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Thread: Bolt Life?

  1. #11
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    Quote Originally Posted by ndmiller View Post
    As much as that's probably great advice, is just looking it over at every cleaning going to see the premature stuff or is it microscopic failure becoming failure? I've never seen a problem (counted lugs before), so not entirely sure what I'm looking for [1]

    Is there a round count or time that just replacing the bolt is just good preventive maintenance? [2] I ask as they are relatively inexpensive ~$50 and that's kind of what I do with pistol magazine springs, every couple years I just replace my carry gun mag springs.
    [1]At least seven locking lugs....

    [2]Why would you want to replace a perfectly good bolt?
    - The two reliability tests started with bolts known to be crack free (all were NDI'd prior to test).
    - The bolts were NDI'd every 6000, 8,400, and 12,000 rounds, most of the cracks found in these inspections were probably visible to the naked eye, and those that weren't were detectable under 10X magnification. Some of the bolts showed developed cracks as early as 6000 rounds, some of the bolts were still crack free at 12,000 rounds. Something I failed to mention was none of the bolts were removed just because they had cracks. Parts were removed only when they were the cause of a stoppage. And, those bolts with cracks detected at 6000 round when on to shoot 10,000 or 12,000 more rounds before they were pulled. Also, there were no rifle casualties, ie, non of the guns "blew-up"...

    I am fairly confident that even if you crack a lug around 5000 rounds, you can go another 10,000 rounds before it becomes a reliability issue (unless you like to shoot really high pressure loads +62,000 psi).

    So, if you feel that you just have to replace bolts on a regular schedule, replace them when the barrel is shot-out.

    (Oh, and I replace springs only when they are below the minimum length. For pistols that I don't have technical data on, when they are about 1/2 inch shorter than they were when I got the gun....I haven't had to replace many springs.)

    Cracks, you are looking for something like this:


    Last edited by lysander; 11-02-19 at 18:28.

  2. #12
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    Quote Originally Posted by lysander View Post
    USAF TO. 11W3-5-5-41 and TO 11W3-5-5-42

    I am a firm believer it regular cleaning of stuff, not white glove cleaning, but a good thorough cleaning so you can inspect to see if you have cracked or broken parts.

    And, I also feel that all parts in one of these type guns can be on an "on condition" replacement schedule. The most safety critical item in the gun is the bolt, and we have seen that up to three lugs can be missing and it holds together. A cracked lug can go a long time before becoming a missing lug, and a missing lugs will go for some time before becoming a safety issue. If you clean the gunk off your bolt after a days shooting, you should be able to catch any problem before it becomes a safety or reliability issue.

    An AR can go 30,000 rounds without cleaning, but why? Are you that lazy just to skip a little cleaning and inspection of parts? In all of the endurance tests ever reported, the usual schedule was a complete disassembly, cleaning and inspection every 1200 rounds (40 magazines).
    Thanks, amigo, I'll see if I can find those. Lazy? If anything I'm likely to be over-aggressive and replace what might still be considered to have some degree of useful life in the name of "better safe than sorry." Sounds like "regular scheduled inspection, replace on finding No-Go condition" is the way to go.
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  3. #13
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    do ak's experience this kind of breakage?

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    Quote Originally Posted by Skeeter98 View Post
    do ak's experience this kind of breakage?
    No, AK bolts are known to last a lifetime. Two huge locking lugs at non-stressed areas.

    Even a normal case scenario of 15k rounds is a very low bolt life for a service weapon to be honest. AR platform seems to be plagued by some really weird and obscure low level issues. I'm still surprised that the base weapon has not been redesigned/overhauled entirely in its lifetime. Probably the closest to redesign is an HK416 iteration.

  5. #15
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    Quote Originally Posted by Skeeter98 View Post
    do ak's experience this kind of breakage?
    The receivers crack at around 50k rounds. I don't know how long a bolt might last if you kept rebuilding.

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    I'm very skeptical about the part where it says M16 bolts break at the same rate as M4 bolts. I've heard of M16 bolts lasting forever.

  7. #17
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    Honestly, if I can afford to fire 15k rounds, I can afford a new bolt at 10k and when I rebarrel the rifle. Great thread for data.

    What is the bolt life when firing M855A1 exclusively?

  8. #18
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    Okay whiz kids, could the AR still be an AR if the bolt had, say, four bigger and thicker lugs? The AK has two big ones, what would change if the AR had 4 stronger ones instead of 7? Obviously a matching barrel extension would be necessary, but is there something inherent in the AR15's design that would preclude fewer but stronger lugs?
    11C2P '83-'87
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  9. #19
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    Quote Originally Posted by okie View Post
    I'm very skeptical about the part where it says M16 bolts break at the same rate as M4 bolts. I've heard of M16 bolts lasting forever.
    14K to 15K is a long time in the military.

    Most non-infantry (a majority of the Army) unit probably go through 500 rounds per year, if that many, two qualification cycles. That's 28 years, almost forever.

    I'll wager that most M16A1s bought in the late 1960's still had original bolts and barrels when the M16A2 started to be fielded.

    A number of things could have caused the up swing in bolt consumption starting in 2000:

    1) a large number of M16s made prior to 1975 entered rework and had cracked bolts.
    2) the Army started shooting more.
    3) the ammunition pressure specifications increased (they did, BTW, in 1999 the M855 pressure limit went from 55,000 psi average/61,000 psi maximum to 58,700 psi/64,700 psi.)
    4) a combination of all of these.

  10. #20
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    Quote Originally Posted by ABNAK View Post
    Okay whiz kids, could the AR still be an AR if the bolt had, say, four bigger and thicker lugs? The AK has two big ones, what would change if the AR had 4 stronger ones instead of 7? Obviously a matching barrel extension would be necessary, but is there something inherent in the AR15's design that would preclude fewer but stronger lugs?
    Well, that would take some doing...but, yes.

    The thing about the AR bolt head is it has relatively tall skinny lugs. Compare them to other weapons. These have very low, wide lugs





    The tall, skinny lugs allow for more bending and stress at the root:

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