LMT enhanced BCG

[Bgd]

I have been looking for an LMT enhanced BCG and can't find one so I backordered one from Brownells. I have never used one and am curious about the longer dwell time and if it increases velocity. If and when i receive it I will run some tests. I use mine primarily for Deer hunting with supers and any velocity increase is a plus.

[Wanderson]

Can’t comment on dwell time or velocity but I’ve had an LMT enhanced 7.62x39 bolt for about 8 years now and I bought it cause 7.62x39 bolts are notorious for shearing sooner than 5.56 bolts and LMT had the best rep for durability among 7.62x39 bolts.
IMHO LMT bolts are hands down the best quality. Google ‘Battlefield Vegas LMT bolt’, it’s a range that rents FA Firearms and they know what lasts and what doesn’t. They use LMT & DD BCGs.

But I’ve not seen any chrono numbers that prove the LMT adds FPS. I thought the purpose of the LMT was to stay closed longer so it unlocks at a lower pressure for more reliable operation in lower pressure setups in Full Auto mode.

[dellet]

There is a noticeable increase in velocity when shooting suppressed, more so with subs. I have recorded as much as 40 fps. The increase with supers probably falls within normal variations.

If you handload, you will be able load hotter loads without running into cycling issues, will have longer brass life and less gas in your face. Better accuracy is debatable.

[Dr.Phil]

As far as availability, it can be challenging.
LMT only makes a few runs a year.
If you call them, they can let you know when they will be available.

As far as function / purpose, here are some resources:
https://smallarmssolutions.com/home/lmt ... lt-carrier

Taken From: http://www.m4carbine.net/showthread.php?t=3379

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

If you haven't seen it, Chris Bartocci made a great video about Enhanced BCGs.
(Chris is the author of "Black Rifle II: The M16 Into the 21st Century")
https://www.youtube.com/watch?v=HZkK70Miz60
https://www.youtube.com/watch?v=18A-9Q__nNY
(The second video goes over the Surefire eBCG designed by Jim Sullivan and Jim personally praises the LMT eBCG.)