Bolt failures

[C4IGrant]

There is an interesting discussion on 10-8 about bolt's breaking. Kevin Lamb stated that he sees 6-8 mil-spec bolts (most likely Colt bolts) break out of every 40! He doesn't say, but am guessing that that the weapons used are MK18 and M4's (which as we know, have carbine length gas systems).

I then get a PM from a Battalion Armorer stating that for almost 10 years, he has only come across one broken bolt.

The interesting part is that they are running A2/A4's. This points directly to what we have always known, that the carbine gas system is MUCH more violent and hard on components than the rifle length gas system.



C4

[Anthony Jacob]

Interesting... When a bolt breaks does it like to take out other components along with it?

[C4IGrant]

Originally Posted by Cybin

Interesting... When a bolt breaks does it like to take out other components along with it?

Most times, either the lugs or the cam pin hole breaks. This generally does not hurt the other components.


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[twl]

3 things:

1) Use an adjustable gas regulator to compensate for progressive gas port erosion that happens over time. And adjust it periodically, as needed, to restore proper gas metering to the system.

2) Use a heavier buffer that takes longer to energize into motion, slightly delaying extraction, and also slows cyclic rate. Improved extraction reliability, slower cycling for better reliability and less heat into the system(lower rpm) in full-auto.

3) Use a D-Fender on the extractor spring. Self-explanatory.

These things can't change the basic gas system, but they do deal with the problematic symptoms in an effective way.

The other way is to change to a midlength or rifle-length barrel, to move back toward the original design specifications.

Been saying this stuff for years.

[LukeMacGillie]

Ive also been following the 10-8 thread, interesting info. I deal with alot of broken bolts. Hard use M4's combined with Frangible ammo makes life hell for me during training cycle.

Ive seen broken lugs jam under the carrier key and cause the carrier and gas tube to get out of alignment. Also seen the broken lug end up down in the lower and cause broken hammer and trigger pins.

Im looking forward to some southern Indiana magic that might help with the lug shear problems................... Dont know if it will also deal with the cracks at the cam hole.

Like Ive said before here and other places you need to have training uppers and deployment uppers, both need to be set up the same, and have all the various and sundry optics and Friggen LaZers zeroed.

Also a round count(and just as important, Type) book just as detailed as you would keep for a Sniper weapon is a very good idea.

Its going to cut into some grip makers profits, and increase bolt and barrel makers profits im sure, but I cant wait to field round counters. Make M4 maintenance round count based(like aviation weapons systems) instead of gauging throat errosion or guessing about gas port errosion.

[kel3at]

I wonder if the LMT enhanced bolt and carrier would have less failure rate in the carbines than the standard ones.

[S-1]

I had a bolt crack in an unusual spot with my 10.5" upper. The crack starting from right underneath the extractor and ran down the bolt face towards the firing pin hole. I've never seen or heard of another bolt cracking this way. Granted, the bolt had about 8-10k through it with the 10.5".

There's no question that carbines are harder on parts, and as the barrel gets shorter the harder it is on them.

[AMMOTECH]

Originally Posted by C4IGrant

Most times, either the lugs or the cam pin hole breaks.
C4

Some pics of that problem. These are are not mine and were taken from another forum.







What to look for:

[USMC03]

Originally Posted by AMMOTECH

Some pics of that problem. These are are not mine and were taken from another forum.







What to look for:

The pictures you provided have nothing to do with this discussion: "The carbine gas system is MUCH more violent and hard on components than the rifle length gas system"............


The broken bolt in the pics that you provided were obviously caused by the ARMS SIR system, and have NOTHING to do with the carbine gas system being MUCH more violent and hard on components than the rifle length gas system

[JaxCatm]

At my CATM shop we have seen alot of broken bolts on the M4 and GAU systems since the introduction of frangible ammo. I don't know what the other services are shooting but the frangible we shoot is a little hot. I attended a supervisors class about a month ago and the instructor said the chamber pressure on our ammo was 73,000 psi plus not the normal 52,000 psi. Add that kind of over pressure to a carbine length gas system and no wonder we are breaking bolts. We are also seeing the cam pin track recieve excessive wear. We are seeing the same thing out of our 9mm frangible ammo. It is loaded to a much greater pressure than the standard ball. As a result we have locking blocks break on our M9 pistols every class and barrels are getting split.

[cz7]

ok the gas system hole in the barrel is at a point where psi is higher then a 20'' barrel and steel alloying maybe a part too... bolt hole were cam bolt slot angle needs to be reworked too

[Dragon 2 Zero]

lol! That last bolt - the one that's cracked-but-not-broken-apart ... that one is mine! It's a Bushmaster with just a few thousand rounds on it - about 3000, IIRC. The weapon had turned into a single shot carbine. I took the thing apart on the range and found that crack. I had a spare with me. Swapped it out and carried on.

Ya know what else I have noticed on my Carbine? ...(get ready for a Master Of The Obvious type statement...) The firing pin retaining pin gets positively Beat To Shit when I run Green Tips through it. I hate it when that happens...

[mike240]

Has any work been done to determine what is the shortest barrel that can be run with a mid length gas system? Would this reduce the some of the problem?

[AMMOTECH]

Originally Posted by Dragon 2 Zero

lol! That last bolt - the one that's cracked-but-not-broken-apart ... that one is mine!

Thanks for the picture!
It's a good reminder for everyone to check all their components when they clean their rifle and clean/inspect it after every trip to the range.
Just think of what the picture would have been of if you did not replace that bolt...

[Low Drag]

A buddy sent me a pdf from a firm that is trying to sell the round counters to the military. They state that bolts fail on Gubmint issued M4s between 3K to 6K rounds under harsh conditions. 6K to 10K on "milder" schedules. Barrels burn out at about the same time.

I've heard all the stories about Bushmaster and "others" failing, but it looks like all M4 type carbines have this problem. I wonder if the size of the gas ports could be changed to decrease the pressure? I've added a heavy buffer but that does not address the pressure coming down the gas tube and the resulting pressure placed on the parts in the BCG.

[jmart]

Originally Posted by Low Drag

I wonder if the size of the gas ports could be changed to decrease the pressure? I've added a heavy buffer but that does not address the pressure coming down the gas tube and the resulting pressure placed on the parts in the BCG.

I've wondered the same, but I suspect if you went much smaller, you'd probbaly run into relaibility issues with weak ammo or when running in the cold.

While adding weight doesn't reduce the pressure it does keep things locked up so you avoid the situation of moving parts while pressures are still way high. Given that extra amount of time, it has to make life easier on the parts.

I also wonder if there's any correlation to running the gun dry vs having it adequately lubed. Again, I suspect that if the weapon is properly lubed, parts life will increase.

[C4IGrant]

Originally Posted by mike240

Has any work been done to determine what is the shortest barrel that can be run with a mid length gas system? Would this reduce the some of the problem?

14.5 I believe.


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[C4IGrant]

USMC03, I thik you are correct! The ARMS SIR DID break that bolt!




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[SinnFéinM1911]

Originally Posted by C4IGrant

USMC03, I thik you are correct! The ARMS SIR DID break that bolt!




C4

Without a Doubt !~

[Seth Harness]

The pressure behind a bullet in a 20" barrel is greater than the pressure behind a bullet in a 16" barrel... correct?