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Thread: Bolt life question.

  1. #1
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    Bolt life question.

    Hey guys I have a question on bolts. I read the BCG PiP for the M4 has been cancelled because none of the entrants offered any improvements, so this got me curious about why they did not offer any extended life and why bolts seem to break so early.

    I did some searching but could not find anything pertaining to my question.

    From what I have read the AR-15 bolt in an M4 under heavy firing schedules is generally good for 6-10,000 rounds, while bolts on piston guns seem to be good for 20,000 or so.

    Im wondering what makes the difference? The bolt even in piston AR-15's seems to be made the exact same way as the standard bolt, but they seem to last longer.

    Is this a material difference, the heat generated from the DI system or is it a little bit of both?

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    Where did you get your numbers? Especially those that relate to piston guns?

    I will say this. In my experience unless someone starts with a virgin gun, there is no way to predict bolt life. Did they start with virgin guns when they did these tests?

    Were the weapons properly lubed? What was the firing schedule?

    The reason I ask is because I have at least one bolt right now that has over 7.5K rounds through it, many of those are suppressed and yet it seems to be fine. As a matter of fact I have only changed the ejector spring.

    Quote Originally Posted by sinlessorrow View Post
    Hey guys I have a question on bolts. I read the BCG PiP for the M4 has been cancelled because none of the entrants offered any improvements, so this got me curious about why they did not offer any extended life and why bolts seem to break so early.

    I did some searching but could not find anything pertaining to my question.

    From what I have read the AR-15 bolt in an M4 under heavy firing schedules is generally good for 6-10,000 rounds, while bolts on piston guns seem to be good for 20,000 or so.

    Im wondering what makes the difference? The bolt even in piston AR-15's seems to be made the exact same way as the standard bolt, but they seem to last longer.

    Is this a material difference, the heat generated from the DI system or is it a little bit of both?



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    Quote Originally Posted by Iraqgunz View Post
    Where did you get your numbers? Especially those that relate to piston guns?

    I will say this. In my experience unless someone starts with a virgin gun, there is no way to predict bolt life. Did they start with virgin guns when they did these tests?

    Were the weapons properly lubed? What was the firing schedule?

    The reason I ask is because I have at least one bolt right now that has over 7.5K rounds through it, many of those are suppressed and yet it seems to be fine. As a matter of fact I have only changed the ejector spring.
    Exactly...

    Shoot it until it breaks.

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    All things being equal, from a mechanical stand point, I fail to see how a bolt in a piston system will have a lower MTBF than a bolt in a DI system.
    Last edited by skullworks; 06-04-12 at 14:15.

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    Quote Originally Posted by skullworks View Post
    All things being equal, from a mechanical stand point, I fail to see how a bolt in a piston system will have a lower MTBF than a bolt in a DI system.
    Heat transfer.
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    Quote Originally Posted by jwperry View Post
    Heat transfer.
    Well, I would agree with you if it wasn't for the fact that in a DI system the gas pressure pushes the bolt forward during unlock (minimizing friction between the bolt lugs and the lugs in the receiver extension); whereas in a piston system the bolt lugs have full engagement during unlock, thereby subjecting the bolt lugs to a higher level of stress throughout unlock compared to a DI system.

    Therefore the question is what is more harmful: heat transfer or mechanical stress? In this case my money is on mechanical stress.

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    Well, no. Extraction occurs in exactly the same manner in an inline system and an op-rod system, as far as the amount of force on the bolt lugs are concerned.

    I thought we covered that in this thread:

    https://www.m4carbine.net/showthread.php?t=101258

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    Quote Originally Posted by Arctic1 View Post
    Well, no. Extraction occurs in exactly the same manner in an inline system and an op-rod system, as far as the amount of force on the bolt lugs are concerned.

    I thought we covered that in this thread:

    https://www.m4carbine.net/showthread.php?t=101258
    Pretty much, to mch pressure at the chamber to press the bolt against it.

    That said I was reading BB's posts on a different website and they were talking about maint. And one big thing was how on M4's bolts should be replaced every 4,000 but can go 10,000. Barrels should be replaced every 12,000.

    They also mentioned how pistons greatly increase bolt life, thats what had me wondering, WHY do pistons greatly increase bolt life from 4,000-10,000 to 20,000?(on the HK and LWRCI guns)

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    Quote Originally Posted by jwperry View Post
    Heat transfer.
    I was pleasantly surprised to learn how little heat is transferred to the bolt, bolt carrier and upper behind the barrel. It was a lot less heat than I thought. I fired 300 rounds in about 20 minutes and none were more than warm. The barrel transfers a hell of a lot more heat to the upper than the DI system. There are claims that DI is easier on a bolt due to the forward thrust of the gasses which works against the rearward force on the bolt. I don't know if this is true or not. I would like to see some hard evidence or calculations.

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    Quote Originally Posted by sinlessorrow View Post
    Pretty much, to mch pressure at the chamber to press the bolt against it.

    That said I was reading BB's posts on a different website and they were talking about maint. And one big thing was how on M4's bolts should be replaced every 4,000 but can go 10,000. Barrels should be replaced every 12,000.

    They also mentioned how pistons greatly increase bolt life, thats what had me wondering, WHY do pistons greatly increase bolt life from 4,000-10,000 to 20,000?(on the HK and LWRCI guns)
    KAC and LMT are making similar claims with their enhanced bolts, so I dont think its piston vs di as much as bolt construction.
    Last edited by VIP3R 237; 06-04-12 at 15:10.
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