11.5” Mid Length Barrel Discussion

[MistWolf]

I've known for some time it's pressure & flow, not dwell time, but it wasn't until I saw Ray's musing that the final piece of evidence fell into place for me

[tb-av]

It's clear that it's not about dwell time at all- It's pressure and flow

Then why do so many people run the LMT Enhanced carrier? I thought it's claim to fame is longer dwell time. btw.. not doubting.. just asking.

[MistWolf]

Then why do so many people run the LMT Enhanced carrier? I thought it's claim to fame is longer dwell time. btw.. not doubting.. just asking.

This is a good question. The confusion comes from using the term "Dwell Time" without defining what we're talking about. The dwell time being referred to in this thread is the time it takes the bullet to travel from the gas port to the muzzle.

The dwell time referred to concerning the enhanced carrier is the time from when the primer is struck to the time the bolt begins to unlock. The increase in time is due to the extra length of the straight portion of the cam track in the carrier. The carrier has to travel further before the cam can start unlocking the bolt. This gives the case a fraction of a second longer to shrink away from the chamber walls before being yanked out. The bolt dwell time has nothing to with the bullet dwell time

[mtdawg169]

This is a good question. The confusion comes from using the term "Dwell Time" without defining what we're talking about. The dwell time being referred to in this thread is the time it takes the bullet to travel from the gas port to the muzzle.

The dwell time referred to concerning the enhanced carrier is the time from when the primer is struck to the time the bolt begins to unlock. The increase in time is due to the extra length of the straight portion of the cam track in the carrier. The carrier has to travel further before the cam can start unlocking the bolt. This gives the case a fraction of a second longer to shrink away from the chamber walls before being yanked out. The bolt dwell time has nothing to with the bullet dwell time

Exactly. The enhanced carrier is probably more appropriately referred to as an increase in unlocking time or delayed extraction.

[tom12.7]

The term "dwell" can be correctly used differently to describe the correlations in time of different parts of the system as a whole. It can be used correctly to describe items like the "time" between the gas port and the muzzle, bolt over travel, initial carrier movement to initial rotational bolt caming, etc.
Action systems, in a basic level are driven by a gas impulse. The system acts on the energy delivered to it and stores it primarily in the action spring. Some energy is used in unlocking and extraction, the amount used depends on the overall timing of the system as a whole, as it it varies. Hammer reset and magazine drag can be considered fairly constant in a range. Unlocking and extraction can be dynamic, or varying depending.
The action system can only operate with the gas pulse applied to it. If it's insufficient it causes issues obviously. If it is excessive, other issues arise. In the balance of the gas pulse presented, a span of operation can be observed.
Let's keep this basic and say that there's no can involved and a given of the base system. For a 20" 5.56, how do you determine the widest span of preferred function for a given gas port size and/or gas system length? You can start with the baseline and test with different combinations. When seeing results, you can determine if the span widens or narrows in terms of function. Most would agree that a wider span that includes reliability and durability would be preferable to one that reduces that span.
With a barrel length of 11.5" in 5.56, what gas system length and port size gives the wider or widest span of reliability and durability without a can? With a barrel length of 11.5" in 5.56, what gas system length and port size gives the wider or widest span of reliability and durability with a can? When comparing the two types, you can trend the data to see when the 2 are more similar or distant in port size and location.
There are other variables that can be added to the mix to improve function overall for the 5.56 11.5" in terms of spring, buffer, and carrier. That is better suited after determining some of the other basics.
Overgassing is a problem that happens with too much frequency in this system as an unfortunate band aid. The unfortunate solution to many is an adjustable gas block, but it is not possible for an end user without the proper equipment that is really needed to see if they can really achieve the point that they actually do have the span of operation for serious use.

[tb-av]

@Mistwolf... I see what you mean now... I didn't realize the two points of dwell. Thanks!

@tom12.7 -- why do you say the adjustable gas block is unfortunate. If the system is usually over-gassed by nature and you can under-gas it to the point of failure then adjust it back up some. that would seem to me to be ideal rather than unfortunate.

IOW, like knowing you have a car that will go 120mph but only need to drive it somewhere in the 35-55 range with driving 5mph simply being too slow. Even if I had no speedo I know the car will go way faster than I ever need. It seems like while I might not be able to define the span of operation I can define the low end ( bolt won't lock back ) and I can guesstimate my high range which most people seem to just say... "give it a 1/4 turn more."

I thought the adjustable gas block was looked at as valid solution and not an unfortunate situation. Especially if you want to go can on, can off, different can, etc..

[tom12.7]

An adjustable gas block may have the ability to lead to a preferable to MRBS and MRBF, but there is no reason to believe that the users can do so with limited resources.
A proper fixed gas port is beneficial over an adjustable port most anytime, if the fixed port is in the proper span of operation.

[mig1nc]

The FN FAL had a great adjustable gas system. I don't see them as a negative at all.

Sent from my HTC One using Tapatalk

[tom12.7]

The FAL can be a brass stretcher with it's own issues. The context in this is the basic AR platform.

[tb-av]

An adjustable gas block may have the ability to lead to a preferable to MRBS and MRBF, but there is no reason to believe that the users can do so with limited resources.
A proper fixed gas port is beneficial over an adjustable port most anytime, if the fixed port is in the proper span of operation.

I guess that was my unasked question as well. Why is fixed beneficial over adjustable? Is it because of the lack of additional mechanical failure points or something else? that's the part I don;t understand... ok well, a part I don't understand.

so let's say Fixed and with adequate test gear you have a desired range of 5 to 10 on a scale of 1 to 20. So you fix it at 7.5 and it's considered the best possible. Now let's say with adjustable you can guess it to 6 or guess it to 8 and you are in the span but obviously you will have to re-adjust before the perfect fixed one... But besides that, is there something else undesirable about adjustable blocks over fixed?