LMT Enhanced BCG question

[ScottsBad]

There's more to it than that, but ultimately the LMT enhanced carrier doesn't require the same level in gassing to properly cycle compared to a standard carrier for most carbine type uses. There's always exemptions and trends in lesser details.

OK, so it doesn't require as much gas. Does the increased velocity of the carrier before unlock indicate that cyclic rate is increased or decreased? In other words, would you run a lighter buffer, heavier buffer or the same weight to achieve the same smooth operating cyclic rate? How would it help? And under what conditions?

As Tom stated, there's a little more to it than that, but... Essentially, yes.

The way the gas propagates into the carrier and the toroidal gas expansion chamber is different, and it more efficiently imparts force into the carrier as well as permitting more efficient and faster venting of the gasses from that expansion chamber. The port that vents from the actual combustion chamber directly is different, changing the pressure curve inside the chamber. The additional momentum created by the longer cam pin track is also different.

The fact that the carrier, as a system, permits case obturation to more closely approach cessation, permits both a larger window of operation and less gas required to drive the system under most circumstances.

However, and Tom has noted this repeatedly in the past, going shorter than 11.5" approaches a precipice where the overall volume and intensity of the gas pulse MAY not be enough to fully drive the system in order to overcome the inevitable stiction force that the not fully obturated case is applying to the chamber walls. This correlates to, among other things, the length of barrel between the gas port and the muzzle, the blow down time of the system, even bullet weight, etc.

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If I understand you correctly, chamber pressure is alleviated more quickly allowing the case to shrink (cessation) more quickly from its expanded (obturation state). This along with a longer cam track might aid extraction. Is this right?

Otherwise, I'm not sure what the advantage is. Requiring less gas might be noble if some of the gas were not entering the BCG anyway. Increasing carrier speed before unlock seems like it would put extra stress on the bolt and cam pin.

We could go deeper into details. Some ask for more info about comparing other portions, as an example, swept volume vs constant volume within that specific portion as a system by itself. The expansion ratio is important, so is the exhausting of that, including items like condensates from that and where they are directed towards, etc. However, the above posting is pretty much a preferred guide.
The less than 11.5" 5.56 DI AR carbine gas limit really comes to a point into where most 5.56 users would either choose between into using that system only unsuppressed, or only suppressed, with specific porting for either.

Perhaps I'm ignorant as to the PROBLEM that the LMT Enhanced BCG is intended to solve. Does it fix over gassed suppressed rifles? Does it provide reliable cycling to under gassed guns? What was it developed for? Because I cannot think of a reason to use it.

I'm seriously curious, and looking for a REAL reason to buy one. Thanks. I'm a big fan of LMT, but I need to understand this better.

[ABNAK]

Otherwise, I'm not sure what the advantage is. Requiring less gas might be noble if some of the gas were not entering the BCG anyway. Increasing carrier speed before unlock seems like it would put extra stress on the bolt and cam pin.



Perhaps I'm ignorant as to the PROBLEM that the LMT Enhanced BCG is intended to solve. Does it fix over gassed suppressed rifles? Does it provide reliable cycling to under gassed guns? What was it developed for? Because I cannot think of a reason to use it.

I'm seriously curious, and looking for a REAL reason to buy one. Thanks. I'm a big fan of LMT, but I need to understand this better.

Chris Bartocci addressed this. First part of the article:

http://www.sadefensejournal.com/wp/?p=3188

[ScottsBad]

No, there are less stresses and strains. Unlocking at a later time allows pressures to reduce further. That allows a reduced torsional load on the cam pin/ bolt/ BE/ carrier/ upper receiver (at that location) during unlocking. This is different than mechanical wear/ cutting during feeding/ locking at the opposite of portions of most of these (not a portion of the upper), similar can happen during the first half of the action sequence.
To help keep the carrier velocities and other timing events in check, the carrier works better with rifle like action systems rather than carbine ones overall. Not that carbine actions will not run ok, but better with a more rifle like action for most all.
Starting with a properly gassed barrel, the LMT enhanced carrier, and a properly sprung and massed A5 action system, you can have a superior combination over the conventional carrier and carbine action by a fair margin for most carbines.
There are some things that I would have hoped that LMT would have done differently, but if used as prescribed, it's the best commercially available option at this time.

Hi Tom, thanks, but how does does it reduce torsional load? Is the angle of the cam track reduced or something? I've noticed on the SCAR 16 that the cam track is longer and maybe slower than an AR, but its obviously not done to reduce gas pressure in the BCG. I'm guessing that the longer cam track in the EBCG, along with additional gas venting ports in the carrier, allows pressure in the BCG and chamber to be reduced before unlock and extraction. Could you confirm this?

If that's the case, I can see how the EBCG could be helpful. Seems funny, however, that the EBCG would be aimed at rifle and mid-length gas systems given that these systems are already put less stress on the operating system than carbine systems. And are already thought o be more reliable. Is this a solution looking for a problem? Sincere questions.

[tom12.7]

In your first part, the increased velocity is simply that the gas expands the BCG chamber more before resistance from unlocking happens. It does tend to accelerate the cyclic rate overall. It would tend to require more mass to get to the same cyclic rate, as less work is required to unlock, and more mass would be needed to sustain the cyclic rate preferably.
Chamber pressure remains the same, doing work at different barrel blowdown time changes. Directing gas into the carrier for operation with a pair of orifices allows enough gas flow for most without venting excess through the gas key during exhaust events, requiring an additional vent on the full bolt stroke to properly exhaust those. There's more to this than that, but that is a portion.
The LMT carrier was designed to add durability to the M4 14.5" carbine H1/H2 using at the time M855.
There are gives and takes when looking at these. There are some possible avenues we can look into if interested.

[ScottsBad]

Chris Bartocci addressed this. First part of the article:

http://www.sadefensejournal.com/wp/?p=3188

Thanks, that helps a lot.

[ScottsBad]

In your first part, the increased velocity is simply that the gas expands the BCG chamber more before resistance from unlocking happens. It does tend to accelerate the cyclic rate overall. It would tend to require more mass to get to the same cyclic rate, as less work is required to unlock, and more mass would be needed to sustain the cyclic rate preferably.
Chamber pressure remains the same, doing work at different barrel blowdown time changes. Directing gas into the carrier for operation with a pair of orifices allows enough gas flow for most without venting excess through the gas key during exhaust events, requiring an additional vent on the full bolt stroke to properly exhaust those. There's more to this than that, but that is a portion.
The LMT carrier was designed to add durability to the M4 14.5" carbine H1/H2 using at the time M855.
There are gives and takes when looking at these. There are some possible avenues we can look into if interested.

Thanks Tom. Sure, I'm interested in understanding as much as possible. I don't have a specific problem to solve, but I'm open to trying something different as I've decided to do yet another high quality build.

[Rayrevolver]

If that's the case, I can see how the EBCG could be helpful. Seems funny, however, that the EBCG would be aimed at rifle and mid-length gas systems given that these systems are already put less stress on the operating system than carbine systems. And are already thought o be more reliable. Is this a solution looking for a problem? Sincere questions.

I think you misinterpreted Toms statement about Rifle vs Carbine action systems, NOT gas systems. He is saying the combo of the LMT E-carrier with a carbine gas system would benefit from a Vltor A5 (rifle-esque) action system or an actual rifle action system (RE/buffer).

From what I can tell, the initial design of the carrier was focused on the M4 Carbine. And people had issues with shorter barreled uppers. Whatever they did they seem to work for what seems like all barrel/gas combos.

EDIT: My last point is to highlight that changes have been made. If you start researching and find yourself in 2007 LMT E-carrier threads the hardware is not the same as the new gens.

EDIT EDIT: Last post in this link is a treasure trove of info.
https://www.m4carbine.net/showthread...cal-Comparison

[MistWolf]

I believe the problems with the early LMT Enhanced BCG was due to the enhanced bolt. In my case, the problem was specifically with weak extractor springs

[ScottsBad]

I think you misinterpreted Toms statement about Rifle vs Carbine action systems, NOT gas systems. He is saying the combo of the LMT E-carrier with a carbine gas system would benefit from a Vltor A5 (rifle-esque) action system or an actual rifle action system (RE/buffer).

From what I can tell, the initial design of the carrier was focused on the M4 Carbine. And people had issues with shorter barreled uppers. Whatever they did they seem to work for what seems like all barrel/gas combos.

EDIT: My last point is to highlight that changes have been made. If you start researching and find yourself in 2007 LMT E-carrier threads the hardware is not the same as the new gens.

EDIT EDIT: Last post in this link is a treasure trove of info.
https://www.m4carbine.net/showthread...cal-Comparison

Yeah thanks, I sort of figured that out after I read the article posted by ABNAK, but that was after I made my post.

I only run an A5 on one rifle, and that's because when I compared two identical rifles (same barrels Noveske, same upper Vltor MUR, same BCGs BCM), one with an A5 (Vltor A5, A5H3 & Sprinco Green) and one with a carbine action (LMT tube, Sprinco Blue, H2 buffer) , I liked the rifle with the carbine action better. It was smoother and had less felt recoil. Both were perfectly reliable with all ammo I shoot (no steel).

I can see how a little longer action would give me a larger operational window. And if the LMT EBCG helps it might be an interesting combination to try.

I spend a fair amount of time trying different buffers and springs, I guess I could try working an Enhanced BCG in to see what happens.

[JG007]

Didn't LMT recommend it for rifle gas + rifle buffers until recently?

I'm 90%+ sure about the rifle buffer