What happens when AR parts break?


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~Abstract~
January 5, 2012, 02:02 PM
I've got a DPMS M4gery which has been 100% reliable from first bang.

Problem is, that first bang was ~3,000 bangs ago. Now, I'm reading where bolts can start breaking around now.

I'm not too familiar with when bolts, firing pins, various springs, etc start to wear out...and what happens when they fail during operation. What type of failures are dangerous?


Also, is there a maintenance schedule for replacing/inspecting worn parts?


Thanks!

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M1key
January 5, 2012, 02:10 PM
I didn't know AR bolts break at around 3000 rounds. :eek:

M

45_auto
January 5, 2012, 02:11 PM
Problem is, that first bang was ~3,000 bangs ago. Now, I'm reading where bolts can start breaking around now.

Do you have any kind of legitimate source for that claim? I've never heard it.

3000 rounds = 3 cases of ammo = 100 x 30 round mags.

Back in the dark ages when I trained with an M16, we went through at least 10 mags a week for months at a time. Never heard any worries about bolt breakage, never saw any bolts replaced.

Friend of mine with a registered M16 easily goes through a case a month without even trying, he's replaced a few barrels, but as far as I know it's still running the original bolt.

~Abstract~
January 5, 2012, 02:15 PM
http://www.thehighroad.org/showpost.php?p=7849284&postcount=15

TimboKhan
January 5, 2012, 02:16 PM
I have never heard that, but its not hard to fix. Basically, you just buy a new bolt from one of the zillion or so places that sells them.

Sent from my DROID RAZR using Tapatalk

firesky101
January 5, 2012, 02:31 PM
Not very dangerous if it does break. Run it till it goes then you know what to replace. Stress fractures in metal are a reality, most part will run forever with them.

CoRoMo
January 5, 2012, 02:39 PM
Buy one now and keep it in your range bag.
http://www.lmtstore.com/bolts-carriers-groups/556-standard-bolt-assembly.html
http://www.spikestactical.com/new/z/bolt-spikes-mp-tested-bolt-assembly-556-p-326.html

~Abstract~
January 5, 2012, 02:45 PM
Is a catastrophic bolt failure dangerous?

What types of failures are dangerous? When do they occur?

snakeman
January 5, 2012, 02:46 PM
The only parts that i would really think about replacing are the various springs and pins in the rifle. I wouldn't worry too much about bolts. As long as everything is tight it should be ok. Inspect all moving parts for burrs and cracks and gouges. Keep it lubricated and consider a new buffer and spring, this may help absorb shock better than one that's been sprung 3000+ times.

firesky101
January 5, 2012, 02:55 PM
Check youtube for ar kabooms. I have seen a few and no one got hurt, they are usually due to torture testing, like firing with a barrel full of water. Never saw a bolt failure unless you count my m1 carbine. Carrier failed, and the bolt flew out and hit me in the head. Nothing too valuable there to break.:D

taliv
January 5, 2012, 03:01 PM
the failures don't usually result in kabooms afaik, just stop the weapon from working. we're talking about parts getting worn out here, not an overpressure situation.

bolts will shear lugs, and afaik, the lugs usually wind up in the receiver extension, which will usually stop the bolt from going back into battery for the next round.
http://www.jprifles.com/photos_new/BadBolt_271.jpg

bolts will also crack in half, usually at the cam pin hole, which will leave the bolt head in battery and the tail in the carrier.
http://www.hunt101.com/data/500/IMG_12541.jpg

this thread also has some interesting pictures and discussion of cracks in a colt bolt http://www.m4carbine.net/showthread.php?t=61467

in either event, people worried about this will carry a spare bolt. magpul even makes an insert for their MIAD grip to carry a spare bolt and firing pin in the grip. the alternative, as you say is preventative maintenance by replacing parts.

what I have read is that most mil-spec bolts start developing cracks around the cam pin hole by 6000 rnds. that doesn't meant the bolt won't last 20,000 rounds... but if you were super-paranoid, 5000ish would be the point to spend $60 to replace the bolt.

it is a good idea to replace the action spring at about 10,000 unless you buy/make a gauge to show exactly when it is worn out. (there's a min/max length for the spec and so you just measure the spring and if it's too short, replace it)

extractors also need replacing occasionally, when they feel slick instead of grippy. extractor spring/inserts may be replaced when they don't have enough tension.

firing pins don't normally wear but if they chip, they will need replacement

gas rings around the bolt tail will also wear.

you should inspect each of those parts when you clean the gun. (you'll need to clean it to inspect it anyway)

~Abstract~
January 5, 2012, 03:14 PM
...you should inspect each of those parts when you clean the gun. (you'll need to clean it to inspect it anyway)

Wait...you're supposed to clean these things?


I keed I keed. :D




Thanks for the info Taliv! That's exactly what I was looking for!

Any more info is certainly welcome.

taliv
January 5, 2012, 03:32 PM
http://www.dtic.mil/ndia/2006smallarms/taylor.pdf

skip to page 44 on that presentation

~Abstract~
January 5, 2012, 03:46 PM
Yep. Read that.

I don't profess to subject my rifle to a "Harsh Firing Schedule"...but since my rifle is just a DPMS, there's no telling if it's manufactured in a manner which would shorten or prolong the life of the part...afaik, it's not "mil-spec"...for better or worse.


So, generally, shooting manufacturer ammo, there's little danger of a kaboom scenario...even if parts are extremely worn?

Orlando
January 5, 2012, 03:54 PM
Just inspect parts during cleaning for cracks etc. You are worrying about nothing

mastiffhound
January 5, 2012, 05:15 PM
I had a 3/4 circle crack start in my ruger sr556 bolt on the face around the firing pin hole. I replaced the bolt with a les baer bolt and sent it in to ruger, they said they are testing it. They asked me if I had used any reloads and I said no as I hadn't, just some cheap underpowered wolf. They said that was fine send in the bolt and they will fix it. I didn't notice the crack untill I was cleaning it after shooting that day, thought it was carbon build up. I'll bet they are magnetic particle testing it right now, what they should have done in the first place before they were ever installed in one of their rifles.

JustinJ
January 5, 2012, 06:16 PM
I'll bet they are magnetic particle testing it right now, what they should have done in the first place before they were ever installed in one of their rifles.

I'll bet they threw it away and will just send you a new one.

Hacker15E
January 5, 2012, 06:19 PM
10,000 is where I'd begin really examining the bolt after each range session. Even with a single lug failure, you are not likely to have a "kaboom". As soon as you see a crack or other damage, I'd replace it.

The thought is that a non MPI'd bolt is either going to fail very early on, or it's going to have a nice long life.

Strykervet
January 5, 2012, 06:29 PM
Just keep an eye on it and inspect it when you clean it. That can't hurt. Look for cracks around the locking lugs. 3000 rounds? I've seen Colt M4's in the army run waaaay over that, easily 20,000 rounds, and they were "fine". It depends on how you clean it though. Using hot water and such, or stripping the oil off regularly to get it "white glove" clean, that will weaken the metal and can cause it to break from the inside out. Instead, use plain #9 and CLP or whatever, and scrub and clean it good. Inspect it. Make sure to replace the extractor spring and donut (if you use them, and you should) and the gas rings regularly though. Use dental picks to get all carbon out of, in and around, inside and on the bolt, carrier, and firing pin. This will all make them last longer.

When purchasing a bolt, get a good one. They should be magnafluxed, well machined, and made by a reputable dealer. Some quality barrels include bolts.

JustinJ
January 5, 2012, 06:43 PM
The average bolt, barrel and just about any other 'part that can wear out' life will be significantly lower for a semi auto than a select fire.

kfgk14
January 5, 2012, 07:02 PM
The shorter, carbine-length guns are a lot harder on parts than the rifle-length AR-variants. My mileage, go to Palmetto State Armory, buy their premium BCG, it's mil-spec/good to go. When your current BCG fails, drop in the PSA bolt. Also, buy a spare complete bolt (different from a BCG) from PSA and keep it with you. When the PSA BCG gives you trouble, swap out for the new bolt. If that doesn't solve your problem, you need to do some thinking.

At 5,000-6,000 rounds, you ought to think about a new buffer spring (if you have the collapsible stock, if not the spring change can wait a few thousand rounds longer).

Your barrel's accuracy will really start to degrade at about the 10,000-20,000 round mark, depending on the ammo you shoot, whether it's chrome-lined, and whether you're shooting rapid fire, or from the bench. When you replace the barrel, buy a new bolt, regardless of whether your old bolt is worn/abused. The BCG can be transferred from barrel to barrel without worry.

Eventually, after 100,000 rounds and many barrel swaps, the upper will wear out. Your trigger springs and such will, after those many thousands of rounds, degrade. As your approach those extreme round counts, anti-rotational pins may be a prudent investment. The buffer will also wear at these extreme round counts, consider its replacement way down the road. It is unlikely you'll get to these extreme round counts, but just in case, I thought I'd give you what I know.

TurtlePhish
January 5, 2012, 07:08 PM
I've actually heard that the AR bolt was designed so that it could still run semi-reliably while missing one or two bolt lugs.
Sounds not-so-safe IMO, and I wouldn't try it. But something to think about.

Liberty1776
January 5, 2012, 11:34 PM
seriously, last thing I'd worry about. For practical life experience, my neighbor/buddy is the guy in charge of weapons (I can't remember the title) for our local SWAT team. On a heavy training weekend, they run 3-6000 rounds through their rifles, with no problems. Lube 'em and shoot 'em. They don't even do a teardown/inspection beyond basic cleaning until the gun hits 30,000 rounds.

Interestingly enough (to me) they've also tested two of the PlumCrazy lowers and have found them to be just as reliable and accurate, with the provision that they are not made to fall down on, or use to bust down a door...

Onmilo
January 6, 2012, 12:03 AM
The parts should should be looking at @ the 3000 round mark are the,
Bolt-well covered already
Extractor (chipped edges-hairline cracks)
Extractor spring and buffer (Compressed-collapsed),
Extractor pin(bent)
Ejector & tunnel (Chip fouled- mashed tip- weak or broken spring)
Firing pin (worn-chipped tip)
Gas rings (fouled-missing-pulling out of bolt channel)
Firing pin retainer pin, "cotter pin" (loose fit-broken arm or arms-excessive spreading to keep in place)

Inspect barrel length for bulges
Inspect flashider for broken tines, bulges, looseness

Inspect fire control group for broken, weak spring, wallowing of receiver through holes @ trigger and hammer pins

Inspect all springs and detent locations making sure spr
ings are not crushed. collapsed and the detents are present

Inspect ejection port door for weak spring and/or latch, bent, ill fitting door assembly

Inspect forward assist assembly, ensure it is tight and functions correctly

~Abstract~
January 6, 2012, 11:25 AM
Great list. Thanks Onmilo!

JustinJ
January 6, 2012, 11:50 AM
seriously, last thing I'd worry about. For practical life experience, my neighbor/buddy is the guy in charge of weapons (I can't remember the title) for our local SWAT team. On a heavy training weekend, they run 3-6000 rounds through their rifles, with no problems

I assume you mean 3,000 to 6,000 rounds between all the members combined. I don't believe for a second any one gun is going be fired 6K times in one weekend.

~Abstract~
January 6, 2012, 11:53 AM
10 guys....48 hours...one rifle.

:D

helotaxi
January 6, 2012, 11:58 AM
I'll bet they are magnetic particle testing it right now, what they should have done in the first place before they were ever installed in one of their rifles.All an MPI does is confirm that there are no cracks at that time. It does nothing to prevent or predict future cracks.

JustinJ
January 6, 2012, 12:01 PM
All an MPI does is confirm that there are no cracks at that time. It does nothing to prevent or predict future cracks.

If a HPT is performed before the MPI it will be a good indication that the bolt will not crack prematurely.

10 guys....48 hours...one rifle.

That must make clearing drills take forever.

helotaxi
January 6, 2012, 01:13 PM
If a HPT is performed before the MPI it will be a good indication that the bolt will not crack prematurely.
Maybe. The HPT again does nothing to indicate potential for stress cracks, though. If the bolt is properly made, with the right material, the HPT/MPI shouldn't reveal anything. That doesn't mean that a stress riser won't cause a fatigue crack after a few hundred rounds.

JustinJ
January 6, 2012, 02:37 PM
Maybe. The HPT again does nothing to indicate potential for stress cracks, though. If the bolt is properly made, with the right material, the HPT/MPI shouldn't reveal anything. That doesn't mean that a stress riser won't cause a fatigue crack after a few hundred rounds.

Its my understanding that the HPT will make any irregularities viewable to an MPI and ensure the bolt can withstand the forces of operation.

Nuclear
January 6, 2012, 03:07 PM
All a HPT (I assume you mean a "proof" round) does is demonstrate that at that point in its life the weapon in question will withstand that overpressure condition (has no gross defects). An MPI will only show if there is any surface (or very near the surface) cracks at the time of the testing. An HPT will not make any irregularities any more viewable via MPI.

I'd feel better about these tests done buy companies if they published their failure rates after these tests, which would give us an idea of how likely their components are to fail (statistically speaking).

JustinJ
January 6, 2012, 03:30 PM
All a HPT (I assume you mean a "proof" round) does is demonstrate that at that point in its life the weapon in question will withstand that overpressure condition (has no gross defects). An MPI will only show if there is any surface (or very near the surface) cracks at the time of the testing. An HPT will not make any irregularities any more viewable via MPI.

I mean HPT as in High Pressure Test. Yes it it done with a Government standard M197, 5.56 high pressure test cartridge per MIL-C-46936.

The HPT and MPI go hand in hand: MIL-C-70599A(AR) 3.4.4 "After proof firing, parts shall be free of cracks, seams and other injurious defects as evidenced by visual and magnetic particle inspection."

kfgk14
January 6, 2012, 04:43 PM
If you have HPT and MPI testing, and the parts fail, you don't sell them the public. Good companies HPT/MPI every bolt/barrel they sell. When a part fails they don't sell it. It's a guarantee you're getting something that will take an over-pressured round and not subsequently kill the operator of the firearm.

helotaxi
January 7, 2012, 01:43 AM
Overload failure is what a proof load and subsequent MPI test. Stress/fatigue fractures are totally unrelated and develop over time. A bolt that tested good with a proof load can, and eventually will, develop metal fatigue. It's an unavoidable fact with metal parts that are subjected to repeated loads below their failure limit. The bolt flexes ever so slightly under the pressure of firing and over time this repeated flexing work hardens the area that is flexing. Over time this work hardened area begins to crack.

Eb1
January 7, 2012, 01:38 PM
What happens when AR parts break?

You replace them for safety and reliability.

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