In this thread (click here to read) there was a mild argument about what the actual bolt life of an M4 Carbine was.  

Some said it was 5,000 rounds, and somebody countered with a post showing a Army Power-Point that showed the average bolt life is 12,000 rounds.  And, let us assume that is a fact.

HOWEVER, we need to look at what the “average” life means.

This is what the bolt life of a large population of bolts would look like:



Now, in order for 12,000 rounds to be the ‘average’ exactly half are going to fail before 12,000 rounds, and exactly half will fail after 12,000 rounds.  

POP Quiz: When do you want to replace your bolt? (refer to the image)

A. At point “A” at 5,000 rounds.
B. At point “B” at 8,000 rounds.
C. At point “C” at 12,000 rounds.
D. At point “D” at 16,000 rounds.
E. When it breaks, or shows signs of breaking.

Well, if you want a less than .01% chance that a bolt will fail in service, you better pick “A”.  That is what is known at the “Safe Fatigue Life”.

If you are willing to monitor your bolt for cracks, and replace it “on condition”, you will pick “E”.

Most civilians, and even the LEO community, will pick “E”, as they can examine their bolts regularly, and will not go for months at a time away from even a simple magnifying glass, to look for cracks.  And, even those that never examine their bolts (“I never clean my AR…”), are not really putting themselves in too much danger.  For them, a lug failure is nothing more than an inconvenience, as lug failure rarely results in serious injury.

For the military, they desire to never have a lug failure in service, and that might be in the middle of a two way shooting match with the bad guys and a failed bolt is one weapon not shooting.  So they replace bolts around 5,000 rounds.

What is the life of an AR bolt, well it averages 12,000 rounds, assuming the standard deviation is around 1300 rounds, with essentially none failing before 5,000 and almost all failing after 20,000 rounds.

Now, all this is based off the Army main diet of M855 and its fairly high chamber pressure averaging 58,700 psi.  Feed yours milder stuff below the 55,000 psi range, yours may last practically forever . . . .

And, one last thing, this does not deal with bolt failure at the cam pin hole, that's another can of worms.

Good post. Thanks.

I usually replace my bolts at F:  When I get bored with the upper, sell it and do a new one.

Can't I potentially have one bolt that last 5 bazzilion rounds and a bunch last only a few rounds, but still have an average life span of 12000?

Option F.

E3 bolt.

Well, I don't know about a bazillion, but if you have two bolts and one cracks on the first round and the other cracks on the 23,999th round, your average bolt life is 12,000 rounds.  But, what are the chances of that happening?

Statistically, it will be very rare for a bolt lug to actually fail at 5,000 rounds (assuming that 12,000 round average life is correct), but that's why 5,000 was chosen as the "safe fatigue life".

Would it really follow a normal bell curve?  

That's kind of important for readers to remember.

FWIW, at a Colt rifle armorers course a few years ago, the instructor was pressed by the class for some longevity figures.  He made the point that Colt would never publish any figures for expected parts longevity for fear that the figures would be seen as some sort of a guarantee.
He offered the following figures as his own personal experience/opinion for serious use carbines:

The extractor assembly should be replaced every 5,000 rounds.
The bolt can be expected to last about 10,000 rounds.
Use a H buffer for a carbine barrel.
Use a H2 buffer for a SOCOM barrel.
Use a H2 or H3 buffer for SBRs with suppressors or for full auto SBRs (to moderate the cyclic rate).

Which the E3 also addresses.

If you want to really stay up on the bolt, you can buy a simple magnet set and mag powder an mag your bolts very easy. Not quite as good as a wet mag but it will pick up cracks you would otherwise not see. Craig

This.

Why did he recommend a different buffer for a carbine barrel versus a SOCOM barrel?

Assuming we're talking M4's here, both are 14.5" carbine gassed barrels, with the SOCOM being a heavier profile between chamber and gas port.

Is the gas port diameter different between the two, and if so, different enough that the SOCOM barrel would be so much more over-gassed that it would require a jump up in buffer weight to slow things down?

This is not GD.  It is a Technical Forum. Cut it out. - Old_Painless

Your misunderstanding why it needs a heavier buffer. The SOCOM needs a heavier buffer because it has more mass - in turn the bcg will experience more bolt bounce when it slams against it. The extra buffer mass isn't to slow the cycle rate, it's to tame the bolt bounce. Colt went with an even heavier buffer on their IAR entry.

Most semi auto guys will never notice the bolt bounce.

Let us assume that it is factual that the military replaces bolts every 5k rounds.

Let us assume, for sake of argument, that there are exactly 10,000 rifles in the current service to be used by every troop.

Let us further assume, for simplicity of argument, that each bolt costs just a small amount of 25 bucks.

Let us further assume that each year, half of the rifles need to replace bolts.

That is 5,000 * 25 bucks = $125,000. That's per year, just simply in weapon maintenance.

That's just a baby estimate. In the active military, there are probably way more rifles in circulation and use.

Does it make sense, from the stand point of use, for the military to continue and pay hundreds of thousands of dollars in just a simple part to a weapon?

Or... does it make more sense that it's purely made up by internet folk tales?

5000 is a SOF number and it doesn't mean they actually get replaced. I believe the majority are run until they break or are identified via PM inspections as having cracks. That would normally happen during pre-deployment workups.

SOF was looking at getting round counters installed in most the rifles and swapping bolts before they failed. But that never happened.

They will also get a new bolt if the barrel gets worn out. 

option F.





never.

In my experience the Army doesn't keep count of how many rounds through a M4.

They only replace bolts if a barrel is switched out, a crack is found or a bolt breaks.

I had one bolt break/lug sheared off and I didn't even notice until the end of the day after the range.

I was cleaning it and said damn my bolt is missing a lug, It fired fine all day so no clue when it broke off.

Might be different for SOCOM units, I am only talking about my experience in the 82nd.

What factors would cause it not to fall into a normal distribution?

And since the general run-of-the-mill Army M4 gets shot about 600 rounds a year.  Changing the bolt every decade wouldn't be a large logistical burden.

Remember, a huge percent of the Army doesn't pull triggers for a living

Gotcha. Now that you've explained it, I do remember reading about this effect of heavier profiled barrels somewhere before. Thanks.

Looking back several decades at my math classes, what you are describing is the "median", not the "average".  Depending on the sample population median and average can have quite a spread between them.

Differences in field conditions, usage, ammunition, cycling rate, amount of full auto fire, cleaning, and lots more. Sure, if all factors were equal, it would likely be a normal or lognormal distribution. However, an active combat unit in Afghanistan will be operating under harsher conditions and shooting a lot more rock 'n' roll than a National Guard unit in Vermont, or a Service Rifle shooter in Ohio. Some yahoo trying to match .22-250 ballistics with his .22 Nosler will go through bolts (and rifles) much faster than normal sane people.

SOCOM/Crane's policy is 6,000 rds not 5,000 rds for bolt replacement.  Generally I replace a bolt after it breaks as round counts are not kept.  Only twice during two years down range did a ODA come to me after I quized them about their firing amounts did I replace all their bolts.  Cost of a bolt is $71,~  Guns get serviced before a deployment generally.


CD

Cost of a bolt is $71? Not debating that, just seems high. Especially considering I can get a bolt that matches the specs of a Colt bolt for about the same or less. I seem to remember reading something a few years ago that said LMT sold their SOPMOD stocks to the military for something ridiculously inexpensive like $12 per unit.

Just relooked into the Fed Log.

Bolt, Assembly 1005-01-505-1035  $72.08  (Colt, FNMI or other subcontactor)

SOPMOD stocks are $16.73 either LMT or B5.

Buttstock, Subassembly  1005-01-544-9825

CD

Yup. Still seems high. I would've guessed much less for a bolt. Thanks for the info. Interesting to see what's paid for what.

Does the army ever shoot their guns full auto?  Do you think a bolt will fail quicker if its run on a full auto gun or simi-auto only?

I'm assuming so, since most M4s are being converted to the full auto M4A1 standard.  We did in SF all the time on the range to an extent.  Most shooting is done in SA of course.  Like driving a car you don't always do the speed limit.  Have to push it to learn the limitations.  Bolts will fail sooner on full auto as its the heat build up.  Remember a cook off with a M4A1 can happen in about 6 mags such as a mad minute during an ambush (happened to me).  

Only time I had a bolt break on me was during a test fire before rolling out on a convoy in Iraq 06'.  Did the test fire the night before by function testing couple controlled pairs and a few bursts.  Bolt broke during a 2-3 rd burst (M4A1).  My armorer was out, so swapped bolts out of the M16A2 above my desk (getting ready for All Army Match).  Got the bolt swapped after the mission, locking lug broke.

CD

This is me.  I'm looking to change that though.  Good post OP.

Yes, every M4A1 is selectable to full auto. Full auto generates more heat, which leads to greater chance of failure.

Those are the exact factors that would cause it to fall into a normal distribution, as we are talking about the entire population of bolts in the Army.

I wonder if some one would catch that . . .

Yes, there is the possibility of a large number failing at exactly 12,000 rounds, since we are talking about integers.  In any case, a normally distributed population, about 50% of the population will fail at or before the average.

Your OP is excellent! I think you nailed it. 

about 2 years ago i had 2 lugs break off on an unknown mfg'ed bolt and one of the broken pieces stayed in the barrel extension causing it to not lock up. it was like having a door stop in there.

would sure hate for that to happen when you need a rifle the most.

i'm considering changing bolts out on some of my older stuff.

Completely pointless. Crack formation to failure is going to be a few rounds, maybe a magazine at most. Your chances of doing your ersatz MPI within that tiny window are basically zero.

I broke a MPI bolt on an XM177E2 after more rounds than I could count. Still have the Bolt carrier with another MPI bolt installed. Do not use it anymore but keep it on hand with new rings because it will work if needed. Have moved on the more exotic and newer thing and have no complaints with any of then so far. In my youth, I am surprised I didn't rupture a barrel with the worst happening was the cracked bolt. Fun Rifle to play with and I have the trashed barrels to prove it.

Is that they one that isn't interchangeable with every other AR/bolt out there? The

Well.....I don't know about that.

In some endurance tests run by the Army a few years back, they MPIed bolts every 3000 rounds, but when cracks showed up the bolts were not replaced, but just put back in to see how long they would go after cracking.  All the bolts that showed early cracking, went over 3000 rounds without failure, and completed the endurance test.

So, the window between the first identifiable crack and loss of a lug is somewhere between 1000 and 4000 rounds, maybe even more, not a "few rounds".

I'll take an LMT SOPMOD stock in Black and FDE at that price please.  

I wonder how much ammo costs the military per round........

IIRC it's about .25/rd and M855A1 is .05 more.

I mag bolts all the time for guys. When a crack first shows up, you most likely have several thousand rounds before it will let go. You would be shocked how many bolts out there have small fractures in them. They will eventually fail at some point but it is not like they are glass. When a lug fails, it has been working at it a while. Craig

According to Fed-Log, M855A1 is about $0.50 more a round . . .

Interesting. Some of the old briefs said it was only supposed to add .05 to the cost when in full production.

I'd use it for HD if I could find it for $1/rd.

Are Carbine and SOCOM barrels not the same length, 14.5? Why the difference?

ETA asked and answered. My apologies.

Yeah, well, that's how they sold it to Congress, et al.

Name a program, and I'll name a program that is over-budget....

The fatigue failure as a result crack initiation and crack growth in metals does not exhibit a normal distribution around the average fatigue life.  The damage it accumulative and the nature of crack growth makes a Weibull distribution more appropriate in most cases.

Weird things happen....I remember once were running live-fire lanes and I got designated the "SAW gunner". But since our M249 SAWs were still months away, I simply got to fire my A1 only on full auto and got to lug about 12-14 mags, some loaded all tracers. I had a lot of fun.

But that was the only time I got to fire full auto, aside from belt feds. You typically go out and qualify 2X per year (40 rounds). Maybe some "familiarization", checking zero, or night shoot. And lots of blanks.

This is back in the 80s and 90's peace time. I have never seen a broken bolt.

Wiebull distributions can be normal, depends on the value of k, low cycle fatigue usually is fairly normal, at least not highly skewed to the right like high cycle fatigue.

In the absence of better data, a normal distribution is pretty good assumption.

For DoD M4/M4A1 both Carbines initial had 14.5" Govt profile.  Later SOCOM adopted the heavier and thicker (under handguards) the SOCOM bbl which is also 14.5".  Both have a twist rate of 1:7, cuts to mount the M203 and chromed lined.  The Mk18/CQBR/URG uses a cut down Gov't profile to 10.3".  The heavier barrel was developed to prevent cook off when fired full auto around 2000.  I've had a cook off (Govt bbl M4A1) during a mad minute firing 6 mags full auto into a kill zone as fast as you could, this was about 97/98' ish.

CD

You were shooting a rifle. Carbine is a leopard of different spots.  

The differences in loads and stresses on locking lugs and bolt bodies between the rifle and carbine systems while in operation are huge.