I've been looking for another 10.5 upper and ran across the BCM article saying "40% more dwell time" being why they highly prefer and make 11.5 over 10.5

This was back in like 2009 I think, just wondering if there has been any additional testing/technology to prove otherwise

Considering going from 10.5 to 11.5 only adds 4% length to the rifle...

A longer barrel is always better, in basically every way, except weight, length and balance.

Also, 11.5 with a carbine plus one gas system is the heat.

I am a fan of something’s BCM.  Great receivers, BCGs, cool hats.  Not a barrel fan.

So, I do t trust their barrel options, but 11.5” is 2x a better barrel than 10.3” and it is all about dwell time.  You can make a 10.3” work, but all the stars have to align. An 11.5” is super forgiving if anything you throw it’s way

Reliability on a 10.5” has been figured out by a few manufacturers like Colt, FN, Noveske, Daniel Defense, LMT just to name a few.  They all offer a damn reliable 10.5” firearm.

If your set on building your own, just use an adjustable gas block to slow things down.  The adjustable gas block “easy buttoned” most of the issues with short barrels.  The only other thing you loose is a little velocity.

A short barrel won’t be as reliable as a longer barrel. The loss is fractional. They also won’t last as long, which is significant but it’s a trade that’s easy to make.

As best I can recall, Dwell Time is the period between the bullet passing the gas port and exiting the muzzle. That makes this a simple math problem.

I don't have the gas port location in inches to verify, but I believe that claim is basic math.
Update - according to THIS the gas port in a carbine is at the 7 inch mark.
In a 10.5, the bullet has to clear 3.5" of barrel before the gas pressure drops.
In a 11.5, the bullet has to clear 4.5" of barrel before the gas pressure drops.
My math says that's about a 33% increase.

A previous Arf Thread has a nice chart in it that says about the same thing in relation to dwell time.

So, it looks like the question is.... Is 33% close enough to 40% to make the claim you're questioning at least passingly accurate?

I appreciate the math, but 33% is a significant amount in any situation, I would accept 33% gains in any situation in life lol

The question really has nothing to do with "is 33% close enough to 40%", the question is more so "is the dwell time an unnecessary unit of measure", as in dwell time is really for reliability, which I never have any reliability issues with a 10.5 but I also dont stray far from a few brands of ammo and mainly use the same brand uppers/bolts

The best 11.5s (KAC) have the gas port pushed out an inch, trading dwell time for lower port pressure.

DD MK18 uppers just work and no stars need to be aligned.

That said, I'm an 11.5" convertbut wouldn't have an issue with a DD MK18 upper.

The gold standard in 11.5" is the KAC Mod 2, IMO.

I agree and so does Green.

That said, 12.5 mid is even better.

My 10.5 LWRCi and Androcorp 10.5’s work well. Thought about switching one of my SBRs to 12.5 because I have a 11.5 KAC and also a 7.75 PWS.

When you add a sound suppressor, it makes that argument irrelevant, which is why 10.3" was chosen for the CQBR (MK18), and is still popular today.

They work unsuppressed too, but were designed with less variety of ammunition in mind, which is why commercial barrels typically start at 10.5" and have larger gas ports.

When you add a suppressor the other trade on overall system length to achieve a given sound level and balance is longer barrel with shorter suppressor vs shorter barrel with longer suppressor vs shorter barrel with shorter suppressor.  Often the longer barrel with shorter suppressor hits the sweet spot, letting the powder do its work in the bore.

Yeah, I’ll take a 12” barrel with a little can every time over a shorter barrel with a longer can.

better yet, a short barrel with little can...

Dwell time means jack shit. The AR15’s unlocking is mechanical, and that is controlled purely by the amount of gas allowed to reach the action in combination with the reciprocating weight.

If the rifle unlocks at the correct time with the correct gas pressure, the gun simply works.

Now if you go back 20 years when the default response to all AR15 problems was more gas, yeah 10.5’s sucked. But the industry has mostly figured it out and dwell time had nothing to do with it.

(I have over 3K through an unsuppressed .070 10.3 that’s never had a single failure and shoots as soft as my midlength)

Dwell time is one of several variables including port pressure. I don’t have any way to quantify it but my sense is that the 10.5 doesn’t have a short swell time issue it has a high port pressure issue. That said the softer combinations on the gun tend to have less port pressure, less gas volume and more dwell time. That’s why a 14.5 carbine with its smaller port and longer dwell time is softer shooting, has lower carrier velocity and demonstrably greater reliability and service life.

You're thinking of the KAC Carbine, 14.5"
The CQB, 11.5", has a carbine gas length and a huge gas port

That’s not true of the Mod 2.

OK. All off my Mod 2 CQB's (11.5") have carbine length gas system. Gas tube is 8 7/16" long, standard carbine tube 9 3/4"

One fits on a nipple at the back of the gas block and one buries 9/10ths into the gas block and you think they are the same length?

My 11.5" KAC CQB and Colt FBI barrels, gas port distance from closed bolt face, in inches:
KAC: 7.8
Colt: 7.8

According to my quick calculations (done while sick), an 11.5" has 35% more dwell length than a 10.5" and 50% more than a 10.3".

Does it matter? I don't think so. 10.3"s have been proven to be very reliable in SOF. Comparing apples to apples a 10.3" and 11.5" with proper gas port sizes I don't think there's a difference anyone will notice. Will the 11.5" barrel and or bolt last longer? Probably. But that isn't going to happen for many thousands of rounds. Way more than the cost of an entire rifle in ammo.

The 14.5" carbine has 78% more dwell length than the 11.5". Why not go with that? Why not 16" carbine? Funny someone in here recommended 12.5" mid which is even less reliable than 10.5" carbine.

I'm a big fan of the 11.5" barrel for the AR15 platform in 5.56mm. Good velocity, short barrel, reliable.

The 10.5 is also good, but you lose some velocity for a tad shorter length. Reliability can sometimes be an issue, but nothing that can't be fixed.

^^^  pretty much

There are certain barrel gas systems and barrel lengths that just sort of hum...  Carbine and 11.5" is great.  The timing is perfect, and forgiving of lots of ammo.  10.3" took a lot of trial and error, and is one of the most unstable barrels on the market, or at least it was.  10.5" gives you a smidge better, but to get everything working with 10.3" meant changing the gas port size, the bullet weight, the buffer weight and everything has to align perfectly.  11.5" just sort of works.

Going up from there, mid-length and 14.5" and rifle and 20".

The irony to me is that most 11.5” barrels seem to have a bigger gas port than their 10.3/5” counterparts. So it has more dwell time and a bigger gas port. Then we put cans on them, and they gas us out, and we take to the internet complaining and seeking adjustable gas blocks. But it will cycle Wolf, so there is that.

I went from a 10.3 to 11.5 and now 13.7.

The 13.7 is just as balanced and handy, but they midlength gas makes for a much better shooter.

YMMV

Tejas is correct. The Mod2 CQB is a standard Carbine length system and, IMO, a pretty big port. She’s a gassy girl suppressed

Now the Mod 2 Carbine, that has an extended gas system and is a pussycat.

That’s so ridiculously stupid... I have a PSA 10.5 kit that has run flawlessly for thousands of rounds. If PSA can figure out how to align the stars, it can’t be that hard.

Real dwell time is the time from when the gas port sees pressure to when the bolt unlocks. BCM doesn’t understand that, and now we have all of this crap. It has nothing to do with gas port location relative to the barrel length. Gas system length itself has a bigger impact on this, which is the same from a 10.3 to a 14.5. The gas is traveling the same distance, and the difference from a 10.5 to an 11.5 is literally thousandths of a second from when the bullet exits the barrel.

An AR15 is tuned to unlock at a specific pressure, at a mechanically defined moment. If it receives too much or too little gas at that moment, is defined entirely by the gas port diameter. The mechanical timing of that moment is dictated by the reciprocating weight, in conjunction with the basic operation of the bolt unlocking.

If you have a rifle built to unlock early, it’s going to have more problems. If you have a gun built to delay unlocking as long as it can while still functioning, it’s going to run better.

So by using a heavy reciprocating assembly (M16 carrier and heavy buffer), you’re slowing the point at which the gun unlocks, allowing pressure to drop and the brass to retract before unlocking begins. It’s at that point the gas port needs to be tuned to match that specific timing. If done correctly, you reduce rate of fire, reduce parts stress, get a softer shooting rifle, and a potentially cleaner running rifle.

My 10.3 with an 070 gas port and a 4.3 ounce buffer shoots soft as a midlength. I’d be curious to see what chamber pressure differences they saw at unlocking when they developed the 070 gas port, because I bet it’s astronomically reduced.

There's a ballance between
1) gas system length
2) port size
3) time between when gas enters the port and the projectile leaves the barrel (commonly referred to as dwell time)
4) the bcg/buffer system.
#4 is an entire discussion in of itself, but it's easiest for the end user to change and swap without taking the upper apart.

The three other things, think of like timing on an engine. There's trade offs and from a manufacturer standpoint you want a system that runs on the cheapest of cheap pump gas and still works every time. That's where bcm is pushing 11.5 because it's more forgiving and won't short stroke with steel case or be over gassed with 556 NATO loads. They don't want every bubba calling them saying their 10.3 won't cycle.

There's no such thing as best when it comes to barrel length. 11.5 is, in theory, more reliable from the standpoint it can probably run weaker ammo and still cycle. But 10.3 is shorter and more manuverable. 1.2 inches shorter is a bigger gap than +1 on a 11.5 so why not go 12.5 and get even more reliability and velocity? But 12.5 there's mid gas barrels out there which can shoot even softer. but short dwell time... See how this argument quickly spirals out of control and before you know it you're at 14.5 with mid gas.

End of the day my only answer is own one of each. Enjoy them all and don't think one is superior than another because each length has strength depending on application. 11.5 is a nice balance but certainly not the "best"

Rule of thumb with ARs is that the closer you get to a 20” rifle the better the gun is in every way except weight, length and balance.

Nothing scientific but my 11.5 is much nicer to shoot than my issued mk18. Less gassy, smoother, way less concussion indoors. The RIS II is outdated and unnecessarily heavy offsetting any advantage of the shorter barrel.

My 10.3 with .070 gas, H3 buffer and SOCOM suppressor shoots nicer than my 11.5 with more gas.

The added velocity of longer barrels is the strongest argument when a focus is put on ammunition efficiency in the 5.56 caliber.