Dwell Time?

[alamo5000]

In other words the guy that invented the AR15 was a few rungs higher on the IQ scale than most folks.

[BWT]

In other words the guy that invented the AR15 was a few rungs higher on the IQ scale than most folks.

Oh I don’t doubt that at all. I don’t see any issue with any configurations - I just was curious as we seemed to be getting into deep technical nuance.

I have several AR-15’s and this one has always just been something I saw the increase in dwell time, perceived that as a benefit, and correlated (perhaps confirmation bias?) with my positive experience with the cheapest ammunition available working pretty much like clock work if I cleaned the chamber once going over 500 rounds.

I’ll re-read your last post again tomorrow and thanks for the detailed reply.

[MistWolf]

In theory those pressures would be the same, but when you have varying gas system lengths it changes everything, which is what I was pointing out. A pistol gas, vs a mid length, vs a rifle all do the same thing but under vastly different conditions.

Of course varying gas system lengths changes pressure at the port. But pressure when the bullet is at a certain point, say 7 inches, is the same whether the barrel is 7 inches or 27 inches.

A longer gas system is more forgiving because you're tapping the barrel at a point where the pressure is lower and the port diameter is larger. Because the pressure is lower, changes in port diameter become less critical. The system will have a larger span of operation and will tolerate variations better.

When the gas port is closer to the chamber, a smaller gas port must be used and because the pressure is so much higher, even the tiniest variation in gas port diameter has a larger impact on gas flow. The system is less tolerant of variations.

[OlongJohnson]

comparing the 14.5 to the 16 is traveled by the bullet in a fraction of a millionth of a second

You've written that a few times now. It's wrong.

Things happen fast, but we can do the math. At 3000 fps, a bullet travels 1 inch in ~0.028 milliseconds, or ~28 millionths of a second. So an inch and a half is 0.042 ms. The bullet's accelerating, so it's going slower at the shorter barrel lengths, increasing the travel time.

Gas port positions from some random web site:
Carbine - 7.3 inch
Mid - 9.3 inch
Rifle - 12.65 inch

So look at those on the chart of pressure vs. barrel position that was posted.
Look up a chart of muzzle velocity vs. barrel length. http://ballisticsbytheinch.com/223rifle.html
Then build a spreadsheet and play with the math, charting out travel time from port opening to barrel uncorking, and pressures at port opening and barrel uncorking.
Run an approximation of the area under the curve (pressure times duration) for various configurations of gas system vs barrel length to see how much motive force is available to put pressure in the bolt carrier cavity behind the bolt.

I guarantee you'll understand this thing better if you do all that correctly.

Read through this first, you might be satisfied enough to skip the work, or it may provide useful inputs.

https://ndiastorage.blob.core.usgovc...hilipDater.pdf

[MistWolf]

Why insist on a false dichotomy? It's both. But mostly the other guy is right. The pressure behind the bullet before it leaves the barrel dominates, but the time of blowdown after uncorking does have an effect.

If you were to try to operate an AR only off the gas that enters the gas tube during the time the bullet travels from gas port to muzzle, the gas port would have to by HUGE to pressurize the expansion chamber that quickly. Even then, I'm not certain it'd work because the bullet covers that distance so quick. The majority of gas used to operate an AR is what's left in the barrel after the bullet departs.

If you study the slow motion vids, you'd see the carrier doesn't even begin to move until the bullet is gone from the muzzle.

[MistWolf]

You've written that a few times now. It's wrong.

Things happen fast, but we can do the math. At 3000 fps, a bullet travels 1 inch in ~0.028 milliseconds, or ~28 millionths of a second. So an inch and a half is 0.042 ms.

Ok, I got confused by mili. I thought mili meant one millionth. Thanks for the correction.

[alamo5000]

If you study the slow motion vids, you'd see the carrier doesn't even begin to move until the bullet is gone from the muzzle.

In reality that is completely irrelevant as the AR15 is designed to have a delay in the bolt opening.

From the internet:

"How the AR-15 BCG Works

The AR's bolt is a delayed locking and rotating bolt. This means the bolt physically rotates and locks itself inside the firing chamber. When a round is fired, the pressurized gas it creates is fed back into the bolt via the gas tube. As pressure builds inside the bolt, it rotates, unlocks, and drives backward to eject the spent shell casing from the round just fired. Upon returning to the firing chamber, the bolt picks up a new round to chamber it. This process happens incredibly quickly. Less than one second is all it takes for the firing pin to ignite the round, gas pressure to build, the bolt to unlock, drive back, eject the spent casing, and chamber a new round. Although this looks quick and simple at first glance, there are a lot of important things happening at once to make the bolt perform all these tasks."

You are mistaking when the bolt actually moves with what everyone else is saying. The primary force that is imparted to move the bolt is done during the time when the bullet passes the port and exits the muzzle.

The fact that the actual bolt moves later on in the sequence is completely irrelevant as that is part of the basic function and design of the gun itself.

[Paul Blackburn]

The following statement is directly from Paul Baffoni at BCM
"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."

However, technically, "DWELL TIME" is the time from primer ignition to bolt unlock.
The AR community commonly refers to dwell time as the length of the barrel forward of the gas port.

[BWT]

The following statement is directly from Paul Baffoni at BCM
"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."

However, technically, "DWELL TIME" is the time from primer ignition to bolt unlock.
The AR community commonly refers to dwell time as the length of the barrel forward of the gas port.

I didn’t mention this yet but my SBR’s are all BCM. That quote and trend is what led me that way years ago.

Thanks for that input Paul!

[ASH556]

Caveat: I do not consider myself an expert of any kind and may be very wrong.

Qualifications and experience from which I speak: twice completed Will Larson's (RIP) Semper Paratus AR15 armorer's course. 20 years total turning wrenches on AR15's; 15 of which behind the counter of one of the busiest shops in Atlanta; point being: not just assembling new stuff from parts, but lots of time working on other people's broken shit.

My primary AR since 2011 has been a 10.3" SBR with a suppressor.

My profession is the sales & engineering team manager for a company that supplies sever service isolation and control valves to Power Plants, Gas Utilities, and Space Coast end users (among others).

Having therefore laid my dick on the table (small though it may be), my initial response to this thread is that one key piece of information missing from this argument is gas port size. In other words, volume.

Ever heard of Boyle's Law? "The absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the volume it occupies if the temperature and amount of gas remain unchanged within a closed system."

In layman's terms, there is a drop in pressure, and increase in velocity, as a fluid (in this case, gas) goes through a change in flow area. Ignoring for a moment the differences in bore volume and gas tube volume and considering the flow coefficient (Cv) of the orifice (gas port) in the barrel, there will be an associated drop in pressure and increase in volume as the gas goes thorugh that orifice.

Speaking again in layman's terms, why is a midlength gas port larger than a carbine gas port? Why is a rifle length gas port larger than both a carbine and a midlength? Why is an ideal 10.3" carbine length gas port the same size as a 16" midlength gas port?

Is is possible (I think it is in fact quite likely) that the size of the gas port has much more to do with the weapon's reliability than the length of the gas tube.
In my humbe opinion, when someone says that they find gas system length "X" to be "sooooo much softer shooting" than gas system length "Y" they are (1) drinking Koolaid, and (2) lack proper knowledge and understanding to compare apples to apples. For sure a properly ported midlength will feel softer than your over-gassed carbine length gun. However, what if you properly ported your carbine length gun? How would it feel then?

I currently have an 11.5" Colt HRT barrel on my gun. Before that I had a 10.3" Colt barrel. Both are factory lengths and both are factory ported at 0.070. Both are over-gassed. How do I know? Because I'm running an H3 buffer and still get lockback on an empty mag without the suppressor attached. On a previous build I had ADCO cut down a 14.5" Colt SOCOM barrel to 10.3, but leave the port at the factory 0.063. Guess what? That barrel ran like a dream. But, it still locked back on an empty mag without a suppressor and with an H2 buffer. So, still somewhat overgassed.

Ok, I'm rambling. My overall point for the thread is that if you're really going to go down the rabbit hole and do the math to figure out ideal, you've got to consider volume, which is mostly affected by the gas port size, assuming consistent ammo pressure; and that volume will likely have a much greater affect than port position/gas tube length on performance and reliability.

ETA: In other words, it is disengenuous to say that X barrel length has more dwell time than Y barrel length without first considering and comparing the gas port sizes used between the two.

Also, I switched from my 10.3 to 11.5 for increased terminal performance by keeping my bullet over the expansion velocity threshhold for longer. Not for reliability reasons.