Yet another AR barrel length thread...

[Mauser lover]

Yeah, I know, this has probably already been covered at least a zillion times, but I couldn't find what I was looking for.

So, I'll start with my question. What barrel length should I get?

Okay, now the details. I am pretty much dead set on a bull barrel (because I want to, that's why!). I understand that all other things being equal a shorter barrel is more accurate, because it is stiffer. Also, I am looking at a .223/5.56 (not quite sure which yet, I'll be handloading, so I'm not worried about a 5.56 being inaccurate).

So, my specific quandary, I can't find any good data about what velocity difference is between a 16 inch and a 20 inch. This is what I am really interested in. If you have data for an 18 or a 24 inch, that is welcome too, but I'm really wanting the 16 and 20 inch data.

I want this info because if it isn't a big velocity difference, I'll get a 16 inch because it is lighter (and cheaper often too!)

Thanks all!

 

[kwg020]

I can't show you a thread but the difference has something to do with the quality of the barrel and twist. But, in general you will probably get about 40-60 feet per second more per inch up to around 22 inches. That's from memory, I do not have the specific research in front of me. But, lot's of folks have done research on it. It's out there, good luck.

 

[Mauser lover]

Yeah, think the same twist rate. The purpose will be the same for either barrel length. Killing vermin (or exploding targets, or steel, or whatever else) at longish range. So, heavy bullets, I think 1:8 or 1:9? I will need to look it up again, but if the twist rate is the same for both barrels whatever their rate is, then the velocity difference will be "close enough" for me to figure what I want to know.

 

[Jackal]

It largely depends on the ammo. Your lighter weight varmint rounds benefit much great from barrel length than your run of the mill 55gr or heavier loads.

 

[Bart B.]

I am pretty much dead set on a bull barrel (because I want to, that's why!) I understand that all other things being equal a shorter barrel is more accurate, because it is stiffer.

Not true at all.

Yes, they are stiffer for given outside and inside diameters. But each barrel whips and wiggles at its own set of several frequencies (several dozen to a few thousand cycles per second) but they are 100% repeatable from shot to shot. How much they wnip and wiggle while bullets go through the mid determined by ammo causing recoil, rifle design and how the rifle is held. The variables in holding rifles causes the bore axis to point at different places relative to the target than most all other things involved; the reason several people shooting the same rifle and ammo have different sight zeros for a given range and shooting position.

Accuracy and precision are best when all things are repeatable. Doesn't matter how much things move while bullets go through the barrel. They only have to do it the same for every shot.

http://www.varmintal.com/amode.htm

One inch of barrel length change causes about 25 fps change in .223 Rem muzzle velocity. The only valid tests proving this is starting out with a long barrel and one lot of ammo. Cut an inch off the barrel clamped in a fixed mount for each 10 or more shots fired.

 

[jmorris]

10.5 is the shortest .223/5.56 barrel I have on an AR.

As above it depends on the load you are using but the 40-60 fps per inch will cover a lot of them.

 

[Buck60]

I have not chronographed my shots out of my 16 and 20 inch barrels .
So I can't say what the difference is in FPS .
I can say that my 20 shoots a tighter group than my 16 for me .
Of course that could just be that I like the 20 better than the 16 too .

I will try to get to the range and throw a few rounds through each and get the numbers for you .
I have been meaning to check this anyway .

 

[MachIVshooter]

I am pretty much dead set on a bull barrel (because I want to, that's why!)............
I want this info because if it isn't a big velocity difference, I'll get a 16 inch because it is lighter (and cheaper often too!)

You're set on a bull barrel, but you're worried about a few ounces with 16 vs. 20" tube?

IMO, 20" or 22" is a good balance for a useful tool that achieves good velocity with all loads.

 

[Varminterror]

If you can't find velocity info comparing a 16" to a 20", you're not looking very hard.

Second time today I'm backing @MachIVshooter's statements - it's very hard to beat a 20" AR for versatility, balancing handing and muzzle velocity.

I'd personally NOT go for a full bull profile, and stick with the National Match or Hbar profiles - meaning heavy under the Handguards, then 0.750" gas blocks. The .836" and .920" block models are excessively heavy out front, where the weight, stiffness, and cooling do almost nothing of any real advantage.

I own a lot of AR's, and more still come and go from my workshop. Personally (again), I recognize one important paradigm - application drives design, and for any application suited by a 16" carbine, I'd be as well or better suited with a 10.5-14.5" model instead. The $200 tax stamp is a one time annoyance, but the advantages are real. So I'm finding myself with rather little satisfaction from a 16" carbine. If I need a working rifle, I run 20", if I want a fast handling fighting carbine, I'm pulling out a 10.5" SBR.

 

[Mauser lover]

MachIV, not just ounces of dead weight, but weight out front changing balance etc.

I guess I'm not DEAD set on bull barrel versus the hbar profiles, but something pretty heavy.

I am planning on getting a SBR... someday.

I'm looking for good velocity differences with minimal variables, I can find a generic 16" velocity and a generic 20" velocity, but I haven't found a study that compared the same load/twist rate/etc. with only differing barrel lengths.

 

[FL-NC]

I can't show you a thread but the difference has something to do with the quality of the barrel and twist. But, in general you will probably get about 40-60 feet per second more per inch up to around 22 inches. That's from memory, I do not have the specific research in front of me. But, lot's of folks have done research on it. It's out there, good luck.

It also depends on the load. A 62 grain M855 (geen tip) is 300+ FPS faster than a 77 grain MK 262. All that said, the military issues 14.5" 1:7 barrels and they do just fine. A 16" 1:9 is a darn good barrel.

 

[MachIVshooter]

I'm looking for good velocity differences with minimal variables, I can find a generic 16" velocity and a generic 20" velocity, but I haven't found a study that compared the same load/twist rate/etc. with only differing barrel lengths.

You're gonna spend a lot of time looking for info that is really pretty anecdotal in the end.

As a rule of thumb, most loads will pick up around 200 FPS going from 16" to 20", and another 80-100 from 20" to 22". Longer than 22", you hit the point of diminishing returns with the small cartridge. I built a 22" bull barrel rifle, and a friend of mine built a 24" with the same 1:8 Wilson 4140 tube. Velocities are within 30-60 FPS consistently, from 45 gr. varmint loads to 75 gr. match loads.

 

[Mauser lover]

As a rule of thumb, most loads will pick up around 200 FPS going from 16" to 20", and another 80-100 from 20" to 22". Longer than 22", you hit the point of diminishing returns with the small cartridge. I built a 22" bull barrel rifle, and a friend of mine built a 24" with the same 1:8 Wilson 4140 tube. Velocities are within 30-60 FPS consistently, from 45 gr. varmint loads to 75 gr. match loads.

This is the kind of info I am looking for. Particularly the part about you and your friend with the same barrel maker and different lengths.

If anyone has a link to someone who has done a more extensive study, please post it! That is what I am looking for.

Looking forward to your testing results Buck60!

 

[Bart B.]

Unless both those Wilson barrels had exactly the same chamber, bore and groove dimensions, the rifle held exactly the same way and shot the same ammo, that comparison is invalid.

I've shot the same ammo through 5 different 24" barrels and saw over 100 fps spread in average velocity for 10 shots at 15 feet.

Did another test comparing a 22 inch barrel to a 24 inch barrel shooting the same ammo. 22" one shot bullets 60 fps faster average velocities.

Not many understand why this happens.

SAAMI's tests on muzzle velocity are probably the best ones conducted with almost zero variables except barrel lengths. They state muzzle velocities in the .223 Rem range have about 25 fps per inch of barrel length change.

 

[Varminterror]

I'll go digging for the origin of this data, but I was sent a link some years ago which compared different barrel lengths with different powders. These plots are the result of that comparison - you can see a remarkably linear relationship between length between 10 and 22" for these 3 common powders, varying 43-45fps per inch.

Bart is free to pontificate about bore diameter, rifling twist, chamber dimensions, etc, and there ARE odd exceptions to any rule - many of us have seen our share of "fast barrels" and "slow barrels"... But the physics remain the same - with as much held equal as practically possible, with typical loads for the cartridge, you'll see about 40fps difference per inch for the 223/5.56.

There's gobs of data online comparing 16" and 20" rifles with mil-spec loads. Again, you're not looking very hard.

 

[Bart B.]

and there ARE odd exceptions to any rule

Good rules have fixed conditions and standards for a given objective. When exceptions set in, that changes the standards, conditions and usually the objective.

but I haven't found a study that compared the same load/twist rate/etc. with only differing barrel lengths.

If you want to see what the numbers are when the only variable is barrel length for a given load, check the 223/5.56 velocities from the same barrel at lengths from 26" to 16" in one inch increments:

http://rifleshooter.com/2015/12/223...el-length-and-velocity-26-inches-to-6-inches/

Numbers for the 308/7.62 ammo and barrel are also good.

 

[grampajack]

From you post, it sounds like you'll be shooting from a bench? If that's the case, no reason not to go with a 24'' bull barrel. It doesn't sound like you could go wrong with a 1:8 twist either. It can handle the heaviest bullets and still leaves you a margin if you want to go lighter.

 

[hdwhit]

Mauser lover wrote:
I can't find any good data about what velocity difference is between a 16 inch and a 20 inch.

Nor are you likely to find any that is realiable or meaningful.

Velocity differences between different lengths of barrels are going to be determined based on how fast the powder burns. The difference between, say, an 18 inch barrel on a Mini-14 and a 20 inch barrel on an M-16 is going to be less when a fast burning powder like IMR-4198 is used as opposed to a faster burning powder.

If you want meaningful "per inch" differences, you would need to determine them in the context of a specific bullet weight and powder.

 

[hdwhit]

Mauser lover wrote:
What barrel length should I get?

I've always been partial to 20 inch with a rifle length gas system. The reasons for this are:

• A 20 inch barrel gives greater flexibility in choice of powders.
• It does not add much additional weight versus a 16 inch barrel.
• It does not affect balance and pointing the weapon very much versus a 16 inch barrel
• The longer gas system means cooler gas hitting the BCG which should marginally improve reliability and longevity.

The weight of bullets you will shoot, not barrel length, determines the twist rate, but since I want to be able to shoot from 35 to 60 grain, the ARs I have bought have a 1:9 barrel.

 

[mjsdwash]

my 1in7 20" shoots the same velocity as a 1in9 16" more or less. Faster twist does slow a bullet. The 20 will shoot faster with lighter powder charges, but near max (R15, H335, BLC2) they even out. Both can get a 55 grain bullet into the 3200 range, and both shoot well with max charges. I rarely shoot the 20" even though its is more accurate, because of the extra weight. Its a A2 profile, and the rifle weights 7.8Lbs, while the pencil weight 16" weighs 5.5LBs The most accurate rifle Ive ever shot was a 24" fluted heavy barrel with a 1in9 twist. That shot 7" at 600 yards with remington's cheap 55gr FMJ. As far as shorter barrels being more accurate, if your getting a bull barrel it wont matter. A .920 20" barrel will be rigid enough to practically remove that from the equation.

 

[Bart B.]

In response to Mauser lover's comment: "I can't find any good data about what velocity difference is between a 16 inch and a 20 inch.' this shows up:

Nor are you likely to find any that is realiable or meaningful.

So, you think my post 16 link to the Rifle Shooters tests showing a test as good as possible with the only variable is barrel length for several types of ammo is, in your opinion, not reliable nor meaningful at all?

 

[Mauser lover]

I'll be shooting from a bench or bipod... Mostly.

Okay varminterror, from your graphs, it looks like after 20 inches the extra length does significantly less than up to 20 inches. But, what happened to the 18 incher in the graph at the top right?

I'll be taking a look at SAAMI research when I get a few minutes...

No, that rifleshooters page was exactly what I was looking for. More of the same would be fantastic, albeit academic rather than practical to a large degree.

 

[Bart B.]

Faster twist does slow a bullet.

That's the first time I've heard of that.

Any tests been done with the same rifle, ammo and two identical barrels except for twist?

Any other comparison's not valid.

 

[JohnKSa]

Although Ballistics By The Inch is primarily focused on handgun calibers, they did some testing with the .223. Unfortunately they didn't test any barrels over 18" or any bullets over 55gr.
http://www.ballisticsbytheinch.com/223rifle.html

 

[mjsdwash]

That's the first time I've heard of that.

Any tests been done with the same rifle, ammo and two identical barrels except for twist?

Any other comparison's not valid.

not sure the exact numbers, but turning a bullet faster takes energy, and that comes from somewhere. I compared chrono results from few loads using a few different powders, and the results were that the 20" shot about 50-100 FPS faster, until the load got to max, where with all loads the velocity started to even out. Factory ammo was about the same from the start. Not an expansive test, but it was only presented as an observation.