LMT Enhanced BCG Compatibility

[Dr.Phil]

Did you get a chance to play with this at all over the weekend Stick?

[Stickman]

I just got done working a project with the 300 AAC yesterday, and was not able to use the LMT E-BCG. I should be hitting another range with it, but with low ammo combined with using it with MIL guys, I'm very doubtful that I'll be dropping it in.


I can say that use of the sub and super sonic ammunition in FA, both with and without the can, that recoil was minimal and the can is squared away. Shooting subsonic ammo is almost like shooting squib loads. You think to yourself that there has got to be more recoil, but that refreshing deep thump as the round hits the berm eliminates any questions. Forget saying that this is quieter than the MP5s that I've run, I could shoot people all day long in a crowded place and never be heard.

[JasonAAC]

I just got done working a project with the 300 AAC yesterday, and was not able to use the LMT E-BCG. I should be hitting another range with it, but with low ammo combined with using it with MIL guys, I'm very doubtful that I'll be dropping it in.


I can say that use of the sub and super sonic ammunition in FA, both with and without the can, that recoil was minimal and the can is squared away. Shooting subsonic ammo is almost like shooting squib loads. You think to yourself that there has got to be more recoil, but that refreshing deep thump as the round hits the berm eliminates any questions. Forget saying that this is quieter than the MP5s that I've run, I could shoot people all day long in a crowded place and never be heard.

So awesome to hear first-hand accounts. The feel of the shooting can not fully be described, it' very smooth.

And yes, the can is a winner.
Thanks Stick!

[Stickman]

I've got an NCOIC (just retired) who specialized in weapons and training who I will be doing some work with today, I'll see if I can get the LMT BCG out today. Round count for it will be limited, but I'll try to remember.

[Dr.Phil]

Excited to hear your results.
Have my upper reciever, PRI Tube, ARMS 38 SPR-PEQ-2 Sleeve, and BUIS in hand.
Getting ready to order the BCG and Charging Handle, so your results will steer that purchase.

Thanks again!

[Dr.Phil]

I came across this write up of AR operations and was very impressed to see the explanation of port pressures and dwell time at the end.
http://www.ar15barrels.com/prod/operation.shtml


This information reinforces my hypothesis that a 9" or 10.5" 300 BLK barrel with a pistol length gas system will be similar to a 5.56 NATO 14.5" barrel with a carbine length gas system.
It would appear to me that the timing of LMT Enhanced BCG would work within a range of .15ms & .25ms of net dwell time.
It would be cool to see what the differences would look like if someone could post up a similar graphic of the 300 BLK from quick load.

[300Blk]

Except that LMTs comments about 'when it will work' are highly undefined.

You cannot make any prediction about what will work from this chart.

Basically, you need to test it for thousands of rounds, like AAC did with the bolt carriers we picked. We ruled out a very popular brand because it did significantly more cam pin damage to the upper than others.

There is not a single bolt carrier group which I feel is better than the AAC. Maybe we should sell them.

[jppd47]

Do you think it will work good in 5.56 as well?

[300Blk]

Yes.

[Dr.Phil]

Other than your mighty super duper extractor, what else is different with the BCG that you developed for the AAC BLACKOUT uppers?

As far as cam pin damage and durability, the best solution I have found is the LWRC Advanced Cam Pin.

I run one in my LMT E-BCG and it nor my upper show any cam pin wear / gouging.

The statements made by LMT are similar to ones that you have made regarding the 300 BLK.
It is just staight forward "It wasn't designed for that" & "It may or may not work".

The best explanation of the LMT Enhanced BCG function that I have seen has come from Bill Alexander.

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

The whole thread that this is posted on can be found here: http://www.m4carbine.net/showthread.php?t=3379