Page 3 of 6 FirstFirst 12345 ... LastLast
Results 21 to 30 of 53

Thread: LMT Enhanced Bolt Question

  1. #21
    Join Date
    Mar 2009
    Location
    Alabama
    Posts
    2,770
    Feedback Score
    0
    Quote Originally Posted by LONGBOWAH View Post
    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.
    The bolt is not ION coated I have never see that in any statement from LMT regarding the bolt?

    If you read Bill Alexanders statement in this thread it explains the feature's of the bolt and carrier better than any other statement I have ever found.

  2. #22
    Join Date
    Jun 2006
    Posts
    11
    Feedback Score
    0
    Quote Originally Posted by Bill Alexander View Post
    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
    As an engineer by trade it always impresses me to see detailed information such as what was posted here. Another reason why I enjoy this board.

    I am interested in the long term reliability and parts sustainability of the entire 'system'. To this end I have adopted a mid-length gas system on a 14.5 barrel (BCM BFH). I have noted decreased bolt carrier speeds which in my opinion reduce stress on the moving parts in the system and therefore should increase parts longevity.

    I am interested in the bolt due to the technical advantages and relatively non-existent disadvantages but am also considering the enhanced carrier as well.

    Question is: Given a BCM BFH mid-length gas 14.5 barrel, H buffer and standard Colt carbine spring (with standard receiver extension) utilizing a wide range of available ammunition and weights how will the enhanced LMT carrier perform?
    Last edited by REDSKY383; 08-12-11 at 16:24.

  3. #23
    Join Date
    Mar 2009
    Location
    Alabama
    Posts
    2,770
    Feedback Score
    0
    I use one in a 14.5'' LMT with a carbine spring and H-buffer.
    I have about 4K through it give or take. Mostly XM193 & XM855
    but I have put 100rds of wolf steel case 62 grain and 600 rds of 55 grain PMC Bronze. Wolf seems to be close to the edge but I didn't have any malfunctions with it. I had to replace the buffer spring around the 2K mark it fell bellow spec. Still going on the factory gas rings and they still pass the gravity check.

  4. #24
    Join Date
    Jun 2009
    Location
    AZ
    Posts
    8,431
    Feedback Score
    9 (100%)
    Huh, this thread raised it's head. I say after all this time, mine (the bolts) have run great on everything. Short long fat skinny barrels. I've not had problems.

    You could justify the extra cash on the coating alone they clean real easy. I have to admit, I enjoy LMT coolaid.
    "Air Force / Policeman / Fireman / Man of God / Friend of mine / R.I.P. Steve Lamy"

  5. #25
    Join Date
    Aug 2006
    Location
    Southern California
    Posts
    1,111
    Feedback Score
    0
    Quote Originally Posted by REDSKY383 View Post
    Question is: Given a BCM BFH mid-length gas 14.5 barrel, H buffer and standard Colt carbine spring (with standard receiver extension) utilizing a wide range of available ammunition and weights how will the enhanced LMT carrier perform?
    You can have too much of a good thing.
    Just like Lmt recommends against the use of the enhanced carrier on carbine gassed 10.5" barrels, you may also see similar short stroking issues on a mid gas 14.5" barrel.
    Randall Rausch
    AR15 Barrel Guru

  6. #26
    Join Date
    Jun 2006
    Posts
    11
    Feedback Score
    0
    Quote Originally Posted by AR15barrels View Post
    You can have too much of a good thing.
    Just like Lmt recommends against the use of the enhanced carrier on carbine gassed 10.5" barrels, you may also see similar short stroking issues on a mid gas 14.5" barrel.
    That's what I was thinking. I was thinking that I could compensate for that by using the AR-15 BCG rather than the M16 BCG (lighten it up) and go to a carbine buffer vice the 'H' buffer (further lighten the mass). But in the long run I may just go with the bolt and forget about the carrier.

  7. #27
    Join Date
    Jul 2011
    Posts
    24
    Feedback Score
    0

    Exclamation

    Quote Originally Posted by Thomas M-4 View Post
    The bolt is not ION coated I have never see that in any statement from LMT regarding the bolt?

    If you read Bill Alexanders statement in this thread it explains the feature's of the bolt and carrier better than any other statement I have ever found.

    Just what is the coating of the LMT bolt? It looks like chrome or nickel boron, or something similar?

    Anyone happen to know? Surprising that LMT doesn't try to market this aspect of the bolt better to attract buyers considering the $150 price tag.

  8. #28
    Join Date
    Jun 2010
    Location
    MN
    Posts
    947
    Feedback Score
    6 (100%)
    Quote Originally Posted by Tyrantresister View Post
    Just what is the coating of the LMT bolt? It looks like chrome or nickel boron, or something similar?

    Anyone happen to know? Surprising that LMT doesn't try to market this aspect of the bolt better to attract buyers considering the $150 price tag.
    It's elctroless nickle.

  9. #29
    Join Date
    Jun 2010
    Location
    MN
    Posts
    947
    Feedback Score
    6 (100%)
    Any ideas how a 16" mid length with the enhanced carrier and bolt with a Vltor A5 stock would compare to a 14.5" mid length? Witch one would be "softer shooting"?

  10. #30
    Join Date
    Mar 2009
    Location
    Alabama
    Posts
    2,770
    Feedback Score
    0
    The carrier was optimized for the carbine gas system using a 14.5" -16" barrel. Anything other than that and you are outside LMT's specifications. So if it doesn't work for you outside of that there is nobody to blame but your self
    Now after saying that I have used it on a 10.5" barrel with carbine gas length system. With a suppressor attached it would lock the bolt back on the last round with out the suppressor it wouldn't lock the bolt back on the last round. I wouldn't bother with it on a middy gas system I think it would be too much of a good thing and you would be likely to have short strokes.
    Last edited by Thomas M-4; 08-27-11 at 08:33.

Page 3 of 6 FirstFirst 12345 ... LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •