View Full Version : What are the most common failure modes of the AR15?
I'm looking into what spare parts to buy to future proof my current set ups.
What are the most common failure modes in an AR15? What spare parts is one most likely to need? What is the best source for these parts?
Right now I'm not very experienced with rifle failure as I've only done 2 classes and my most used rifle only has 5k through it.
MC_Oper8or
01-25-21, 01:20
Bolts. Bolt parts. Barrels. One of every spring and detent at least.
Find then wherever you can at this point. Primary Arms is my go to.
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titsonritz
01-25-21, 01:22
Focus on the bolt/BCG, that is most of the heavy lifting is done, get gas rings, extractor, spring & insert, firing pin, FP retaining pin, cam pin, ejector, extra bolt or two if possible. Beyond that, spring and detents. And barrels.
ETA: My go to place for parts would be Primary Arms, BCM and Forward Controls.
vicious_cb
01-25-21, 02:11
I'm looking into what spare parts to buy to future proof my current set ups.
What are the most common failure modes in an AR15? What spare parts is one most likely to need? What is the best source for these parts?
Right now I'm not very experienced with rifle failure as I've only done 2 classes and my most used rifle only has 5k through it.
User error...
I keep enough parts to build up another set of receivers.
Extra gas rings, extractor w/spring and insert, firing pin, firing pin retaining pins, bolt, buffer springs, and hammer springs.
MikhailBarracuda91
01-25-21, 08:38
Nobody mentioned gas tube yet.
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The most common stoppage inducing item is the magazine.
PMAGs are the most reliable, so stock up on those.
The easiest way to get a bunch of spare parts is to pick up a complete BCG.
All good suggestions above. I would say the first and most likely problem area would probably be related to extraction, so extractor spring, insert, extractor, o-ring (if you use it), etc. An easy way to address this is to simply have a spare bolt (with extractor, gas rings, etc. already installed). An even better way would be to have a spare bolt carrier (complete with bolt etc.) because that would address some other common sources of malfunctions.
If you don't know, there are many different types and brands of extractor springs. It would be a good idea to find what works in your guns, and stock up on the exact same ones.
Don't overlook good magazines and good ammo for your guns. You didn't ask, but those are areas that are much more likely area to cause malfunctions in my humble opinion.
Joe Mamma
All good suggestions above. I would say the first and most likely problem area would probably be related to extraction, so extractor spring, insert, extractor, o-ring (if you use it), etc. An easy way to address this is to simply have a spare bolt (with extractor, gas rings, etc. already installed). An even better way would be to have a spare bolt carrier (complete with bolt etc.) because that would address some other common sources of malfunctions.
If you don't know, there are many different types and brands of extractor springs. It would be a good idea to find what works in your guns, and stock up on the exact same ones.
Don't overlook good magazines and good ammo for your guns. You didn't ask, but those are areas that are much more likely area to cause malfunctions in my humble opinion.
Joe Mamma
Thanks for the additional suggestions. I mostly use pmags or OKAY surefeeds. I have some spare bolts but I will look into extraction related parts.
Thanks so much for your help.
User error...
That's valid. We are always a failure point. Once hunting season ends I'll get back to more carbine classes. I'm always interested in learning more.
Nobody mentioned gas tube yet.
Is that really a common failure point?
Is that really a common failure point?
I've never had one fail but I want to let more experienced people discuss it
MikhailBarracuda91
01-25-21, 11:42
Is that really a common failure point?I've never personally had one fail. But I know it's a maintenance item, if you put lots of rounds through a rifle
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Gas rings and buffer springs are really the only items I've changed out over the years as a true replacement.
So the most common maintenance items are going to be gas rings and extractors. And maybe to a slightly lesser extent, ejectors and main springs.
Then the bolt and the cam pin can both fail without warning. You don't have to worry about it at all with a 20", a 16" you can count on many thousands of rounds, but a 10" is a little bit of a crap shoot, especially depending on what bolt you have.
You should keep an entire spare bolt carrier group. You probably don't need the carrier itself, but gas keys do occasionally come loose, and it's no more expensive to buy an entire BCG than to buy the bolt and all the small parts separately.
Barrels. Again, kind of depends on barrel length and lining. The gas port eroding is the issue with short barrels. You will most likely end up with excessive bolt speeds on a 16" carbine before anything actually breaks or starts keyholling or anything like that. So you'll want to keep some tungsten weights on hand to heavy up your buffer for when that starts to happen, which will extend the life of the barrel for probably another 5-10 thousand rounds.
Now that's chrome lined of course. With stainless or CMV you're going to shoot out the barrel before or around the same time that your gas port is getting to be toast. And of course with 20" barrels gas port erosion just isn't a problem. And it's a lesser problem with 16" mid, but at the expense of some ruggedness in terms of being able to power through the extraction if the chamber gets a little bit dirty. A properly timed carbine with the correct extractor parts is a workhorse in terms of chewing through ammo in less than ideal conditions, like running dirty because of a suppressor, for example.
I'm a big fan of Colt because they are super durable and not much more expensive than the cheapest out there, but of course LMT's enhanced stuff can't be beat, though it is pricy. Like you can get several Colt BCGs for the price of one LMT. And the LMT does still need replacement parts, and you'll be paying 60 dollars each for extractors, which would buy an entire milspec bolt on a good day.
The gas port eroding is the issue with short barrels. You will most likely end up with excessive bolt speeds on a 16" carbine before anything actually breaks or starts keyholling or anything like that. So you'll want to keep some tungsten weights on hand to heavy up your buffer for when that starts to happen, which will extend the life of the barrel for probably another 5-10 thousand rounds.
Port erosion happens in the bore. The actual diameter of the port on the outer reaches of the barrel does not open up, and thus the gas flow is still the same or very close to what it originally was.
Port erosion happens in the bore. The actual diameter of the port on the outer reaches of the barrel does not open up, and thus the gas flow is still the same or very close to what it originally was.
I don't know where you heard that, but it's inaccurate. It's true that you get more erosion on the inside, but the length of the orifice has as much to do with gas flow as the diameter. That's why gas ports are corrected for size if the diameter of the barrel is changed. The gas cuts into the edge and erodes it like a rivulet in clay soil, resulting in higher flow, in turn resulting in earlier unlocking and higher bolt velocities. The bolt velocity in a carbine (chrome lined) will become an issue before keyholling. ARs that start out with H2s often end up with H3s before the barrel is retired, and the shorter the barrel the more of a certainty that becomes.
Parts I've replaced due to wear or failure-
-Extractor spring
-Action spring
-Hammer spring
-Gas tube
-Gas rings
-Bolt catch
-Bolt catch roll pin
A friend replaced a worn barrel
SonOfAGunn
01-25-21, 16:33
It's mainly bolt parts that are wearing for me. Gas rings seem to be showing the most wear since they aren't holding the weight of the carrier anymore. I can't tell that they are worn by looking at them, the just don't hold the weight. The Cam Pins look to be taking a beating as well, but I've never seen one break. There's a visible "seam" or "edge" on the cam pins as well as finish wear.
I've heard that firing pin retaining pins go bad, but I haven't had one break yet. I've got spares of all of these parts, especially the firing pin retaining pins.
It's mainly bolt parts that are wearing for me. Gas rings seem to be showing the most wear since they aren't holding the weight of the carrier anymore. I can't tell that they are worn by looking at them, the just don't hold the weight. The Cam Pins look to be taking a beating as well, but I've never seen one break. There's a visible "seam" or "edge" on the cam pins as well as finish wear.
I've heard that firing pin retaining pins go bad, but I haven't had one break yet. I've got spares of all of these parts, especially the firing pin retaining pins.
This is the first I'm hearing of the retaining pins. Would like to get more info on that if anyone can elaborate. Do they just lose their spring from being taken in and out so much, or do they actually break?
Parts I've replaced due to wear or failure-
-Extractor spring
-Action spring
-Hammer spring
-Gas tube
-Gas rings
-Bolt catch
-Bolt catch roll pin
A friend replaced a worn barrel
A few questions if you could give more details:
On the hammer, was that on a blowback PCC by chance?
The gas tube, how did that fail?
And the bolt catch, did that fail at the same time as the roll pin? What happened to it?
CrowCommand
01-25-21, 18:29
User error...
Thanks for your useful input.
I keep a LPK, gas tube, barrel and spare bolt on hand. Worst failure I've experienced as of late was a blown gas ring (rifle still functioned), and a double feed. I try to shoot 2 times amonth, at least 200 rounds per session. 11.5" pistol (no SBR here) for drills. 14.5 for longer range. The gas ring failure was under 3k rounds, but was a poverty pony (Anderson) rifle.....lesson learned, I run Colt now.
Sounds like the makings of a new rifle!
I keep a LPK, gas tube, barrel and spare bolt on hand.
Early gas ring failure is often associated with a rough finish on the gas ring run portion of the bolt carrier bore.
The rough finish is technically "out of spec" and tends to show up in lower quality units.
Worst failure I've experienced as of late was a blown gas ring (rifle still functioned), and a double feed.
The gas ring failure was under 3k rounds, but was a poverty pony (Anderson) rifle.....lesson learned, I run Colt now.
SonOfAGunn
01-25-21, 20:54
Sounds like the makings of a new rifle!
Early gas ring failure is often associated with a rough finish on the gas ring run portion of the bolt carrier bore.
The rough finish is technically "out of spec" and tends to show up in lower quality units.I've got a DD bcg that stopped having the rings support the weight of the carrier after less than 500 rounds. It's the chrome version of their bcg. Not sure if that matters.
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A few questions if you could give more details:
On the hammer, was that on a blowback PCC by chance?
No. Both were on a 16" 5.56 AR different guys at different ranges had light strikes. I had my parts kit with and replaced both for a nominal fee and got them back up and running.
The gas tube, how did that fail?
The knob end that goes into the gas key was worn beyond allowable limits. I've replaced one or two of mine and one or two of my brother's.
And the bolt catch, did that fail at the same time as the roll pin?
Yes. Yes, it did.
What happened to it?
The pivot pin hole in the bolt release was slightly over large. With use, the hole elongated and the roll pin started to shear. This was exacerbated by the mag release being not tightened enough (Mag release was on me. I was experimenting). Replaced the bolt release and roll pin and tightened the mag release. Hasn't been a problem since.
This is the first I'm hearing of the retaining pins. Would like to get more info on that if anyone can elaborate. Do they just lose their spring from being taken in and out so much, or do they actually break?
I think they do break, but I think the issue is the different levels of quality out there. I think any firing pin retaining pin from a reputable manufacturer/seller will rarely be a problem unless there is a problem with the gun.
Some people use regular cotter pins which I believe don't fit as well (because of a rounder head) and are not as strong (because some/all are not heat treated). I think this is where some of the reported problems come from.
I've never replaced a firing pin retaining pin, but keep some on hand in case I lose one when cleaning. But, I've never lost one either.
Joe Mamma
No. Both were on a 16" 5.56 AR different guys at different ranges had light strikes. I had my parts kit with and replaced both for a nominal fee and got them back up and running.
The knob end that goes into the gas key was worn beyond allowable limits. I've replaced one or two of mine and one or two of my brother's.
Yes. Yes, it did.
The pivot pin hole in the bolt release was slightly over large. With use, the hole elongated and the roll pin started to shear. This was exacerbated by the mag release being not tightened enough (Mag release was on me. I was experimenting). Replaced the bolt release and roll pin and tightened the mag release. Hasn't been a problem since.
Any idea what kind of round count on those hammer springs?
And what was the round count on the gas tubes? I've always heard that those will outlast the barrel. Also, what did the gas keys look like? I'm assuming too much gas was escaping and you were getting short stroking?
Early gas ring failure is often associated with a rough finish on the gas ring run portion of the bolt carrier bore.
The rough finish is technically "out of spec" and tends to show up in lower quality units.
Does this smooth out with time (and new gas rings) or does such a carrier continue to cause premature ring wear throughout its life?
No, don't expect this to resolve itself with use.
It is essentially a chrome plated file or rasp with surface harness over 60HRC.
Chrome is very wear resistant and gas rings don't stand a chance.
This is another example of the importance of starting with a quality BCG.
Does this smooth out with time (and new gas rings) or does such a carrier continue to cause premature ring wear throughout its life?
Early gas ring failure is often associated with a rough finish on the gas ring run portion of the bolt carrier bore.
The rough finish is technically "out of spec" and tends to show up in lower quality units.
No, don't expect this to resolve itself with use.
It is essentially a chrome plated file or rasp with surface harness over 60HRC.
Chrome is very wear resistant and gas rings don't stand a chance.
This is another example of the importance of starting with a quality BCG.
I want to scream every time someone tells me their $80 BCG is milspec and therefore just as good. There are a million little things they don't know they don't know.
titsonritz
01-26-21, 14:55
I've never replaced a firing pin retaining pin, but keep some on hand in case I lose one when cleaning. But, I've never lost one either.
Loss is the main reason to have spare retainer pin, IMO.
Have seen post on sites of broken and bent handles. A spare charging handle in the spare parts. Imagine trying to use the gun without one.
Have seen post on sites of broken and bent handles. A spare charging handle in the spare parts. Imagine trying to use the gun without one.
Good point. They stretch over time and will eventually break, though it takes a really long time to happen. What can also happen is the dimensions can be a little off and the carrier will eventually snap them that way. Shouldn't be a problem ever with decent ARs, but could be an issue with shoddy parts cobbled together.
I think the moral of the story of this thread is that a second rifle is the most economical spare parts kit.
Gatorgrizz27
01-26-21, 19:13
A spare BCG is likely to be a “quick fix” if you’re doing some type of training and just want to get back to it rather than diving into diagnostics. I keep most of the common small parts on hand, but I need to stock up on a few more.
I have an issue where my “cheap” parts like mil-spec charging handles, triggers that come in LPK, stack up deep enough to justify building a cheap beater rifle. Then I realize I don’t like cheap crap on my rifles, it gets upgraded, and the spare parts are free once again.
Several of them have taken up residence in a recent 7.62x39 build that I’m treating like it’s an AK though, so they might live there awhile.
titsonritz
01-26-21, 19:32
I think the moral of the story of this thread is that a second rifle is the most economical spare parts kit.
Then you need twice the quantity of spare parts. :D
If there's no time to diagnose and fix an issue, my spare parts is just grabbing another AR & go.
In more general sense, spare parts should be springs, detents, pins, trigger group, bcg, handle, bolt catch, safety, mag release, castle, buffer, sight(s) of some sort, magazines, maybe a stock, oil. Much of this can be neatly stuffed into a compartment grip and many fwd grips.
Even with all the parts, still need tools to do the work.
Any idea what kind of round count on those hammer springs?
And what was the round count on the gas tubes? I've always heard that those will outlast the barrel. Also, what did the gas keys look like? I'm assuming too much gas was escaping and you were getting short stroking?
The first hammer spring was in a brand new AR the owner paid to have assembled for him. Don't know the round count of the second, but it too was an AR the owner paid to have assembled.
The gas tubes had a couple thousand rounds through them. Mine was shot with a suppressor and binary trigger. Second was in an AR owned by someone with access to free ammo. Both gas tubes were from the generic parts bin.
Nothing wrong with either gas key. Yes, short stroking was involved.
The first hammer spring was in a brand new AR the owner paid to have assembled for him. Don't know the round count of the second, but it too was an AR the owner paid to have assembled.
The gas tubes had a couple thousand rounds through them. Mine was shot with a suppressor and binary trigger. Second was in an AR owned by someone with access to free ammo. Both gas tubes were from the generic parts bin.
Nothing wrong with either gas key. Yes, short stroking was involved.
So maybe shoddy materials combined with higher than normal heat leading to accelerated wear?
Combat_Diver
01-30-21, 03:17
During my 24 yrs in the Army (all SF) I experienced only one part ever breaking that wasn't my fault. Had a locking lug on my M4A1 bolt break during pre-mission test fire before op (Iraq 06'). Swapped with bolt out of other rifle a M16A2 at the time. My fault was doing a Parachute Landing Fall on my brand new M16A2, smashing its handguards (87'). Since 14' till today been a SOCOM armorer in Afghanistan and Iraq. Number one thing called to fix is a broken bolt. During inspections number 2 is shot out barrels and number 3 is worn gas rings. During rebuild of 10.3" and 14.5" URGs, only thing I prefer to re use is the upper receiver, FH, CH and bolt carrier. New barrel, bolt (complete), gas tube, gas block
Malfunctions on range is usually operator error ie magazine not locking in due to loading 31 rds in a mag or magazine issues. About once a year have a trigger issue not resetting. Cause is a blown primer under disconnector tail. Did have disconnector tail break off once. Have come across two broken ejection port door spring broken (short leg) These guns are all run hard suppressed, unsuppressed, semi and full auto in combat zones. That's about over a thousand guns that I've serviced since 14'.
CD
During my 24 yrs in the Army (all SF) I experienced only one part ever breaking that wasn't my fault.
Had a locking lug on my M4A1 bolt break during pre-mission test fire before op (Iraq 06').
Swapped with bolt out of other rifle a M16A2 at the time.
CD
Sounds like a lot of rounds down range with minimal parts failures, confirming generally excellent durability with the weapon system.
Since 14' till today been a SOCOM armorer in Afghanistan and Iraq.
Number one thing called to fix is a broken bolt.
number 2 is shot out barrels
number 3 is worn gas rings.
CD
Did you notice any general trends in weapon configuration that correlated to increased bolt breakage?
Mk18 vs M4 vs M16?
Suppressor use?
During rebuild of 10.3" and 14.5" URGs, only thing I prefer to re use is the upper receiver, FH, CH and bolt carrier. New barrel, bolt (complete), gas tube, gas block
CD
This is a very practical approach.
The barrel, bolt, gas tube and gas block are wear items with a finite life.
The gas tube and gas block can theoretically last longer, but they're inexpensive enough that they should just be replaced with the barrel to avoid any possible issues.
Spares for your spares. I had a spare complete BCG that failed first time out. Over fat cam pin had to be turned with pliers and levered with much force with a large screwdriver. Sure enough, no extra pin in parts bins.
Combat_Diver
01-30-21, 10:50
Sounds like a lot of rounds down range with minimal parts failures, confirming generally excellent durability with the weapon system.
Did you notice any general trends in weapon configuration that correlated to increased bolt breakage?
Mk18 vs M4 vs M16?
Suppressor use?
This is a very practical approach.
The barrel, bolt, gas tube and gas block are wear items with a finite life.
The gas tube and gas block can theoretically last longer, but they're inexpensive enough that they should just be replaced with the barrel to avoid any possible issues.
Never seen a M16 bolt break in my time. Not saying it doesn't happen, just never seen one break. But then again we are no longer using those daily in SOF. Haven't noticed a difference between the 10.3 or 14.5" with carbine gas. They do break but not often and very high round counts. That's why I change them out during rebarrel. Far as suppresed/unsuppressed depends on the team and their preference of use. But none noted. The newer URGI with its 14.5" mid gas, no issues yet but they are still too new, only being out for about 1.5 yrs.
Course all this doesn't include battle damaged guns.
CD
Focus on the bolt/BCG, that is most of the heavy lifting is done, get gas rings, extractor, spring & insert, firing pin, FP retaining pin, cam pin, ejector, extra bolt or two if possible. Beyond that, spring and detents. And barrels.
ETA: My go to place for parts would be Primary Arms, BCM and Forward Controls.
Do they offer a all in one kit for the heavy lifting parts one should keep on hand?
During my 24 yrs in the Army (all SF) I experienced only one part ever breaking that wasn't my fault. Had a locking lug on my M4A1 bolt break during pre-mission test fire before op (Iraq 06'). Swapped with bolt out of other rifle a M16A2 at the time. My fault was doing a Parachute Landing Fall on my brand new M16A2, smashing its handguards (87'). Since 14' till today been a SOCOM armorer in Afghanistan and Iraq. Number one thing called to fix is a broken bolt. During inspections number 2 is shot out barrels and number 3 is worn gas rings. During rebuild of 10.3" and 14.5" URGs, only thing I prefer to re use is the upper receiver, FH, CH and bolt carrier. New barrel, bolt (complete), gas tube, gas block
Malfunctions on range is usually operator error ie magazine not locking in due to loading 31 rds in a mag or magazine issues. About once a year have a trigger issue not resetting. Cause is a blown primer under disconnector tail. Did have disconnector tail break off once. Have come across two broken ejection port door spring broken (short leg) These guns are all run hard suppressed, unsuppressed, semi and full auto in combat zones. That's about over a thousand guns that I've serviced since 14'.
CD
Those ten inchers are hell on bolts. Do the guys ever carry spare bolt parts in the field in case a weapon goes down while they're out and about?
Never seen a M16 bolt break in my time. Not saying it doesn't happen, just never seen one break. But then again we are no longer using those daily in SOF. Haven't noticed a difference between the 10.3 or 14.5" with carbine gas. They do break but not often and very high round counts. That's why I change them out during rebarrel. Far as suppresed/unsuppressed depends on the team and their preference of use. But none noted. The newer URGI with its 14.5" mid gas, no issues yet but they are still too new, only being out for about 1.5 yrs.
Course all this doesn't include battle damaged guns.
CD
As far as I know, an M16 bolt will end up failing the field gauge before it breaks.
You have some good info here!
Combat_Diver
01-30-21, 23:06
Those ten inchers are hell on bolts. Do the guys ever carry spare bolt parts in the field in case a weapon goes down while they're out and about?
The teams 18B usually as a spare bolt in his kit for the team. Then the do have the other upper to rob the bolt from.
CD
So maybe shoddy materials combined with higher than normal heat leading to accelerated wear?
My guess is it's more the quality of the parts than the heat. I've got a suppressed Colt shorty I've put a few rounds through and have not had to replace any parts yet.
My guess is it's more the quality of the parts than the heat. I've got a suppressed Colt shorty I've put a few rounds through and have not had to replace any parts yet.
It's tragic seeing all these people buying "milspec" ARs thinking they're "just as good." I guess most people just have to learn the hard way.
nightchief
01-31-21, 18:12
During my 24 yrs in the Army (all SF) I experienced only one part ever breaking that wasn't my fault. Had a locking lug on my M4A1 bolt break during pre-mission test fire before op (Iraq 06'). Swapped with bolt out of other rifle a M16A2 at the time. My fault was doing a Parachute Landing Fall on my brand new M16A2, smashing its handguards (87'). Since 14' till today been a SOCOM armorer in Afghanistan and Iraq. Number one thing called to fix is a broken bolt. During inspections number 2 is shot out barrels and number 3 is worn gas rings. During rebuild of 10.3" and 14.5" URGs, only thing I prefer to re use is the upper receiver, FH, CH and bolt carrier. New barrel, bolt (complete), gas tube, gas block
Malfunctions on range is usually operator error ie magazine not locking in due to loading 31 rds in a mag or magazine issues. About once a year have a trigger issue not resetting. Cause is a blown primer under disconnector tail. Did have disconnector tail break off once. Have come across two broken ejection port door spring broken (short leg) These guns are all run hard suppressed, unsuppressed, semi and full auto in combat zones. That's about over a thousand guns that I've serviced since 14'.
CD
Interesting info. Thank you for sharing.
titsonritz
02-01-21, 15:10
Do they offer a all in one kit for the heavy lifting parts one should keep on hand?
BCM sells a bolt rebuild kit, basically and extractor and gas rings. I typically keep a spare complete bolt, firing pin, FP retaining pin and cam pin on hand, I figure that covers a lot of ground.
ETA:
https://bravocompanyusa.com/bcm-sopmod-bolt-upgrade-rebuild-kit/
BCM sells a bolt rebuild kit, basically and extractor and gas rings. I typically keep a spare complete bolt, firing pin, FP retaining pin and cam pin on hand, I figure that covers a lot of ground.
ETA:
https://bravocompanyusa.com/bcm-sopmod-bolt-upgrade-rebuild-kit/
Thanx. Probably worth just keeping a additional BCG on hand.
This is the way...
Probably worth just keeping a additional BCG on hand.
extractor.
primer showing up in the FCG.
firing pin getting damaged from blown-out primer.
These types of failures can occur but are normally very rare in a properly setup AR.
If they do show up in combination, they're generally signs of over pressure and over gassing.
Loose primers are caused by a combination of high chamber pressure and over gassing.
The firing pin damage is from primer piercing, which is caused by a combination of 1) large FP hole in bolt 2)high chamber pressure 3) thin (non-mil-spec) primers
extractor.
primer showing up in the FCG.
firing pin getting damaged from blown-out primer.
Lubrication.
I haven't cleaned my main range toy in 1.5 years / thousands of rounds, but she runs without a hiccup wet.
Buddy brought a new in box DPMS out. First round out of the mag, failed to go into battery. He despaired. I inspected. It was dry. Generous CLP anointment and it ran just fine.
Holy crap everybody. Thanks for all the responses. It seems like the general consensus is to keep spares of just about anything related to the BCG. I keep some spare BCGs and rifle so I should be good for now. Again, thanks everyone.
Holy crap everybody. Thanks for all the responses. It seems like the general consensus is to keep spares of just about anything related to the BCG. I keep some spare BCGs and rifle so I should be good for now. Again, thanks everyone.
Yep, that's because all the explody stuff is happening at the BCG/barrel extension/chamber interface...that's where things get stressed.
Spare bolt, maybe a spare extractor as well. Carriers just don’t break but it’s usually just as cheap to buy a complete BCG. I bought a bolt back up kit from AIM Surplus (bolt, cam pin, firing pin & cotter pin) years ago then a year later found a complete Toolcraft BCG on sale for the same price.
In my experience these are the parts most likely to fail with use. An extractor is usually more likely to fail but if your gunsmith skills are minor it’s easier to swap the whole bolt. Swapping a BCG is easier.
Any other firearm I’d say a firing pin too, but I rarely see firing pins break. But they are cheap.
I do recommend keeping spare pins & springs if you gunsmith or build, just because I’ve lost a tiny detent retaining pin or sent a spring flying. But these are parts that don’t see a lot of use/abuse.
Best spare part is another AR.
I've seen extractor break the tooth off, a few locking lugs on bolt cracked one missing M16a1's. Worn out hammer and disconnecter (s) and carrier key on my first AR "87" vintage (that one went to Will's collection) Cam pin, Gas rings not often, Extractor springs and hammer spring old rifles aka M16a1's. A2s shot out barrel's. Shitty magazines due to AWB some A.D units couldn't even get magazines even not in budget or available.
Just have alot of good magazines, good LPK, gas tube n roll pin and bolt carrier group that would cover 99% of most anything. If more a second rifle. Most problems are lost parts.
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