Optimal barrel length for 5.56 mm


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no_problem
July 22, 2007, 11:31 PM
What's the ballistically optimal barrel length for a 5.56 mm. The criteria is as follows:

1. Long enough to stabilize the current military round (64 grain SS109?)

2. Long enough to be effective at up to 300 meters

3. Can be regular profile or heavy barrel profile.

What are some thought/rationale behind your conclusion

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Andras
July 22, 2007, 11:42 PM
1- Length has minimal effect on stabilizing a bullet, it's mainly twist rate (1:9 for SS109, 1:7 for 69gr+ bullets)

2- No ammo is going to fragment at 300m, so accuracy is your concern here, not velocity.

3- Profile really doesn't effect ballistics, accuracy perhaps, weight definately.

Anything 14.5" or longer should meet your needs (with appropriate flash-hider)

Ps- SS109 is 62gr.

MisterPX
July 23, 2007, 03:56 AM
If you're concenred about the M855 being effective at 300M, it's coming down to shot placement. Even with a 20" barrel, your "most likely to frag" range is about 200M.

raubritter
July 23, 2007, 10:01 AM
I would personally want it at 18in. Still handy, but not so short it's spraying my unburned powder everywhere. 1-7in twist, fluted barrel ridgitity.

rangerruck
July 23, 2007, 10:41 AM
Im going the opposite way here, i want max speed, I'll take it with a 26 in bbl.
I'm going to get a velocity of about 3600fps out of my bbl, with the 62 grainers, and that exta 6 to 800 fps, is worth about 150 extra yards or so of max damage.

BeJaRa
July 23, 2007, 01:18 PM
what is the deal with flutted barrels? seems that you are implying it adds strength to the barrel, is that true? I always kinda figured it was more decoration like a hexagonal barrel on lever actions.

wdl1
July 23, 2007, 01:25 PM
Fluting a barrel won't stiffen/strengthen a barrel per se. However, they will lighten the barrel, thus allowing a thicker/stiffer barrel to be used. In other words, if you have two barrels of the same weight and length, the fluted barrel will be stiffer (just because the fluting lightens the barrel and allows for a thicker barrel to be used).

Heavy Metal Hero
July 23, 2007, 01:38 PM
Fluting a barrel won't stiffen/strengthen a barrel per se. However, they will lighten the barrel, thus allowing a thicker/stiffer barrel to be used.

Does it not also cool it down quicker?

Zak Smith
July 23, 2007, 01:49 PM
Ballistically optimal would be 24" or longer to get max velocity.

The problem with "Optimal..." threads is that they usually fail to define the intended use and trade-offs the shooter is willing to make.

For a general purpose defensive AR-15, 16" is a good length because it's still above the knee in the curve where the velocity starts to drop fast (SBRs), and it's reasonable short and not yet in the realm of NFA, which adds logistical annoyances.

AR-15 accuracy is mostly a function of: barrel quality, upper "build" quality, and ammunition.

Profile is relevant mainly for heat issues. That's why the "SOCOM M4" had a heavy barrel-- to sustain higher rates of fire.

I prefer a profile that is about 0.70", made from a good blank. I would personally rather have a high-quality blank left normal than have one fluted, to less residual stress is in the steel.

For a practical rifle, the best reason IMO to go longer than 16" is to get the rifle-length gas system, instead of a mid.

wdl1
July 23, 2007, 02:10 PM
Heavy Metal Hero,

Yes, fluting should technically improve cooling with increased surface area for heat dissipation although I'm really not sure if it's by a significant margin.

aspade
July 23, 2007, 02:35 PM
The only way a 26" barrel is going to pick up 800fps is if the one it's being compared to is chopped off at 8" even.

Gewehr98
July 23, 2007, 02:40 PM
1- Length has minimal effect on stabilizing a bullet, it's mainly twist rate (1:9 for SS109, 1:7 for 69gr+ bullets)

I strongly disagree. If you don't have enough barrel length to accelerate the bullet's velocity into the right range to make use of the twist rate, stability will indeed suffer. The Greenhill Formula is a good place to start, but it isn't the only factor in calculating bullet spin rate. Heavier bullets need a faster spin, hence 1-7" twist, but the 14" barrel of the M4 means you have to get that bullet spun up with less velocity, a trade-off on long-range performance for the compact handling qualities of that particular carbine. The M4 is not considered a long-range rifle compared to its full-length M16 siblings, so it is best used for the purposes it was designed for, accordingly.

You'll see the effects of poor velocity/twist matching first-hand as keyholing in the targets downrange. I ran into the same problem working up loads for the 7.62x45 Czech cartridge, and had to cross a transitional velocity to produce stability.

As an aside, folks who cut down gain-twist Italian Carcanos discovered that phenomenon even more quickly.

More here:

http://www.exteriorballistics.com/ebexplained/5th/24.cfm

Quintin Likely
July 23, 2007, 02:57 PM
1- Length has minimal effect on stabilizing a bullet, it's mainly twist rate (1:9 for SS109, 1:7 for 69gr+ bullets)
Side note - a 1:9 will stabilize 68-69ish grain bullets and some 75s.

benEzra
July 23, 2007, 03:14 PM
Ballistically, the longer the better, up to at least 26".

Accuracy-wise, the shorter the better, down to 16" or so (shorter barrels flex less for the same barrel thickness).

Handling-wise, shorter is better, 16" minimum unless you go the NFA route.

Noise-wise, longer is better.

Plot all those on your mental graph, and weight them accordingly.

For me, I think a 16" would be about right, and 20" would be definitely too long.

MrDig
July 23, 2007, 03:24 PM
Most of what has been said is spot on,
Wd1 "In other words, if you have two barrels of the same weight and length, the fluted barrel will be stiffer (just because the fluting lightens the barrel and allows for a thicker barrel to be used)."
Zak Smith "Ballistically optimal would be 24" or longer to get max velocity."
Federal tests all of the factory rifle ammo in 223 with a 24 in Bbl.
Odd that they don't mention rate of twist in the Bbls but the are a Stadard 24 inch Bbl.

Werewolf
July 23, 2007, 04:27 PM
Fluting a barrel won't stiffen/strengthen a barrel per se.Fluting is designed to do two things. One more than the other.

1. Primary Purpose of Fluting: Fluting (grooves on outside of a barrel running parallel to the cylindrical axis of the barrel) are cylindrically shaped. Tubes are stronger (harder to bend) than solid cylinders because of the increased surface area a tube has compared to a solid cylinder. The tube shape of a barrel is inherently strong but putting tube shaped grooves on the outside of a barrel increases further the surface area and adds the strength of the tube shaped groove to the barrel thus stiffening it. Firing a bullet thru a barrel creates a shockwave which results in a sinusoidal whipping/flexing of the barrel. By fluting and thus stiffening the barrel this whipping/flexing effect is reduced and accuracy is increased. What is commonly referred to as a blood groove in a knife or sword is there for exactly the same reason - it aids in stiffening the blade.

2. Increasing the surface area of the barrel increases the rate at which heat is dissipated from the barrel.

wdl1
July 23, 2007, 05:48 PM
Werewolf,

I'm going to say my disclaimer first that I'm not a materials engineer and I really don't have any business trying to explain this technically, but here it goes:

I'm pretty sure that stiffness is directly correlated to cross section area and not surface area. If surface area did increase stiffness, then wouldn't it make sense for barrel makers to flute it so that it looked more like a heatsink with multiple fins rather than just cutting a few flutes in?

Additionally, I don't think comparing a knife fuller to barrel fluting is a good comparison simply for the reason the forces are applied. A knife fuller stiffens when the knife is used perpendicularly (as a knife should be), but cutting away material will reduce its gross strength when using the knife in a prying fashion (lateral forces). Apply the same logic to a barrel in which the forces are applied in a 360 degree fashion and it's not quite comparable to the knife where the force is directed perpendicular to the fullers.

I remember reading on Lilja's webpage that stated that fluting does not increase stiffness. Here's a good site explaining the logic behind fluting:
http://www.varmintal.com/aflut.htm

Again, this is just what I've gathered myself so please let me know if I'm wrong.

Deer Hunter
July 23, 2007, 06:27 PM
As others have said, optimal performance of the .223 is realized with a 26" or so barrel, but I'm guessing you're not looking for an AR-15 with that kind of barrel. For that kind of gun, in my opinion (yours will probably vary), I'd much rather have a solid 16" barrel (read, no pseudo 16"/perm flash hider) or longer. I love the 18" barrel on my FAL, so if I were to ever get an AR-15, it'd most likely have an 18" barrel.

Werewolf
July 23, 2007, 06:34 PM
Wdl1:
From the article you cite:The flutes are positioned for maximum stiffness in the vertical plane with a land both top and bottom. This puts a flute on each side.
The author supports my contention that fluting impacts stiffness.

But the main emphasis of the article is not whether stiffness is increased or decreased (though the body of the article and the data collected show that stiffness is increased) rather the article is a study of whether fluting has more effect on a heavy barrel than a light barrel.

The summary statement by the author is:Fluting the baseline barrel showed a reduction in group size of about 21%. Keeping the same weight and fluting the barrel with a 1.222" muzzle diameter reduced the group size by 23%. If fluting is done without introducing large residual stresses in the barrel it should improve accuracy by as much as 20% over a solid barrel of the same contour.
So in a nutshell fluting a barrel increases accuracy. The author talks about sag, time of bullet exit etc etc which are the details behind the whip action I reference.

I stand by my statement that fluting increases stiffness (the author of the article you cite agrees) and improves accuracy (which the author of the article you cite seems to prove).

rangerruck
July 23, 2007, 07:01 PM
my 12 inch bbl, keltec plr gets about 2600 fps, I guarantee with a 26 inch bbl, i get you another 800fps out of it.

wdl1
July 23, 2007, 07:26 PM
Werewolf,

PM sent since I don't want to hijack this thread.

Thanks :)

DMK
July 23, 2007, 07:58 PM
Ballistically, the longer the better, up to at least 26".

Accuracy-wise, the shorter the better, down to 16" or so (shorter barrels flex less for the same barrel thickness).

Handling-wise, shorter is better, 16" minimum unless you go the NFA route.

Noise-wise, longer is better.

Plot all those on your mental graph, and weight them accordingly.
As benEzra illustrated so well, everything is a compromise. Where do you want to make your concessions?

Acera
July 23, 2007, 08:00 PM
So Werewolf you are telling me that taking away stiff steel will make the remaining steel stronger.....no F'ing way. The poster that said same size and weight is closer to the truth. Think about a arch bridge, it is designed to use as little material as possible (keep costs down) and support the needed load. If you wanted the strongest possible bridge, just fill in all of the space between the road bed and the ground with a solid material, and have no arch, that is the strongest way..........not practical most times, but strong. With fluting, you have got to make sure it is done precisely or you will destroy the harmonics of the barrel.

Werewolf
July 23, 2007, 11:25 PM
So Werewolf you are telling me that taking away stiff steel will make the remaining steel strongerLook guys - I can't find any references on the internet to prove that tubes are structurally stronger than solid cylinders and that fluting a barrel is the equivalent of increasing the surface area of a tube and thus the strength of a barrel AND I tossed my engineering texts 35 years ago.

So I'll take this tack... :D

Fluting a barrel must have some benefit or everyone of us (including me) who's paid that extra $50 bucks to get it is a flaming maroon suckered in by shyster gun company marketing weasels. We all wasted our money and those same weasels are laughing all the way to the bank where they're stashing the money they conned us out of to be used later buying unfluted barreled guns, speed boats, houses in the country and gorgeous women to go with it all. :neener:

Personally...
I'll go with fluting has some benefit. :)

rangerruck
July 24, 2007, 04:42 AM
"Come , let us reason , together" said the Christian to the Muslim!!!
Let us end the fluting debate , once and for all time.

http://www.snipercountry.com/Articles/RealBenefitsBarrelFluting.asp

DMK
July 24, 2007, 09:48 AM
Fluting a barrel must have some benefit or everyone of us (including me) who's paid that extra $50 bucks to get it is a flaming maroon suckered in by shyster gun company marketing weasels.It does have a benefit. It makes the barrel lighter without giving up as much stiffness. In the right context, it is not snake oil. But it does not defy the laws of physics either.

A fluted barrel is not stiffer than an HBAR of the same diameter and length. It is stiffer than a thin barrel of the same weight and length.

Again, everything is a compromise. Know what you are giving up to get what you want.

RecoilRob
July 24, 2007, 07:45 PM
I have a 16" Colt w/pencil barrel and a Bushmaster Varminter w/fluted 24" heavy contour. Chose the fluted version because the un-fluted was too heavy for me. Fluting allowed me to have the extra barrel length without being so front heavy that the rifle was uncomfortable to shoot offhand.

Oh, Werewolf....you are misreading the article referenced previously. Look at the graphs for .8 dia barrels, both fluted and non-fluted. The fluted one is MUCH less stiff.

The computer model only projected in the vertical plane...not very realistic. And, the full barrel diameter was left in place by placing the flutes to the sides....again to retain maximum rigidity in the vertical plane. If fluting made the barrel 'more stiff', wouldn't it seem reasonable that the flutes could be placed anywhere to achieve this benefit?

The conclusions drawn by the author are also a bit mis-stated. His contention that the fluted barrel is more accurate is based on the assumption that they are the SAME WEIGHT. This means, of course, that the fluted version must be greater in diameter....hence the greater rigidity.

jmr40
July 24, 2007, 10:16 PM
Yes, fluting a barrel will make it more rigid. A 1/2" pipe is much harder to bend than a 1/2" solid bar of steel because of the greater surface area from the inside of the pipe. This can easily be proven. Steel I-beams are used in construction because the greater surface area makes them much stronger than comparable sized solid beams.

RecoilRob
July 24, 2007, 10:39 PM
Oh boy, please don't take this the wrong way, but I hope you aren't designing any bridges or the like anytime soon.

If a steel pipe gets its' strength from the 'greater surface area', would a 3" pipe with a .005 wall thickness be stronger than a solid steel 3" rod? The hollow pipe has MUCH greater 'surface area' but, I think you would agree that the thin steel would buckle pretty easily.

SaMx
July 24, 2007, 11:01 PM
jmr40, I'm telling you from the point of view of an engineering student, that you are wrong. Adding surface area does not make a rod stiffer, and a pipe is not stiffer than a solid rod of the same diameter.

I beams are used because they are almost as stiff as a solid rectangular block of steel, but they are much lighter.

SaMx
July 24, 2007, 11:16 PM
EDIT: weird double post.

SnakeEater
July 24, 2007, 11:19 PM
I'm hearing the 75gr or 77gr loads may actually fragment out to 250 yards if you get the velocity right.

SaMx
July 24, 2007, 11:20 PM
http://www.fulton-armory.com/fluting.htm
Conclusion: the fluted barrel is much lighter, much less rigid, and has much more surface area than a solid barrel of the same diameter.

Conclusion: the fluted barrel is significantly more rigid, and has much more surface area than a solid barrel of the same weight.

fluting will reduce the weight and rigidity of a bull barrel, but are still more rigid than a thin lightweight barre.

no_problem
July 24, 2007, 11:34 PM
Of all the barrel lengths in the world, why did the Army's M4's choose to use 14.5" barrels?

jmr40
July 24, 2007, 11:50 PM
SaMx, the link you provided proves that I am wrong. Thanks.

Gtscotty
July 25, 2007, 12:19 AM
Interesting, most of this discussion on fluting and stiffness transpired while I was at the library studying for my Deformable bodies final :rolleyes:. I beams and wide flange beams are not used because their added surface area contributes to stiffness. The youngs modulous (stiffness) multiplied by the moment of inertia = a term called the flexural rigidity, aka the resistance of a beam to bending (or barrel whipping). A bull barrel will be stiffer than a fluted barrel with the same nominal diameter, but the fluting allows you to get an increased moment of inertia for a given weight. This in turn contributes to a higher rigidity. I know people have already effectively said this, but after hours upon hours of studying this crap, I could not pass up a chance to regurgitate it.... it makes me feel better about the time spent :( . (Btw, I beams are used because for a load coming from a singe direction, they too effectively maximize the rigidity for a given weight.)

DMK
July 25, 2007, 07:19 AM
I'm hearing the 75gr or 77gr loads may actually fragment out to 250 yards if you get the velocity right.75gr or 77gr loads will generally fragment reliably out to about 200 yards with a 20" barrel and 160-175 yards with a 16" barrel. I believe they need to be in the 2300-2400fps area to reliably fragment. They may fragment farther out, ...or maybe not. So with a 24" barrel, you might could push that out to 250ish yards.

Bartholomew Roberts
July 25, 2007, 07:49 AM
Of all the barrel lengths in the world, why did the Army's M4's choose to use 14.5" barrels?

The original Colt Commando started with a 10.5" barrel and a moderator that reduced the sound to normal 20" rifle levels as well as creating additional backpressure/dwell time that helped the rifle function. The 10.5" didn't function as reliably as desired (short dwell time, big gas port) so they lengthed the barrel to 11.5" to get better reliability and left the moderator in place. The moderator was considered an NFA item and was a pain from a record keeping perspective; but if you removed it you lost some of the additional backpressure that helped the weapon function. One way to get around that is to lengthen the barrel past the gas port so that you have roughly the same dwell time as the rifle (about 4" of barrel). This gives you a 14.5" barrel.

MechAg94
July 25, 2007, 09:30 AM
rods vs tubes
In bending a rod or tube, the materal on the outside diameter sees most of the stress. The material in the middle is doing very little and might actually add to failure by cracking or deforming. So, a thick tube will retain most of the strength of a rod (since the outside diameter does most of the work anyway )with a lot less weight. How thick the tube is depends on your application.

Werewolf is mostly right but leaving out some details that confuse the issue. A fluted barrel is not stronger or stiffer than a non-fluted barrel of the same diameter. It may be stiffer than a smaller diameter barrel of the same weight. In other words, fluting lightens a barrel while maintaining much of the strength and stiffness of the heavy barrel.

jkingrph
July 25, 2007, 09:54 AM
Steel I-beams are used in construction because the greater surface area makes them much stronger than comparable sized solid beams.

I disagree, I beams are used because the shape gives them probably more structural strength than a comparable sized solid beam. Consider the weight of a solid beam, in a span situation it would have more tendency to bend( sag) of it's own immense weight than an I beam of the same "size", if by size you mean cross section demension( L x H) . You also must consider web thickness, beams can be made with the same Lxh but with different web thickness for different strengths. Almost any I beam of a comparable weight for the same length will be much stronger than a solid beam.

SaMx
July 25, 2007, 10:22 AM
yes, that's true, but the lower sag comes from lower weight. I'm not talking about an I beam of the same weight as a solid beam, I'm talking about an I beam of the same dimensions, as if the sides of the beam were filled in.

MechAg94
July 25, 2007, 10:32 AM
That is because the flats on the top and bottom are what hold the load. The fact that they are separated by the web gives them better load bearing ability without the high weight. I don't think the web itself sees a lot of stress.

It has been a long time since looking at I-beam stress. Not something I have seen since college.

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