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Optimal barrel length for 5.56 mm

Discussion in 'Rifle Country' started by no_problem, Jul 22, 2007.

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  1. no_problem

    no_problem Member

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    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
     
  2. Andras

    Andras Member

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    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.
     
  3. MisterPX

    MisterPX Member

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    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.
     
  4. raubritter

    raubritter Member

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    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.
     
  5. rangerruck

    rangerruck Member

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    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.
     
  6. BeJaRa

    BeJaRa Member

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    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.
     
  7. wdl1

    wdl1 Member

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    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).
     
  8. Heavy Metal Hero

    Heavy Metal Hero Member

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    Does it not also cool it down quicker?
     
  9. Zak Smith

    Zak Smith Moderator Emeritus

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    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.
     
  10. wdl1

    wdl1 Member

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    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.
     
  11. aspade

    aspade Member

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    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.
     
  12. Gewehr98

    Gewehr98 Member

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    You'd be surprised.

    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
     
  13. Quintin Likely

    Quintin Likely Member

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    Side note - a 1:9 will stabilize 68-69ish grain bullets and some 75s.
     
  14. benEzra

    benEzra Moderator Emeritus

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    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.
     
  15. MrDig

    MrDig Member

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    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.
     
  16. Werewolf

    Werewolf Member

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    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.
     
  17. wdl1

    wdl1 Member

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    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.
     
  18. Deer Hunter

    Deer Hunter Member

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    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.
     
  19. Werewolf

    Werewolf Member

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    Wdl1:
    From the article you cite:
    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:
    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).
     
  20. rangerruck

    rangerruck Member

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    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.
     
  21. wdl1

    wdl1 Member

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    Werewolf,

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

    Thanks :)
     
  22. DMK

    DMK Member

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    As benEzra illustrated so well, everything is a compromise. Where do you want to make your concessions?
     
  23. Acera

    Acera Member

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    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.
     
  24. Werewolf

    Werewolf Member

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    Look 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. :)
     
  25. rangerruck

    rangerruck Member

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