Seems like the AR platform plateaus at 20" from my informed but limited knowledge.

Never met a guy who had a longer barrel.

Talk to me about longer barrels, specifically as they relate to the AR15.

Is it as simple as diminishing returns after a certain point, or are there other benefits I'm not aware of?

All the powder in a 5.56 or .223 charge burns within 20" unless you get into handloads with slower burning powders. Thus, with ordinary loads velocity reduces beyond 20".

Beyond 20", a dedicated handloader can get some extra velocity. Certain kinds of match rifles will use longer barrels to increase sight radius too.

Ever did any prairie poodle or long range shooting? 24" is the starter length. Ever shot any .20 cal stuff?  They would laugh at you with a 20" or shorter tube.

I have both a 20" and a 24" upper that I use for coyote hunting and I can use factory loads in the 20, but if I want maximum performance out of the 24, I have to reload, but I like both them and they do what I ask them to do.

If I am going after prairie rats, I use my 26" bolt gun with hand loads.

The Colt 6724 HBAR ELITE is a 24" stainless barrel. I have one and it shoots good.

Odd that every test of barrel length out there (with factory Ammo) shows that 223 keeps gaining velocity out to 26" barrels...

Sometimes reloaders are just so fulla shit it's ridiculous.  I reload but I don't pretend my shit doesn't stink

WOW!

I enjoy reloading for the various guns that I own, it lets me tailor things to what I am trying to accomplish with that particular gun, I really don't buy much factory loaded ammo these days and as I am retired it keeps me busy these days, which my wife really is happy about.



Have you always been an angry person? or is this a new thing that has come along?

I have shot both 20 and 24" barrels in my yote guns. For bench shooting or out of a vehicle the 24" bull barrel works great, for a over the hill walking barrel the 20" is easier to handle. There my be a slight velocity difference in the two but I have not noticed a big difference in the two. I am currently using a 20" Christensen Arms barrel on my gun now and enjoy the length and less weight. If you want the greatest speed you can get, go for the 24". But if you are going to pack it around all day on foot going from fence line to fence line you may want to look at some lighter.

Great info, thanks. I remembered there were gains after 20", just not as much.

Crazy, on the link you provided, the UNC load LOST velocity from 24" to 26". Did not expect that.

I had a 24" bull barrel, but it was heavy so I had the barrel turned down and cut to 20".

Now I have 24" and 26" bull barrel guns, but they're both in 6.5 Creedmoor.  Even then velocity gains are tapering off with the extra length.

Pics from before I added Radius range finders:


Pics with the Radius range finders:
26" Krieger with Razor II 4.5-27


24" Criterion with Bushnell XRS 4.5-30.

I have a match 24" .223 SS bull barrel that I experimented with for a bit.  It was deadly accurate and consistent.  It was fun to play with, but I'm not a set hunter or bench shooter with a .223/5.56...I use other calibers for that.  To me the AR shines as a portable weapon for hunting, defense, etc.  The 24" barrel does improve ballistic performance but not at a level worth the weight penalty for me.  Most 24" barrels are heavier profile, and for those need/want that niche, it is a viable option for sure.  The 18" and 20" barrels in lighter profiles seem to offer more than enough performance for me and provide a wider application of use.  Still, as the original question about ballistic performance goes, longer is generally better.  It's just a matter of weight, mobility, handling, etc. that one has to decide upon for their application.

One additional observation about barrel length and powder burn.  I see this mentioned occasionally about how this or that barrel length allows full powder burn and therefore there's no advantage to a longer barrel.  I reload, and I'm no ballistic or reloading engineer by any means.  However, that concept about powder burn, barrel length, and ballistic performance isn't as simple as the powder doing a complete burn by the end of the barrel.  There's often a lot more going on here that affects ballistic performance.  Velocity can still be increasing even if there is what most might refer to as a complete powder burn before the end of the muzzle.  There are pressure characteristics, bullet design/weight, and other variables that come into play that continue to improve velocity.  Using the generic concept of "complete powder burn" as it relates to muzzle length isn't a set-in-concrete element.

I own 10.3" through 26" AR-15's and can make a generalized that you gain a little speed going longer, you lose a lot of speed going shorter. Depending on the ammo used, speed generated falls off dramatically once you go under 20" in length.

My 200/300 yard NRA match load is 24.5 grains of Varget, a 69 grain SMK seated @ 2.250", Winchester or Lake City brass and Remington 7.5, CCI-BR4 or Russian kvb-5,56m primers. I have recorded velocity so many times I can repeat it in my sleep.

26" generates 2850 fps.
20" generates 2750 fps.
16" generates 2550 fps.

This loss of velocity isn't as pronounced when shooting M193 or M855 surplus ammo, probably because their pressure levels are substantially higher than my match ammo. I suspect, but can't prove anything without actual pressure testing equipment, that my match load generates something close to 52,000 psi. NATO spec ammo is closer to 62,000 psi. I'm 50 fps and .5 grain of Varget below my published .223 data sources. 2800 fps is a working .223 SAAMI maximum when fired from a 20" barrel.

Using SAAMI .223 data as my limit, that load should produce the following from the barrels already listed.  

26" = 2900 fps.
20" = 2800 fps.
16" = 2600 fps.

Some of the ammo being sold as M272 clones scares the crap out of me when I read the chronograph results. Many people have reported blown primers with IMI 77 grain Razor ammo. It's running much faster than my 69 grain ammo from the same length barrels.

While longer barrels may serve a bench shooter, prairie dog hunter or F-Class competitor, they are a pain in the posterior for everyone else. 100 fps higher velocity is irrelevant on target. Not only are the too long to be handy, they almost always come in profiles resembling a bull barrel more than a fighting rifle. I had John Noveske build a custom varmint barrel a long time ago. I had him turn it to .812" under the handguard and forward of the gas block. It's still a pig, but no where near as bad as most. It's amazing on target.

I think a varmint hunter could be well served with a medium profile 1/12 Wylde chamber 24" barrel. Loading lightweight 50 to 55 grain match and varmint bullets he would have very little drop and drift out to 300 yards. Everyone else needs a 16" to 20" for general purpose use.

Seen a few varmint variants with 24" stainless barrels.

I'd like to know what M855A1 chronographs at out of a 24" barrel.

with factory ammo the amount of velocity gain per inch starts to drop off considerably around 18.5" with .223. so say you average 25ft/sec an inch from 16-20" it may drop to 10-15ft/sec an inch from 20-26 and so on.

depending how my first barrel turns out (making my own) I'd like to make a 26-28" 1:8 twist barrel one day. I have been enjoying the long range shooting sport as my new form of plinking due to 22lr costs. and once my kids get a little older I'd like to let them get into it.

I've been debating between a bolt gun in .223 ai throated for 75+gr, a 6x45 throated for 105's,  or a AR in .223 with the long barrel. they already have lowers thanks to Hilary

with some of the new higher bc bullets and powders like cfe223 and 8208xbr a long barreled .223 can be a decent introduction into the sport. cheap to load, and recoil would be nill with that heavy of a setup.

Spot on with the blown primers.

My SR15 does not like CBC (Magtech) 77gr or IMO 77gr RazorCore. Destroys primers, then they get jammed in the bolt and cause stoppages.

KAC also noticed premature bolt wear and replaced it. Don't know if related to the ammo or not, they wouldn't say.

For some reason my 20" BCM has no problem with either load.

HAHA - Interestingly, when I built my 7.5" pistol, I expected to get an enormous fireball when fired due to unburnt powder ignition.  Much to my disappointment, there is virtually no fireball at all, so it appears that factory round powder is fully burnt within that 7.5".  It's too bad though - I was really looking forward to having 4' diameter fireballs belching out of the end of my pistol.

Powder used, muzzle device and daylight all effect seeing a fireball. Why a different load at night with no muzzle device (just put a thread protector on it)... I promise you will get what your looking for.

Try shooting it after the sun sets!!!, I only run A2's on all of my guns and the first time I shot it, my wife thought I had set myself on fire!

No kidding.  The first short 5.56 gun I owned was a Kel-Tec PLR16 that has a 9.25" barrel.  They come with just a thread protector.  Wow!...almost impossible to see a followup shot in the dark after that blinding fireball.  I was amazed at how well a standard A2-style birdcage flash hider tamed that light show.

You're right. I checked the tests. A couple here and there peak at 24" or 25", but mostly 26" is fastest.

24" for my Grendel, I wanted max velocity for long range.

I'm a believer that if you need more than 20" to get the performance you are looking for you simply need a different caliber. That said, I do not care for long barrels because they are a pain in the ass to carry around, a pain in the ass to get in a vehicle, a pain in the ass to get a case for and a pain in the ass to store. So much so I had my 24" Kreiger chopped down to 22" then finally just sold it and switched calibers.

Of course there are exceptions to that. If you are shooting competitively and rules restrict you to your current caliber then yes, you can see advantages going longer. Its not a lot, but when you start reaching way out there every little bit helps. 

But if you arent restricted to a caliber and are looking to get better performance why not jump to a caliber that gives you what you want in a more easily handled package? Makes no sense not to as far as I can see.

Good points.  To stay in the AR15 platform if I want longer range accuracy and effect, I'd be tempted by a 6.5G.  Otherwise I'm sticking with my chassis bolt rifle for truly reaching out for distance, accuracy and effect.  My longest AR15 right now is a 20", and it mainly gets used as a test mule for reloading as much of the data for AR15 5.56/.223 reloading is done with a 20" barrel.  About as long as I seem to prefer for toting in the field where I have to walk at least some distance is in the 18" range.  I have a nice, home built 18" fluted barrel setup that's about as equal to most 16" barrel weights, and it's not painful to carry around or maneuver with in tight brush/terrain.  It's just about equal in accuracy at distance to the 20-incher.  For defensive, anti-personnel applications, I can see little advantage to anything over 16"...and frankly I have a 14.5" pinned barrel setup that is deadly accurate beyond what I would be shooting ISIS attackers at...LOL!  Everything's a compromise.

Good points.

Hmmm? $200-300 barrel vs. getting into a new caliber. Getting into 6.5G cost a hell of a lot more than than if I would have just got a longer barrel for my .223.  The performance difference of a 18-20" 6.5G vs a 24" .223 is not huge. ETA: if you only shoot for fun.

Hey, just because this is tech doesn't mean we can't still be enablers.

You are right, its an enormous price difference. Lets compare. If you get a 20"+ barrel you are probably buying a high end barrel. Lets say (For sake of comparison) we look at the following:

PF 223 Wylde barrels
A Bartlein barrel starts at $615 and includes the bolt.

PF Grendel barrels
A Bartlein barrel starts at $575 and includes the bolt.

Of course, we need mags! Yes, mags. 

AA magazines

So 20 bucks per mag. Lets say you pick up 3. So you save 40 bucks on the barrel and spend 60 in mags. Well, you are right. The Grendel conversion would cost you a whopping 20 bucks over sticking with your 223. I can see how 20 bucks is a make or break price difference. No doubt. Better stick with your "just as good" 223. 

Why would you assume a high end barrel? I spent $300 for my 24" Grendel barrel and bolt. I could have spent $300 on a 24" .223 barrel and been done with it at a barrel change. 

I have done this I know what it costs to get into Grendel. See my post above with the 24"Grendel . Grendel is more expensive unless you just plan on shooting a few hundred yards with cheap Wolf ammo.
We are talking about longer barrels here so I  assume that means longer distances.

Cheapest 6.5 bullets are about $0.22 each.
Cheapest .223 bullets are about $.07 each

Long range bullets
80eldm = $0.22
140eldm=$0.33

Brass
6.5G = $0.55
.223= $0.30 or less each


It would have cost $300 period with a 24"for better performance than my 18". 

The Grendel cost me $500 to put the upper together, $900 for the scope, a little over $200 for loading gear and more expensive components.
So $1650 (w/mags)($550 for essentials) vs. $300 initially for me and more expensive ammo.

Now I did go with a long barrel Grendel for max velocity so my .223 loads even with a long barrel wont match it but I am not sure it was worth the expense for my use.

To add, your points are valid. My point was that I can see where it makes sense to go with a 24" .223 rather than going to another cartridge.

Adding a new cartridge is more costly, that cost may outweigh any negative a longer barrel may have for some. My 24" Grendel with a couple inches of brake has no negative effect in my shooting or use.
I would have no issue with a 24" .223 and would enjoy the added performance from the extra 125-150fps.

I was slightly busting your balls. 

The cost difference isnt really enough to sway one whos serious about long range work. Given you are handloading I doubt you are shooting thousands of rounds a year. Which is to say even though each round is more expensive it shouldnt add up to a deal breaker. But I highlighted the most important bit in red.....

It absolutely comes down to defining the requirements and building to meet them. If on an individuals list his number one priority is minimizing costs and he owns a 223 and he owns reloading equipment for a 223 then definitely stick with that and dont switch calibers. If the top requirement is exceptional long range performance in a AR package, and an increased cost to get that isnt a concern then it doesnt make much sense to stay with the 223. If the requirement is exceptional long range performance in a semi auto go with the Creedmore. So on and so on.

It all comes down to what the requirements are. In your case it sounds like staying with the 223 is the better choice. I dont keep very tight controls on my shooting costs (Not that I just throw money away though) and I hate longer barrels so for me the Grendel was unquestionably the right choice.