LMT enhanced bolt/carrier question

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dellet
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Re: LMT enhanced bolt/carrier question

Post by dellet » Mon Nov 09, 2015 1:44 pm

Just a bit of an update on this. Same issue different load.

This load is a lighter bullet, sub-sonic load.

I worked up the load to cycling and almost locking back. The BCG would lock back with the latch catching on the carrier instead of the bolt face.

I felt this was a good point in load development to try the LMT enhanced BCG since the effect positive or negative could be seen easily. The only problem was the LMT is a F/A carrier instead of S/A that I was using.

Two things happened, one expected the other was not. The fired round ejected, but short stroked and jammed the next round. I expected that that due to the weight difference.

The curious thing that happened was the rounds I fired went super sonic. Unfortunately the chronograph was not working due to clouds, but the same rounds the day before were 1000-1030fps.

I ended up reducing the charge weight by .3 and the COL. by .020 and ended up with a subsonic round, that would lock back correctly. This was the same load that had failed to do so the day before and before switching the BCG.

I have not measured the headspace difference between the two bolts, which could be a contributing factor also.

So the questions are,

Why the significant velocity gain with a loss of bolt speed?

Why was it corrected with less powder, but essentially the same air space between the powder and bullet?

Is this caused by the increased time the bolt is closed, carrier weight or combination of the two?
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Re: LMT enhanced bolt/carrier question

Post by rebel » Mon Nov 09, 2015 2:37 pm

You know, now that you have put that question in writing that way, I think I see at least why the rounds went super. By delaying the bolt it creates a situation similar to a bolt action, more gas available for the push. I may be wrong, but that seems to be what happened.
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Re: LMT enhanced bolt/carrier question

Post by dellet » Mon Nov 09, 2015 3:20 pm

rebel wrote:You know, now that you have put that question in writing that way, I think I see at least why the rounds went super. By delaying the bolt it creates a situation similar to a bolt action, more gas available for the push. I may be wrong, but that seems to be what happened.
More gas pressure for the bullet would also mean more to operate the bolt.

Solving the problem with less powder, less seating depth, points to a more efficient powder burn that might have created a higher gas volume while not reaching the same velocity?

In theory everything I did should have reduced the chance of success and that's what confuses me.
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Re: LMT enhanced bolt/carrier question

Post by Dr.Phil » Mon Nov 09, 2015 4:48 pm

dellet wrote:Solving the problem with less powder, less seating depth, points to a more efficient powder burn that might have created a higher gas volume while not reaching the same velocity?
This is my best hypothosis.
Percent of powder burned prior to reaching the gas port would create a velocity change that coresponds to with your description.

The LMT e-BCG likely has less to do with the issue.
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Re: LMT enhanced bolt/carrier question

Post by dellet » Mon Nov 09, 2015 5:15 pm

Dr.Phil wrote:
dellet wrote:Solving the problem with less powder, less seating depth, points to a more efficient powder burn that might have created a higher gas volume while not reaching the same velocity?
This is my best hypothosis.
Percent of powder burned prior to reaching the gas port would create a velocity change that coresponds to with your description.

The LMT e-BCG likely has less to do with the issue.
I think it shows how on the edge of working the load was. I understand changing BCG's was a major function change, but some of the load changes were almost in the window of what you would change trying to adjust for accuracy.

Might have to try a regular F/A carrier just to understand better, too many variables.

Hopefully that means it will just burn cleaner :P
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Re: LMT enhanced bolt/carrier question

Post by steven11b » Sat Nov 14, 2015 7:11 pm

This is a interesting thread as I just purchased a Lantac e-bcg. I think its design is along the lines of the LMT version. It is 2 ounces heavier then a standard FA bcg at 11.7oz weight and I was wondering if my loads would change after installing this because of increased lock time it advertises. I currently use 4227 with190gr Nosler CC and when using the other bcg I have had no problems developing loads with this 10.5 Noveske barrel. So seeing your results do you think I should add a few grains of powder and work down again with the sub loads? Then changes to the gun are the upper receiver itself and the bcg.
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Re: LMT enhanced bolt/carrier question

Post by dellet » Tue Nov 17, 2015 10:10 am

steven11b wrote:This is a interesting thread as I just purchased a Lantac e-bcg. I think its design is along the lines of the LMT version. It is 2 ounces heavier then a standard FA bcg at 11.7oz weight and I was wondering if my loads would change after installing this because of increased lock time it advertises. I currently use 4227 with190gr Nosler CC and when using the other bcg I have had no problems developing loads with this 10.5 Noveske barrel. So seeing your results do you think I should add a few grains of powder and work down again with the sub loads? Then changes to the gun are the upper receiver itself and the bcg.
I have had more time to work the load I am working with, and I would just run what you have and see what happens. No changes for at least 100 rounds.

The first rounds after installing the bolt mysteriously picked up speed. After chasing velocity back and forth for about 100 rounds I have settled back in within a .100 of a grain with a slightly shorter seating depth.

It does seem to run cleaner, less unburned powder in the upper and on the shooting table.

Your experience could be completely different, I am actually working up a sub sonic load that functions using a much lighter bullet.
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Re: LMT enhanced bolt/carrier question

Post by steven11b » Tue Nov 17, 2015 12:00 pm

Ok, my new upper reciever is currently getting cerakoted so I am still waiting for its return. I have probaly 300 rounds of 110gr varmegeddon supers that I plan to run through it before testing out subs, just to break it in as it will be new reciever and BCG while using my same barrel. I had no sub loads left after taking the gun apart so I did load up 50 in a ladder went .5 grains above and .5 grains below in .2 gr increments off my load I settled with before the changes. I use IMR4227 and I think it was decently clean in my opinion so if it cleans up even more with this e-bcg I would be happy.

Using the bcg with advertised longer lock time do you feel it adds the capability to load a lighter sub round? I have 175gr smk that would he intersting to try if that's the case.
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Re: LMT enhanced bolt/carrier question

Post by dellet » Tue Nov 17, 2015 12:49 pm

steven11b wrote:Ok, my new upper reciever is currently getting cerakoted so I am still waiting for its return. I have probaly 300 rounds of 110gr varmegeddon supers that I plan to run through it before testing out subs, just to break it in as it will be new reciever and BCG while using my same barrel. I had no sub loads left after taking the gun apart so I did load up 50 in a ladder went .5 grains above and .5 grains below in .2 gr increments off my load I settled with before the changes. I use IMR4227 and I think it was decently clean in my opinion so if it cleans up even more with this e-bcg I would be happy.

Using the bcg with advertised longer lock time do you feel it adds the capability to load a lighter sub round? I have 175gr smk that would he intersting to try if that's the case.
I don't have enough experience with enough different loads. I have not even ran more than five rounds of an old load though it yet, and was developing this one when I changed over.

My concern was that it would need a little more gas volume and pressure to operate a borderline load and that has not been a problem. If I use a load that worked before and it no longer cycles, locks back or goes super then I might have a theory.

As for cleanliness, if anything I thought it should get worse, but for the same reasons can't say accurately. The other load I ran a few through this is with AA #9 and it burns very clean compared to the 4227 load I am working with now.

One of the main advantages of the longer un-locking time is it give the brass time to shrink before it is extracted. This is an advantage in super sonic firing. In a suppressed system this will allow more gas to leak by the brass into the upper before the bolt opens. Allowing more of a load that tends to run dirty back in your face. Without a suppressor it may be less than without the enhanced bolt, but there is plenty for me the way it is working now.
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Re: LMT enhanced bolt/carrier question

Post by steven11b » Tue Nov 17, 2015 1:08 pm

Well I guess there are many variables so I'll just shoot alot of these supers like I said to get a solid break in. Then I'll move to testing out subs. I am really hoping this bcg helps with gas mitigation as I use the standard novekse gas block. Plus the amount of money I invested in it!
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