spending $600 on replacement parts is about 10% of your costs.

On commercial grade weapons I have seen broken bolts at the cam pin hole, trigger springs snap, safeties break, gas keys come loose (pretty common), and most commonly popped primers using 5.56 ammunition in out of spec chambers that were marked "5.56" but were not.  Most of these issues happened under 1000 rounds the first time the weapon was put to some hard use in a training environment.  

If you only plan on shooting 100 rounds a month your weapon will probably last forever if you can can get through 500 rounds in one afternoon to test it out.

The #1 reason for malfunctions with the AR platform are lube related.  The #2 reason is bad mags that need some hammer love.  Keep your weapon lubed (wet) and throw away your old mags when they start to fail.

If you are really worried then I would get a complete spare bolt and call it a day.  Action springs and extractors fail over use and time, I have never seen them just suddenly fail and cause the weapon to not work.  I have seen that with broken bolts and loose gas keys, but you should be able to avoid that by purchasing quality parts from the start.

YMMV,
Rob

Here's my parts maintenance schedule for my AR, the goal is to prevent problems from occurring.

2.5K rounds interval, inspect and replace as necessary:
extractor spring
extractor insert
extractor O-ring (if in use)

5K round interval replace:
extractor spring
extractor insert
extractor
extractor O-ring
bolt gas rings
action spring

10K round interval replace:
bolt (bolt assy only not the bolt carrier)
lower receiver parts

15K round interval replace:
barrel

You could just get a bolt assembly not including the bolt carrier, but if you have extra coin get a complete BCG.

I had never broken any AR part with my 41 years rifle and 27 years carbine mostly Colt experience with my previous 4 pre ban rifles and 2 carbines, usually I sold mine at 10K rounds because I just wanted to have a newer configuration model.

I had one rifle with 15K and another 20K rounds when I sold them, I just replaced the worn parts with my noted schedule.

+1 The best spare is another complete AR, but if you only have one rifle now get at least your minimum spare parts then get another one (or several).

Ref: Here's some links of the thread "What Parts Break in a Carbine Course" and the ARMY TM 9-1005-319-23&P tech manual (pdf file).


http://www.ar15.com/forums/t_3_118/360169_What_Parts_Break_in_a_Carbine_Course_____Thread_summaries_are_on_pages_48_49.html&page=1


http://www.ar15.com/content/manuals/TM9-1005-319-23.pdf

The operator is usually what fails first…

shooter