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Thread: LMT Enhanced Bolt Question

  1. #11
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    Let us perhaps be a little more precise with regard to the details of the LMT enhanced bolt and carrier system. As shown the details are very often misinterpreted.

    In the first instance one must understand why the system exists, more particulaly in the context that the design is brilliant in its execution, but as reported frequently on the errornet is not always reliable. The design basis for the assembly is specifically to enhance the durability of the M4 configuration as currently issued. This gun is to some extent unbalanced. Port pressures running military grade ammunition are well in excess of the design levels which leads to very high carrier acceleration rates (+22 f/s/s). this combined with chamber pressure drop often creates the undesirable condition that the bolt commences to unlock while still subject to a degree of head thrust from the cartridge case. The result is that the lugs are subject to a biaxial loading of both shear and bending.

    The LMT system being both the carrier and the bolt seeks to operate directly in this gun and elleviate symptoms. The carrier is set up with a longer delay during the initial portion of its movement. To facilitate this longer cam path and movement of the bolt forwards in the carrier, the front edge of the carrier is extended such that it still continues to retain the extractor pivot pin. Additional exhaust vents act to drop the piston pressure faster and to relieve any blowby at the tail of the bolt.

    The bolt itself is of particular interest. The function of the dual spring extractor is frequently misinterpreted as an attempt to add spring force to the extractor claw. Rather it reduces the fatigue that the extractor spring(s) undergo by allowing the use of longer springs with lower K values; the % relative compression during the movement of the extractor is reduced. Remember that additional extractor force is not required now that the carrier is slowing the extraction cycle. The mitigation of stress in the bolt is accomplished in several ways. Material is the least visible change but is important to the design. The traditional Carpenter 158 is abandoned, being replaced by a significantly tougher grade from a different manufacturer. The lugs themselves are generously radiused between lugs and at the rear the diameter is actually reduced to allow a larger transition radius to be machined. The incorrectly identified sand cuts on the lugs are stress relief cuts. These allow any individual lug to elastically deform and give a smoother load over the contact patch. While this type of feature is very difficult to calculate and even more difficult to implement it helps to place the lug in a true shear load rather than amplify the bending moment. As noted the lug opposite the extractor is relieved. This feature prevents the unequal transfer of load to the two opposite lugs but I would argue that the stress relief groove already in place largely accomplishes this purpose. This is a academic quibble so I will bow to LMT in this respect. There is one additional feature that can be found in the bolt, but I am not at liberty to disclose the detail.

    When considering wether to use the LMT parts one must consider the weapon. Correctly ported guns will derive little or no benefit from a carrier that is specifically set up to absorb excessive port pressures and some degree of residual case pressure. If not sufficiently gassed the reliability will suffer. This is not a fault of the carrier rather a mistake made in the application so be careful with simple substitutions. The bolt itself is exemplary. If not constrained by a $ value the bolt is a worthy addition to any rifle and will do nothing but enhance the durability of this part of the system.

    Bill Alexander

  2. #12
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    Quote Originally Posted by Dano5326 View Post
    I noted the decreased tension of the extractor (in comparision to the current SOCOM 5coil copper washed extractor spring, black insert, and Oring)
    When you say '5 coil' do you mean the 3.5 coil Colt spring? Or is there some 5-coil spring that is used by SOCOM? Do you have a photo of one?


  3. #13
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    Further to Bills excellent description of the Enhanced Bolt, you can see from the picture the generous radius and stress relief cuts



    I don't use my ARs for work so I don't feel I need a super duper bolt like this. Having said that I have broken 2 DPMS bolts in my 7.62X39 AR. The only bolt that keeps chugging away in this upper is the LMT Enhanced 7.62X39 bolt

    You would think the lobster tail extractor would more tension but it does not ( for the reason Bill outlined above )

    It would be be wrong however to assume it has insufficient tension. See pic below of a stuck 7.62X39 case that I managed to pull out using the charging handle...and a block of wood ! The extractor had no problem holding on to this case. As you can see the build up from the laquer coating grabbed on to this case

    Last edited by trg42; 07-28-10 at 10:39.

  4. #14
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    Would the LMT enhanced bolt be good to go in a BCM 16" recce middy?

  5. #15
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    I have the Enhanced Bolts and they run great on all ammo and even when I put them in "other" carriers. From everything I have read, Bottom line, you did buy one of the finest BCG available. You did good.

  6. #16
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    so if the LMT enhanced bolt was designed to counter the imbalances of the carbine gas system would it be correct to assume there is a diminished advantage using it in a midlength system?

  7. #17
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    Not for use with barrels less than 14.5" according to LMT.

    http://www.lewismachine.net/product....0a3dcdc25c31c5

  8. #18
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    Quote Originally Posted by ForTehNguyen View Post
    so if the LMT enhanced bolt was designed to counter the imbalances of the carbine gas system would it be correct to assume there is a diminished advantage using it in a midlength system?
    You are confusing the carrier with the bolt.
    The CARRIER is designed to counter the problems of the carbine gas system.
    Randall Rausch
    AR15 Barrel Guru
    California Precision Rifle Club founding member

  9. #19
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    Quote Originally Posted by markdeerhunter View Post
    I thought the enhanced since it is chromed would be smoother cycling and easier to clean.Maybe I'm dense but what could cause a bolt upgrade to be finnicky pertaining to ammo?
    The carrier is ported differently to increase dwell time.
    Last edited by LONGBOWAH; 08-27-10 at 10:48.

  10. #20
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    The four big (and maybe an interrelated 5th) things you get with the LMT enhanced BCG are:

    1. Carrier is ported differently to increase dwell time and thereby ease extraction (5th thing).
    2. Bolt is "ion" coated.
    3. Cartridge case is fully supported by bolt head.
    4. Bolt heat treat is "stronger".

    You most likely will not have ANY problems with your LMT bolt/carrier.
    Last edited by LONGBOWAH; 08-27-10 at 10:48.

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