The "best" barrel length for AR

The question is vague and so is the answer. You mention "threats" at distances from CQB to 100+ yards. You will find that a 7.5" AR15 is quite capable of destroying paper "threats" out to 300m.

If your "threats" are something other than paper, then you need to revisit your ballistic reports and realize that effective terminal ballistics is a function of a specific bullet with velocity at point of impact. The velocity at the point of impact is a function of muzzle velocity vs the distance to target. And finally, muzzle velocity is a function of barrel length and the specific ammunition.

As you can see, the variables are:
Barrel length (affects muzzle velocity)
Barrel twist (to stabilize the bullet)
Ammunition (and specific bullet)
Distance to target
Target composition (paper, steel, glass, deer)

Generally we strive to obtain the greatest muzzle velocity possible to ensure our desired terminal ballistics are achieved for the greatest distance possible. Usually this means a longer barrel. However it is possible to achieve the same terminal ballistics with a much lower impact velocity if choosing a more suitable bullet (and therefore a shorter barrel is possible).

The more variables you set in stone, the easier it is to answer your question. If you want to stay versatile and be able to handle "threats", then a 20" AR15 is your vague solution.
 
FrankieT said:
The more variables you set in stone, the easier it is to answer your question. If you want to stay versatile and be able to handle "threats", then a 20" AR15 is your vague solution.
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ha: FrankieT for the win! Well stated, it's pretty much why I have a 20" tube. It's my only range gun, and likely will be going forward. Long Range Precision (sort of), Service Rifle (hopefully), and CQB, yes it's a compromise, but it works.
 
CF uses 15.7" barrel 1:7 twist (C8SFHB with simon sleeve) good to stabilise the 62gr ball ammo we have (yes its 62gr, we pulled one appart to weight it)

US army uses 14.5"

Accuracy wise:

I have tested DDMK18 (10.3 HB in 1:7) and Core15 (16" M4 profile 1:7) from 3m to 400m. 2 different optics were used: aimpoint PRO and a bushnell 10x40 with ammo being reloaded 62gr and 69gr. Grouping were similar between 1.5 and 2 moa from 100 to 400m.

I didnt notice any "handicap" with longer barrels in CQB/3guns environment. Obviously, shorter barrel equal to less velocity, but paper dont see the difference.
 
redshooter said:
I have to disagree here. The military uses 1 in 7 twist to stabilize bullets in cold weather. This is an assumption on my part, but you will probably get crap accuracy with 62gr ball out of a 16" barrel in very cold weather even with a 1 in 7 twist barrel. Also according to Bryan Litz the 77gr SMK daddy will stabilize better in a 1 in 9 than the 62gr LC FMJBT.

My experience with 68gr Hornady HPBT Match supports this. Great accuracy out of a 24" 1 in 9 bolt gun in the summer, -20c, total crap, even adjusting the powder for the cold. The 68gr Hdy and the LC 62gr have almost identical stability factors.
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What? That is a first. I've never heard the stabilize in cold weather argument, although you then contradict yourself immediately after. I've been on many a range with sub zero temps and I can't say I've noticed a real practical diference.

1/7 is to stabilize tracer rounds, FACT. A 61 gn NATO L110 is longer than a Sierra 80grn. Why don't you call up FNH and ask them why they settled on 1/7? I believe it was largely because of their work on the Minimi that NATO standardized on 1/7.
 
Leg said:
What? That is a first. I've never heard the stabilize in cold weather argument, although you then contradict yourself immediately after. I've been on many a range with sub zero temps and I can't say I've noticed a real practical diference.

1/7 is to stabilize tracer rounds, FACT. A 61 gn NATO L110 is longer than a Sierra 80grn. Why don't you call up FNH and ask them why they settled on 1/7? I believe it was largely because of their work on the Minimi that NATO standardized on 1/7.
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I'm not saying your wrong about the tracer, but I KNOW I'm right about stability issues in cold weather. There are many factors relating to bullet stability, MV, and air density have a huge effect. The latter being influenced by temperature, altitude, and humidity. If you haven't read it yet I suggest you get a copy of Bryan Litz's book Applied Ballistics For Long-Range Shooting.

Extreme cold can make a load capable of sub MOA performance open up to 3-4 MOA.
 
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guess a more accurate question is: "If you were to have only one AR15 for a multitude of purposes such as CQB to urban environments to extended environments and threats at 100+ yards, what barrel length would you choose?"
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For comp, 14.5 or 16 is great for a average sized human being. If you are a bigger and taller guy, 18 should be considered. It is not just for ballistics, but for shooting postures as well.

If you have a lot of things like light and laser, a shorter gun in 10 and 11.5 bring all the weight closer to the center and it is a bit easier to use, especially there is a need to maintain readiness for a long time.

It is like kayaking or snowboarding, you pick the paddle or the board according to your height.
 
The theoretical question posed by the OP is choose one....
Luckily I'm not hamstrung to one barrel, so I have a variety of lengths and profiles, and with the exception of owning an 1:8 and 1:7.7 twist, the rest are 7s.
The development of 7s may well owe their beginnings to tracer rounds, but that niche was quickly surpassed early on by custom barrel makers making quick twists for match bullets in numerous quality, quantity and variety.
The 1:7 twist is common now, can be had in varying quality from a run of the mill 'mil-spec' to full custom with pretty much every manufacturer producing one.
In my experience they will work with everything from 45gr to 90 grain bullets.

Aside from general army issue 62 Gr. Ss109s, everyone has moved to 77gr. OTM - get a barrel that will for sure keep it going to 500-600m - that 9 twist in my experience might not get it done.
 
With such a vague question such as "what is best" without delving into the details and specifics, the simple answer is "whatever works best for you".

Quite frankly if I HAD to choose just one, I'd probably go for a 13.7". Considering only a reasonable degree of accuracy and slightly better maneuverability over the 16".

I don't mean this in a rude way OP, but this really seems like a stupid question when the joys of the AR platform lie in its modularity.
All you really need to quickly swap between barrel lengths is a second upper, barrel and handguard.
 
Thanks for all the input so far. I am glad I at least sparked a "lively" conversation. Of course my question is "vague" and for the most part meant to be theoretical as it was meant to open up the conversation and hear many peoples opinions. I know there isn't really a "one size fits all" solution, clearly every situation is different and each of these situations poses a multitude of variables, but I am looking for the best solution if you only had one choice. Why ask this question? Because that's what forums are made for - conversation :)
 
I had a custom made 14.7253" it could hit a unicorn at 2000m

The extra 0.2253" gives just enough velocity for a perfect self silencing concussive wave to be a steath bullet... no need for a suppressor (I could explain but you wouldn't understand)


and.....




enough of that. I like 16"
 
I've owned pretty much everything from a 6.5 inch 7 twist through to a 20 inch 8 twist and if I were to build just one rifle and could never have another I would put together a 12 inch 8 twist.
I've found that the muzzle blast from anything shorter than a 10 inch gets old real fast. I've printed sub moa groups with a 12 inch 8 twist and 2 moa from the 6.5 inch so accuracy can be had from any length with the right ammo. I have no problems with a 7 twist and could be happy with either the 7 or 8 but I just personally like the 8 twist for some unknown reason.
That being said, I just sold off a couple AR's and what I have left is my 12 inch 9mm and a 16 inch 9 twist 223 (found a great deal on a good barrel).

I'm not sure what the point of the thread is or what the intentions of the OP are because this is such a vague question that leaves the answer completely up to ones personal preference and is almost sure to end in squabbling. Just another silly thread to increase everyone's post count and answer nothing.
 
For me, mine is just for stationary target play from 25-300 yards. I always thought a 20" tube made sense because in .223rem you wont gain much more velocity with a longer tube, and muzzle blast is a LOT NICER than with the12.5" bastard I use to own. Just nasty. I would champion a shorter tube if it was "run n gun" for the handiness.
I bought a Mossberg MMR and couldnt be happier. Loves a bipod and the Vortex Sparc.
 
If you care about rule #1 of the internet then it's 10.5 and under as they tend to have that cool guy look. I have a 12.5 as I wanted to have my linear comp just poking out of the end of my handguard. Looks good but having shot others stuff I'm glad I didn't go shorter and I also wish I had gone with a 14.5. Since you can get fluted 14.5 barrels that are lighter than my current 12.5. That and the better dwell time on the 14.5 with a mid length compared to the 12.5's carbine gas tube. Can't comment on twist rates though.
 
redshooter said:
I have to disagree here. The military uses 1 in 7 twist to stabilize bullets in cold weather. This is an assumption on my part, but you will probably get crap accuracy with 62gr ball out of a 16" barrel in very cold weather even with a 1 in 7 twist barrel. Also according to Bryan Litz the 77gr SMK daddy will stabilize better in a 1 in 9 than the 62gr LC FMJBT.

My experience with 68gr Hornady HPBT Match supports this. Great accuracy out of a 24" 1 in 9 bolt gun in the summer, -20c, total crap, even adjusting the powder for the cold. The 68gr Hdy and the LC 62gr have almost identical stability factors.
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yes
 
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