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Thread: Bolt Life?

  1. #61
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    double tap
    It is missing the point to think that the martial art is solely in cutting a man down; it is in killing evil. It is in the strategem of killing the evil of one man and giving life to ten thousand -Yagyu Munemori

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    Thanks for starting an interesting thread. I'm really surprised there is not a test that has focused on this issue, if it is an issue. I think it is.... talking about the lug failure, not the cam pin hole failure, but my belief is that both happen more with carbine-length gas systems. I have had several people agree with me on this, and no one has disagreed, but at this point it's all anecdotal as far as I know. It could in part result from "everybody's got a carbine-gassed gun" and fewer people have and shoot 20" anymore. If 90% of AR15 owners painted their guns blue with green stripes, that group of guns would see more breakage.

    Lysander, you opined that cam pin hole breakages might be in part due to less-good bolts on the commercial market. I'm comfortable with calling that one a fact. I don't believe I've ever seen a Colt bolt break that way; conversely on the civilian side (which includes police), the ratio of cam pin hole breaks to lug breaks is probably at 30:1. From that I conclude that lug breaks come from high round counts that are seldom see on the civilian side, and that even high mileage .mil bolts that break lugs don't break at the cam pin hole first so they just might be better. There are exceptions of course-- I saw a House-O'-Parts bolt break a lug in under 100 rounds. That was the "1" in the above 30:1 ratio and I consider it an outlyer due to a poorly made bolt.

  3. #63
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    Quote Originally Posted by Ned Christiansen View Post
    Thanks for starting an interesting thread. I'm really surprised there is not a test that has focused on this issue, if it is an issue. I think it is.... talking about the lug failure, not the cam pin hole failure, but my belief is that both happen more with carbine-length gas systems. I have had several people agree with me on this, and no one has disagreed, but at this point it's all anecdotal as far as I know. It could in part result from "everybody's got a carbine-gassed gun" and fewer people have and shoot 20" anymore. If 90% of AR15 owners painted their guns blue with green stripes, that group of guns would see more breakage.

    Lysander, you opined that cam pin hole breakages might be in part due to less-good bolts on the commercial market. I'm comfortable with calling that one a fact. I don't believe I've ever seen a Colt bolt break that way; conversely on the civilian side (which includes police), the ratio of cam pin hole breaks to lug breaks is probably at 30:1. From that I conclude that lug breaks come from high round counts that are seldom see on the civilian side, and that even high mileage .mil bolts that break lugs don't break at the cam pin hole first so they just might be better. There are exceptions of course-- I saw a House-O'-Parts bolt break a lug in under 100 rounds. That was the "1" in the above 30:1 ratio and I consider it an outlyer due to a poorly made bolt.
    Given my limited knowledge on the subject, my observations mirror yours.

    With nearly 4 years as a contract armorer in the ME, I never saw a broken or cracked A2/A4 bolt. Not that I was able to work on them anywhere near the extent I worked on the M4s. I probably had them come through the shop at just about the same ratio of 30 M4's to 1 A2/A4...ish. probably closer to 40 or 50 to 1 actually. And granted, the vast majority of those were used by support units that carried them around on post and rarely actually used them.

    When it came to M4 bolt failures, the actual causes were not something that could really be nailed down as I didn't have access to how the guns were run prior to arriving in the shop. I didn't know what kinds of firing schedules they had been run through, or if a unit had extra ammo to burn up one range day (gotta expend it all so to make sure you get the same amount next time!..... :-/ ) and had burned up a couple of guns getting rid of it all.

    We would have some units who had been in country for some time bring in guns that all checked out fine and another unit that had literally just hit the ground for a deployment would bring in a lot of guns that I had to replace 2 per 10 bolts for cracked lugs.

    I'd say a lot of the frequency we found issues had to do with the competency of the armorers at different levels who were responsible for maintaining the guns prior to them arriving. I have to say it was pretty sad that it appeared that guns worked on by .mil 20/30 level armorers seemed to have more issues whereas guns worked on by contract 20/30 level armorers where worked to a higher standard.

    Some of the shit I saw happen with former 91F's that came out on contract was ****ing tragic. Watching a guy who had just flown in for his first contract with a nice resume, 91f trained and all, try to headspace and time an M2 without a barrel installed was what I call a ****ing eye opener.
    It is missing the point to think that the martial art is solely in cutting a man down; it is in killing evil. It is in the strategem of killing the evil of one man and giving life to ten thousand -Yagyu Munemori

  4. #64
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    Quote Originally Posted by Ned Christiansen View Post
    Thanks for starting an interesting thread. I'm really surprised there is not a test that has focused on this issue, if it is an issue. I think it is.... talking about the lug failure, not the cam pin hole failure, but my belief is that both happen more with carbine-length gas systems. I have had several people agree with me on this, and no one has disagreed, but at this point it's all anecdotal as far as I know. It could in part result from "everybody's got a carbine-gassed gun" and fewer people have and shoot 20" anymore. If 90% of AR15 owners painted their guns blue with green stripes, that group of guns would see more breakage.

    Lysander, you opined that cam pin hole breakages might be in part due to less-good bolts on the commercial market. I'm comfortable with calling that one a fact. I don't believe I've ever seen a Colt bolt break that way; conversely on the civilian side (which includes police), the ratio of cam pin hole breaks to lug breaks is probably at 30:1. From that I conclude that lug breaks come from high round counts that are seldom see on the civilian side, and that even high mileage .mil bolts that break lugs don't break at the cam pin hole first so they just might be better. There are exceptions of course-- I saw a House-O'-Parts bolt break a lug in under 100 rounds. That was the "1" in the above 30:1 ratio and I consider it an outlyer due to a poorly made bolt.
    I think one reason there is not a test specific to the life of a bolt, there are a number that deal with it in a secondary manner. The baseline reliability test, for example, gave an answer that suits the Army's needs, namely the average number of rounds before a lug pops off. Then seeing as rifle/carbine will more than likely pass through a rebuild long before the round count gets that high gives them confidence that inspection during rebuild will find anything bad. Then to top it all off, even if you do break a lug or two there won't be a catastrophic event, so no serious safety issue.

    So, it is now a logistics issue, the cost of spare bolts over the decades. They just look for ways to make them last longer, in general.

    The other thing is the Army seems to have really lost interest in the 20" barrel, since the late 1990's almost no reports deal with the M16 specifically. All studies are mainly focused on the 14.5 inch barrel variants.

  5. #65
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    The one bolt failure at the cam pin hole I've seen happened in a LEOSA 1033 rifle that had been re-barrelled with, I believe, a 10.5 and suppressed. The BCG was original to the rifle as received. Obviously no idea of round count, the officer using it shot a lot based on his competency.
    Patriotism means to stand by the country. It does not mean to stand by the President... - Theodore Roosevelt, Lincoln and Free Speech, Metropolitan Magazine, Volume 47, Number 6, May 1918.

    Every Communist must grasp the truth. Political power grows out of the barrel of a gun. Our principle is that the Party commands the gun, and the gun must never be allowed to command the Party Mao Zedong, 6 November, 1938 - speech to the Communist Patry of China's sixth Central Committee

  6. #66
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    I don't know if any of you have seen this NAVSEA SOPMOD overview document from 2006.

    https://ndiastorage.blob.core.usgovc...rms/taylor.pdf

    With regard to AR bolt failure, it mentions (on page 44):

    "Most Bolts Subjected to Harsh Firing Schedules Will Show Initial Cracking Around 3,000 to 6,000 Rounds"

    "On Milder Schedules, Nearly All Bolts Will Show Initial Cracking at 6,000 to 10,000 Rounds"

    "Once the Crack Is Initiated, the Bolt May Fail at Any Time, or May Last for Thousands of Rounds, Depending on Crack Propagation"

    Joe Mamma
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  7. #67
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    IIRC, Crane's policy is swap at 6K rds.

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    Quote Originally Posted by Joe Mamma View Post
    I don't know if any of you have seen this NAVSEA SOPMOD overview document from 2006.

    https://ndiastorage.blob.core.usgovc...rms/taylor.pdf

    With regard to AR bolt failure, it mentions (on page 44):

    "Most Bolts Subjected to Harsh Firing Schedules Will Show Initial Cracking Around 3,000 to 6,000 Rounds"

    "On Milder Schedules, Nearly All Bolts Will Show Initial Cracking at 6,000 to 10,000 Rounds"

    "Once the Crack Is Initiated, the Bolt May Fail at Any Time, or May Last for Thousands of Rounds, Depending on Crack Propagation"

    Joe Mamma
    I think the "...once the crack is initiated, the bolt may fail at any time..." is a bit of hyperbole. Namely because the crack propagation seems to be fairly consistent in testing.

    By the way the firing schedules in just about all the the Army testing I have mentioned was as follows:

    A 30-round magazines fired at a rate of one round per second, followed by a 30-round magazines fired in automatic at a rate of three to five round bursts every 5 seconds. Repeat cycle for two more magazines.

    After the 120 round cycle the barrel was allowed to cool to the touch.

    After five 120 round cycles (600 rounds), a wipe and re-lubricate.

    After two 600 round cycles (1200 rounds), complete cleaning and inspection, to include headspace and throat erosion. Dispersion data was collected from the first 10 rounds fired in semi auto after the 1200 round cleaning.

    At 6000 rounds detailed inspection, including NDI of bolt, carrier, cam pin and barrel.

    That is a pretty harsh firing schedule. And, half of the M4 test weapons made it to 18,000 round on the original bolt.

    I have noticed that you have to be careful about comparing Crane reports to Aberdeen reports, they are not one-to-one comparable. For one thing I have never seen the actual firing schedule used by Crane. For another thing, that is a power point presentation. PPPs are not technical reports, they are advertisements given to Generals and Admirals to 1) justify the money you have already been given, or 2) justify giving you more money for some reason, so view them as you would a TV commercial. If you notice that after page 44 where the author gives these dire predictions on bolt life and other woes, on page 52, they talk of wanting to put an electronic rounds counter in the grip to accurately schedule maintenance (so, give us some more money, please).
    Last edited by lysander; 11-05-19 at 21:56.

  9. #69
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    Quote Originally Posted by lysander View Post
    I have noticed that you have to be careful about comparing Crane reports to Aberdeen reports, they are not one-to-one comparable. For one thing I have never seen the actual firing schedule used by Crane. For another thing, that is a power point presentation. PPPs are not technical reports, they are advertisements given to Generals and Admirals to 1) justify the money you have already been given, or 2) justify giving you more money for some reason, so view them as you would a TV commercial. If you notice that after page 44 where the author gives these dire predictions on bolt life and other woes, on page 52, they talk of wanting to put an electronic rounds counter in the grip to accurately schedule maintenance (so, give us some more money, please).
    All good points lysander.

    One other thing that should be considered is that the military M4 is a design which is probably easier on bolts than the typical civilian "carbine" AR. Both have a carbine length gas system, but the typical civilian version has a 16" barrel (and longer dwell time than the military M4 with a 14.5" barrel). Of course, there are lots of other variables. But this is one reason why extractor spring problems are more common in the civilian carbine. I apologize if any of this was mentioned earlier.

    Joe Mamma
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    Quote Originally Posted by Joe Mamma View Post
    All good points lysander.

    One other thing that should be considered is that the military M4 is a design which is probably easier on bolts than the typical civilian "carbine" AR. Both have a carbine length gas system, but the typical civilian version has a 16" barrel (and longer dwell time than the military M4 with a 14.5" barrel). Of course, there are lots of other variables. But this is one reason why extractor spring problems are more common in the civilian carbine. I apologize if any of this was mentioned earlier.

    Joe Mamma
    A 16" barrel adds around 1-1/4 to 1-1/2 inches to the length from the port to the muzzle, depending on whether the barrel is actually 16 inches from breech face to muzzle or 16.2 inches. Therefore, the bullet is in the barrel for, at most, an additional 45 to 50 millionths of a second, so the gas system is pressurized for an additional 50 millionths of a second. That minuscule extra time does not make any difference in bolt loading. In addition to the silly small time interval, consider that due to the the relatively large volume in the gas tube, the gas tube acts as a damper to the gas system. If you added an extra six or seven inches, and had a short (pistol) gas tube, you would see some big difference, but 1-1/2 inches and less that 50 millionths of a second, nah....

    Personally, the effects of going from 14.5 to 16 inch barrel on the operation are far overblown, if you keep the M4 standard 1/16" (0.0625"-0.0630") gas port. Seeing as some 16" carbine barrels have bigger gas ports than 1/16 inch (which is the wrong way to go if this were actually a problem), this is a more likely culprit. There are some factory 16 inch carbine gas-system barrels out there with 0.073" to 0.088" gas ports, absolutely ridiculous, unless you shoot under-powered Tula exclusively.

    As to extractor springs, until recently, most of the bolt assemblies I have seen had the old-style M16 type extractor, with or without an O-ring, as opposed to the gold, or copper hue M4 extractor spring used by the military since round the 2000s.
    Last edited by lysander; 11-06-19 at 10:38.

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