LMT eBCG repository of information

[HammeredSole]

The ebcg seems to be popular here, and I’ve personally used them in most of my builds. However, the information seems to be spread out into a lot of different threads, which each address specific issues or attributes.

I like to use them for their durability and handling of gas, especially with non-standard setups using short barrels or lightly gassed systems (ie BRT exc and ext). I’d like to get a thread going to address some major bullet points with this, and maybe highlight where this BCG is highly effective, performs worse than a standard BCG, or where it just increases the cost of the build and offers no real performance benefits.

I’ll start with a follow up thread of my experiences when I get some time in the coming days, but it would be great if people could start posting their anecdotal or technical assessments other than what we see in SmallArmsSolutions discussion and the like.

[Pappabear]

There was a very technical thread writing years ago, full-page and detailed killer info. I'll try to find it. Found it, it was so interesting I made a copy for such needs. Via Bill Alexander:

This is what Bill said: way above my pay grade,

Bill Alexander Bill Alexander is offline
Industry Professional
Join Date
Jun 2006

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 particularly 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 alleviate symptoms. The carrier is set up with a long 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 an 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 whether 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


PB