LMT enhanced bolt/carrier question

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dellet
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LMT enhanced bolt/carrier question

Post by dellet » Wed Oct 14, 2015 11:02 am

I installed one of these in a setup that I use mostly for supers. It is something that I run pretty hard. The full bolt face and delayed unlocking time of the bolt has definitely cut down on the abuse of brass in hot loads and I am very happy with that application.

The question is more about sub-sonic use.

It would seem that the improvements that I am seeing for supers, might actually be a detriment to subs. At least for my application. I have been working up loads with the fastest powders that would cycle and not worrying about if they worked un-suppressed. Current load is the 194 grain Lehigh ME and AA#9. It's very quiet, very accurate and cleaner than most loads.

The question I have can probably only be solved by trying, but I am wondering if delaying the unlocking of the bolt, might allow the port pressure to drop enough to cause cycling issues.

In my mind, it seems that what I wanted for supers, is exactly what I don't want for subs. Any thoughts or experiences?
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plant.one
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Re: LMT enhanced bolt/carrier question

Post by plant.one » Wed Oct 14, 2015 11:17 am

ive got one in my pistol build, and it doesn't seem to bother anything - 9" aac barrel, pistol gas. runs subs with 1680, d063 and 4227 just fine.

havent tried it in my carbine gas upper.
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dellet
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Re: LMT enhanced bolt/carrier question

Post by dellet » Wed Oct 14, 2015 11:31 am

plant.one wrote:ive got one in my pistol build, and it doesn't seem to bother anything - 9" aac barrel, pistol gas. runs subs with 1680, d063 and 4227 just fine.

havent tried it in my carbine gas upper.
The pressure drop with faster powders happens very quickly, and gas volume is generally less, so timing of the bolt release is more important the faster the powder. This is why the pistol length is easier to work with than carbine and a smaller port is used, the bolt unlocks during a higher point in the pressure curve.

I am already at the trailing end of the curve.
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Dr.Phil
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Re: LMT enhanced bolt/carrier question

Post by Dr.Phil » Wed Oct 14, 2015 12:37 pm

Good question...

First, AA#9 is a tough powder to work with in 300 BLK.
The functional range for both supersonic & subsonic loads is very narrow.
(The following comment is not for you, but for anyone in general that comes across this thread.)
For that reason, it is one of the more dangerous powders to use in 300 BLK because things can go bad very quickly.

Back to the LMT e-BCG, I don't think that you will have that much of an issue.
Reason being, the unlock cycle is already in progress prior to the gas pressure being significantly lowered.
That generates inertia that continues the unlock cycle of the BCG.
(The most recent version of the LMT e-BCG has the tiny weep hole in the main gas chamber and slows it down a bit more than ones without.)
Image
I run the LMT e-BCG on all of my ARs, a Gen 3 version on my 14.5" 5.56 NATO upper and the Gen 2 version on my 10" Noveske 300 BLK upper.
Because the Gen 2 model doesn't have the weep hole, I do not have any negative effects using subsonic loads.

Looking at it from an engineering perspective, I don't thing you would see an issue unless it was with loads that were already on the bubble of functioning.
The one factor that will impact how the LMT e-BCG functions is dwell time.
Since subsonic loads already have a longer dwell time, this issue becomes less critical.

Bottom line:
You may have to tweak your AA#9 subsonic load to work properly if it was barely functioning the gun previously.
The advantages of the LMT e-BCG are significant across the board and IMO, warrant any changes in loading.
I would argue that it is one of the best BCG setups around and in fact, its features have showed up on LWRC & Knights "Enhanced" products.
LWRC's Version / Copy
Image
Knight's Version / Copy
Image
LMT's Original Design
Image
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dellet
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Re: LMT enhanced bolt/carrier question

Post by dellet » Wed Oct 14, 2015 1:31 pm

Thanks Doc,

That's kind of where I am on this load, a little on the edge. I had not thought about the inertia being a constant instead of a variable. I have some wiggle room in velocity, and running a captured spring so I can also make adjustments there if needed.

Actual cycling is flawless, bolt hold open took some work. As an example of how close it was, about 50% of the time, bolt hold open, was by the catch locking on the carrier instead of the bolt on the 174 grain Lehigh. I had loads that worked with 175 SMK's, but the brass bullet just does not seem to create the same backpressure.

I have also been lucky enough to be able to try different suppressors. Working in one certainly does not guarantee working in another. More learning.

I have a much better understanding of the frustration the project must have been for Robert getting this to work in all four configurations and barrel lengths.
300 Blackout, not just for sub-sonics.

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Re: LMT enhanced bolt/carrier question

Post by Dr.Phil » Wed Oct 14, 2015 2:34 pm

dellet wrote:I have a much better understanding of the frustration the project must have been for Robert getting this to work in all four configurations and barrel lengths.
For sure!
Regardless of how people feel about Robert, you have to admire the work he did on 300 BLK.
(Being in the public spotlight with a project like 300 BLK can bring out the haters.)

It didn't occur to me until I read your reply, the other thing you are working against is the bullet weight.
Anything under 190 grains in a subsonic loading can be very tricky to get cycling properly in an AR.
You are tripling up by throwing AA#9 and an LMT e-BCG into the equation.

If it can be done, I'm sure you will find a way.
You might have to mess around with seating depths, magnum primers, and crimp tension to get there though.

Please let me know if I can be of assistance.
I have some AA#9, 175 SMKs, & some Lehigh Defense 174 CF bullets in my inventory.

Cheers,

Phil
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dellet
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Re: LMT enhanced bolt/carrier question

Post by dellet » Wed Oct 14, 2015 3:39 pm

Dr.Phil wrote:
dellet wrote:I have a much better understanding of the frustration the project must have been for Robert getting this to work in all four configurations and barrel lengths.
For sure!
Regardless of how people feel about Robert, you have to admire the work he did on 300 BLK.
(Being in the public spotlight with a project like 300 BLK can bring out the haters.)

It didn't occur to me until I read your reply, the other thing you are working against is the bullet weight.
Anything under 190 grains in a subsonic loading can be very tricky to get cycling properly in an AR.
You are tripling up by throwing AA#9 and an LMT e-BCG into the equation.

If it can be done, I'm sure you will find a way.
You might have to mess around with seating depths, magnum primers, and crimp tension to get there though.

Please let me know if I can be of assistance.
I have some AA#9, 175 SMKs, & some Lehigh Defense 174 CF bullets in my inventory.

Cheers,

Phil
One of the things working to my advantage is a 7" barrel, I can use a little more powder without a severe gain in velocity.

Depending on your setup the 174's will probably work with your 30P. It has more back pressure. not sure if it is the baffle design, length or combination. I prefer the GMT 300 on this setup, as it seems quieter to me as the shooter. The 30P had full lockback with less powder, but was noticeably louder at the back end. I would really like to be able to try an Ultra 7, basically the same length as the Gemtech so it would be a better comparison. I just have not had one available to try. And i may have to revisit the 174' now that I have the 195's going.

It certainly exercised the grey cells on this one.
300 Blackout, not just for sub-sonics.

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Re: LMT enhanced bolt/carrier question

Post by rebel » Wed Oct 14, 2015 10:10 pm

I think with what dellets doing, with the set up he has, it's doable. Hell, he is almost there. Good to see a technical post on this operating system from two guys that know their stuff. I have considered running this bolt on my SBR as it is a bit overgassed and a bit tougher on brass. There are other ways to do this as dellet and I have discussed, but I think he can get there with the set up he has. Doc, I completely agree once you are knee deep into it, you recognize Robert's vision and why they set up their rifle the way they did. Leave it to us to push those parameters to the limit :lol:
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Re: LMT enhanced bolt/carrier question

Post by MMA10mm » Thu Oct 15, 2015 12:28 pm

Great conversation. Thank you guys!

I've long wished to get AA#9 running in a sub load, and have on my plate getting to it with Sierra 220MKs, but this thread is inspirational. I love the Lehighs, have a small stockpile of them, and think they are the way to go for a sub.

Dellet - I'd love to know what you've done with the Lehighs and AA#9. If you don't feel good about posting the load publicly, could you PM me?

I'm running a 10.5" barrel, pistol gas, .105" port, a 1978-era chrome Colt M16 B&BCG, and an H2 buffer (although I keep the standard buffer in the gun bag and can switch that out easily).

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Re: LMT enhanced bolt/carrier question

Post by dellet » Thu Oct 15, 2015 3:11 pm

MMA10mm wrote:Great conversation. Thank you guys!

I've long wished to get AA#9 running in a sub load, and have on my plate getting to it with Sierra 220MKs, but this thread is inspirational. I love the Lehighs, have a small stockpile of them, and think they are the way to go for a sub.

Dellet - I'd love to know what you've done with the Lehighs and AA#9. If you don't feel good about posting the load publicly, could you PM me?

I'm running a 10.5" barrel, pistol gas, .105" port, a 1978-era chrome Colt M16 B&BCG, and an H2 buffer (although I keep the standard buffer in the gun bag and can switch that out easily).

I am reasonably comfortable with the load data, but want to get one more trip to the range for accuracy and velocity. Basically want to confirm it can be repeated. A lot of the work for load development was done with a 190 smk to save money.

Last load tested with the Lehigh's was a three shot group at 45 yards, avg velocity 1030. Cold bore shot was 3/4" higher than the next two shots that over lapped, using an ACOG.

I would think you could get it running with the barrel you have, just how much velocity you might pick up might be an issue. The other caution would be about running it through a suppressor and stability. I needed the can for full function, your port is slightly larger.

Last thing to note is stability, I had no problems but, MY BARREL IS A 1/6 TWIST. Be sure to check it before you run it through a can
300 Blackout, not just for sub-sonics.

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