Gas port location thoughts

[mark5pt56]

This a quote one can find when searching "dwell time in AR15" it's from Paul @ BCM

QUOTE
Why BCM chose the 11.5" SBR over the 10.5"

I get this question a lot. It is a good one. When we spec our program we can build anything we would like, so we sat down and looked at the pros and cons.

First Statement: I know a good 10.5" SBR can run very well. I personally own a semi MK18 type and it runs great. No problems.

A: Dwell time.
Dwell time is the time that your gas operated weapon maintains pressure to continue the cycling of the weapon. It primarily exsists from the time the bullet passes the gas port in the barrel to the time the bullet exits the muzzle. When you pull the trigger and fire the weapon the movement of the bolt carrier group unlocks the bolt, extracts, and ejects the spent casing. Then it cocks the weapon, feeds, chambers the next round, and then locks again. One of the thing that can make SBRs finicky is the dwell time (or lack of).

The 11.5" carbine is approximately 4% longer weapon than its' 10.5" counterpart, but this extra inch gives the barrel a 40% increase in length for dwell time. IMHO, this is an excellent trade off. This additional dwell time (all other things being equal) will allow the carbine to be more forgiving to different ammo types, extremes in air temperature, weak or worn extractor springs, worn extractors, buffer weights, etc.

Last Statement: For those folks who have a 10.5" that works very well, I would reply; me too. (see first statement)

If I had to "go to war" with a AR15 Carbine, I would grab the BCM 11.5".
The BCM 11.5" Runs Like a Dream.

Hope this info helps,
Paul

[Gödel]

These are great technical discussions but there is a lot information we don't know about when it comes to timing. I would like to see a timeline of when the bolt starts to unlock, when the gas tube is clear of the carrier key (and vents), bullet uncork etc.

You can find a report called "Comparison of a theoretical and experimental study of the gas system in the M16A1 rifle" August 1971, BRL Report 1548, somewhere on the internet. Lots of good background on the firing sequence and things they tested.

I started this thread below which did have some good discussion about port size and dwell:
https://www.m4carbine.net/showthread...rel-Discussion

I want to say Tom12.7 mentions a +1" gas system on the 11.5 as being beneficial. He is a good resource for these discussions.

For me, I would be interested in why gas trap systems went out of vogue. The Vltor FOG system was similar and it never went anywhere. I had one of these uppers and sold it before my suppressor came in... kicking myself!

Dwell time can't really be directly compared for different gas system lengths because the port pressure is different. The balancing act is similar to calculating recoil/bullet inertia - velocity x mass. You can maintain the same inertia by increasing velocity while decreasing mass. That's like how you need a shorter dwell to make up for a higher port pressure. So in the thread you started, you point out that the 16M has nearly the same dwell as the 20R - which means that the impulse is going to be larger from the 16M because it has the same dwell but more pressure. The ideal would be less dwell than the 20R to make up for the higher pressure.

[Rayrevolver]

Dwell time can't really be directly compared for different gas system lengths because the port pressure is different. The balancing act is similar to calculating recoil/bullet inertia - velocity x mass. You can maintain the same inertia by increasing velocity while decreasing mass. That's like how you need a shorter dwell to make up for a higher port pressure. So in the thread you started, you point out that the 16M has nearly the same dwell as the 20R - which means that the impulse is going to be larger from the 16M because it has the same dwell but more pressure. The ideal would be less dwell than the 20R to make up for the higher pressure.

Did you read through the whole 11.5M thread or just my first post?

Its not about the port pressure but the gas flow/total volume of gas. Remember the size of the port and the pressure will determine the total volume of gas delivered to the BCG during the dwell time (and blow down).

A 16M with the same dwell as a 20R, but obviously a smaller port, means a similar gas drive. The 16M will start to unlock earlier than the 20R, so that means it is unlocking at a higher chamber pressure, which requires more energy.

In my mind, the dwell time for the 20R is the benchmark so you should absolutely try to compare to it. You start to reduce that time and you reduce the time you have available to overcome things like a dirty system, lack of lube, debris, manufacturing tolerances, system drag, low powered ammo etc.

So no, its not as easy as increasing pressures and reducing dwell. There is more to it than simple math and engineering.

[MistWolf]

Dwell time is the time that your gas operated weapon maintains pressure to continue the cycling of the weapon. It primarily exsists from the time the bullet passes the gas port in the barrel to the time the bullet exits the muzzle.

Dwell time is short. A tiny fraction of a second short. Too short to have any affect on the pressure entering the gas system. There's a chart that's been posted over and over again, showing just what a small fraction of a second dwell time is. It's so short, that the bullet has exited the muzzle before the gas can positive the system. The system operates strictly off of the residual pressure remaining in the bore after the bullet has left the barrel.

If the pressure operated the action before the bullet left the muzzle, as longer dwell time would make no difference. Adding a suppressor would make no difference. Why? Because the bullet would be in the same place in the bore the action operated with each shot. The action would operate before the bullet (and the gas behind it) reached the suppressor.

The length of the barrel after the gas port determines how quick the gasses in the bore vents and pressure drops to atmospheric.

[Gödel]

Did you read through the whole 11.5M thread or just my first post?

Its not about the port pressure but the gas flow/total volume of gas. Remember the size of the port and the pressure will determine the total volume of gas delivered to the BCG during the dwell time (and blow down).

A 16M with the same dwell as a 20R, but obviously a smaller port, means a similar gas drive. The 16M will start to unlock earlier than the 20R, so that means it is unlocking at a higher chamber pressure, which requires more energy.

In my mind, the dwell time for the 20R is the benchmark so you should absolutely try to compare to it. You start to reduce that time and you reduce the time you have available to overcome things like a dirty system, lack of lube, debris, manufacturing tolerances, system drag, low powered ammo etc.

So no, its not as easy as increasing pressures and reducing dwell. There is more to it than simple math and engineering.

I think you didn't understand the OP. My point was that that "total volume of gas" is the product of three dependant variables - pressure, port size and dwell - and that it would be best to adjust the port location rather than the port size.

So while you want to keep the 20R dwell time, I'd rather see the port location adjusted so a 20R port size could be utilized. Moving the port forward decreases pressures and uses shorter dwell to decrease gas volume rather than a restrictive port size that will wear over time, while dwell will never change with wear.

I think a big port is "healthier" for the rifle's function, so I would change the port location/dwell to get the correct volume. You would change the port size to get a particular dwell, but you haven't explained why you think dwell is important in and of itself - especially when it requires the use of small gas ports which seems to lead to haphazard results when you change ammo.

[MistWolf]

Again, port location has a greater impact on gas drive than "dwell time". Port location has a much greater impact on port diameter than "dwell time". How much gas is generated has a greater impact on gas port location than "dwell time". Bore diameter has a greater impact on gas drive than "dwell time".

What we are calling "dwell time" is really blow down time. The shorter the distance from gas port to muzzle, the less time it takes for pressure in the bore to drop to atmospheric at the port location.

[Gödel]

Again, port location has a greater impact on gas drive than "dwell time". Port location has a much greater impact on port diameter than "dwell time". How much gas is generated has a greater impact on gas port location than "dwell time". Bore diameter has a greater impact on gas drive than "dwell time".

What we are calling "dwell time" is really blow down time. The shorter the distance from gas port to muzzle, the less time it takes for pressure in the bore to drop to atmospheric at the port location.

What do you mean it has a "greater impact"? Size, location and duration all have a major effect on the volume of gas that gets to the carrier. Too little barrel after the port and it won't matter how big the port is - the gun won't cycle. I don't see how you can discount any one of the three factors that pressurize the gas system.

[Rascally]

I'll just leave this here...be sure to read the description. A rendition of the (never issued) Colt 605.

https://www.brownells.com/firearms/r...rod124180.aspx

[Clint]

Chamber pressure has a curve during firing.

This curve includes both the time the projectile is in the bore and time it takes for residual pressure to blow down after the projectile has exited the bore.

The total impulse or area under the curve determines the ability of the pressure to do work, such as accelerate the projectile or cycle the action.

The area under the curve is really a summation (integration) of the pressure at each instant, or to state it simply, PRESSURE X TIME.


In a straight blowback weapon, the entire pressure curve is used to power the action.

In a gas operated, locked breech weapon, placement of the gas port relative to the muzzle determines which later portion of the pressure curve is used to power the action.

A port placed at the muzzle (i.e. gas trap) uses only residual pressure, while a port placed at the case mouth uses the entire curve.

Obviously, a port placed somewhere in the middle uses uses a combination.


Once a portion of the curve is selected by the port placement ( the Available Gas Drive ), the port size scales that down to the Actual Gas Drive used to power the action.


There are many combinations of port placement and size that can all yield the same total number for Gas Drive at the front end of the system.

If this was a simple system, you could select any of these combinations and they would produce identical results at the back end of the system.


But it's not, and they don't.

[Sparky5019]

Chamber pressure has a curve during firing.

This curve includes both the time the projectile is in the bore and time it takes for residual pressure to blow down after the projectile has exited the bore.

The total impulse or area under the curve determines the ability of the pressure to do work, such as accelerate the projectile or cycle the action.

The area under the curve is really a summation (integration) of the pressure at each instant, or to state it simply, PRESSURE X TIME.


In a straight blowback weapon, the entire pressure curve is used to power the action.

In a gas operated, locked breech weapon, placement of the gas port relative to the muzzle determines which later portion of the pressure curve is used to power the action.

A port placed at the muzzle (i.e. gas trap) uses only residual pressure, while a port placed at the case mouth uses the entire curve.

Obviously, a port placed somewhere in the middle uses uses a combination.


Once a portion of the curve is selected by the port placement ( the Available Gas Drive ), the port size scales that down to the Actual Gas Drive used to power the action.


There are many combinations of port placement and size that can all yield the same total number for Gas Drive at the front end of the system.

If this was a simple system, you could select any of these combinations and they would produce identical results at the back end of the system.


But it's not, and they don't.

So I’ll resurrect this to ask…

What is the minimal distance from port to muzzle that is reliable? I’ve seen various Dissipator configs with seemingly different lengths past the port.

Does this change based on gas system length or port size?