Barrel length vs. accuracy - again...

The #1 community for Gun Owners of the Northeast

Member Benefits:

  • No ad networks!
  • Discuss all aspects of firearm ownership
  • Discuss anti-gun legislation
  • Buy, sell, and trade in the classified section
  • Chat with Local gun shops, ranges, trainers & other businesses
  • Discover free outdoor shooting areas
  • View up to date on firearm-related events
  • Share photos & video with other members
  • ...and so much more!
  • moose&squirrel

    Banned
    BANNED!!!
    Aug 25, 2018
    253
    I have a new AR10 with a 16" barrel, a free-floated stock and a Timney single-stage trigger. Shoots pretty darn good, at least as far out as I've been able to test it, which, at present is 200-300 yards. The same lower is also available for many hundreds of dinero more with an 18" barrel. My meager firearms knowledge sort of tells me that the 18" barrel will "launch" the rounds at a slightly higher speed, but up to XXX yards distance, not more accurately. (I have no idea what that XXX yards number might be).

    Are my assumptions near accurate, that a 16" barrel popping out .308 rounds is pretty much as accurate as the same rifle with an 18" barrel?

    Thanks!
     

    danb

    dont be a dumbass
    Feb 24, 2013
    22,704
    google is your friend, I am not.
    you will get about 50 fps more speed out of the 18" barrel. Rule of thumb is 25 fps/inch for 308, but it will depend on your load.

    The principal reason 18" barrel is "more accurate" is that higher speed means less time to be impacted by wind, etc. There is a small contribution from additional spin, but its small.

    Out to 300 yards, you will probably not notice any difference between 16" and 18" that's my 0.02. You wont notice until you get farther out. You might notice a difference between a 16" and 20" or 22" barrel at 300 yards, but again it will depend on your load. Odds are,with a 16" barrel to 300" yards, the shooter is the limiting factor not the rifle.
     

    E.Shell

    Ultimate Member
    Feb 5, 2007
    10,245
    Mid-Merlind
    The term "Accuracy" is often interchanged with "Precision".

    A longer barrel may prove more accurate, in that holding a consistent point of aim may be easier due to increased mass and/or sight radius.

    A shorter barrel is often more precise, because for any given diameter or profile, making the barrel shorter makes it stiffer and thus less reactive.

    You can see evidence of this by noticing most benchrest rifles have shorter barrels that are stiffer and more precise, while most position rifles are longer and heavier so the shooter can control them better and/or to provide a longer sight radius, both helping to achieve better accuracy.

    That said, I have seen several 16" .308s shoot sub-MOA groups out to 1,200 yards. It is mostly a function of barrel quality, but the shooter really has to be focused to shoot a short rifle from field positions with repeatable precision.

    One aspect of long range accuracy one will give up with a shorter barrel is that the reduction in muzzle velocity means longer flight time and more sensitivity to conditions. On a known distance course, extra drop isn't a big issue. Conversely, on an unknown distance course, when shooting your own range estimates, increased drop places greater emphasis on your ability to correctly range targets and tends to compound your errors. Wind drift will be greater on any type course and giving up velocity means your drift will be worse and your calls have to be better.
     

    Major03

    Ultimate Member
    you will get about 50 fps more speed out of the 18" barrel. Rule of thumb is 25 fps/inch for 308, but it will depend on your load.

    The principal reason 18" barrel is "more accurate" is that higher speed means less time to be impacted by wind, etc. There is a small contribution from additional spin, but its small.

    Out to 300 yards, you will probably not notice any difference between 16" and 18" that's my 0.02. You wont notice until you get farther out. You might notice a difference between a 16" and 20" or 22" barrel at 300 yards, but again it will depend on your load. Odds are,with a 16" barrel to 300" yards, the shooter is the limiting factor not the rifle.

    One could argue that the shorter barrel will tend to be "stiffer" and thus have less flex when fired. Less flex means more predictable and repeatable which means more precise.

    But practically speaking, the shooter is the limiting factor. Shorter barrels give the propellant less time to completely burn and thus you have less velocity. Less velocity generally means two things:

    - slightly more time for gravity to act on your projectile for a given range (and therefore more "drop" that you need to account for / a more "rainbow like" trajectory). The more drop you have, the less forgiving it is for shooter error in range estimation.

    - as previously stated, more time for the wind to push the bullet around. How much depends on the environmental conditions at the time of the shot. Again, just less forgiving for shooter error / misjudgment.

    At extreme ranges, the loss in projectile velocity has the bullet transition from super sonic to sub sonic sooner than with a more complete propellant burn. Depending on the projectile, that transition can incur yaw, which begins to pull the bullet off of it's axis and isn't a good thing for accuracy. Sometimes so much so that bullets will begin to keyhole.

    All this said...at ranges of 200 - 300 yards I doubt you will notice any difference in those two inches of less barrel.
     

    Major03

    Ultimate Member
    The term "Accuracy" is often interchanged with "Precision".

    A longer barrel may prove more accurate, in that holding a consistent point of aim may be easier due to increased mass and/or sight radius.

    A shorter barrel is often more precise, because for any given diameter or profile, making the barrel shorter makes it stiffer and thus less reactive.

    You can see evidence of this by noticing most benchrest rifles have shorter barrels that are stiffer and more precise, while most position rifles are longer and heavier so the shooter can control them better and/or to provide a longer sight radius, both helping to achieve better accuracy.

    That said, I have seen several 16" .308s shoot sub-MOA groups out to 1,200 yards. It is mostly a function of barrel quality, but the shooter really has to be focused to shoot a short rifle from field positions with repeatable precision.

    One aspect of long range accuracy one will give up with a shorter barrel is that the reduction in muzzle velocity means longer flight time and more sensitivity to conditions. On a known distance course, extra drop isn't a big issue. Conversely, on an unknown distance course, when shooting your own range estimates, increased drop places greater emphasis on your ability to correctly range targets and tends to compound your errors. Wind drift will be greater on any type course and giving up velocity means your drift will be worse and your calls have to be better.

    Yes, great points. One should differentiate between precision and accuracy. They are different.
     

    E.Shell

    Ultimate Member
    Feb 5, 2007
    10,245
    Mid-Merlind
    you will get about 50 fps more speed out of the 18" barrel. Rule of thumb is 25 fps/inch for 308, but it will depend on your load...
    ...and on the exact area we are cutting.

    An inch off an already short barrel is worse than an inch off an already long barrel.

    The difference between a 24" .308 and a 26" is barely worth carrying the extra steel, and almost no one with a standard chamber (long throated 190s notwithstanding) shoots a 28" .308 - there's just not enough return on it.

    The difference between a 20" barrel and an 18" barrel, or an 18" compared to a 16", is relatively greater, especially with long range loads.

    While the reduced velocity can be an problem, that isn't the issue I am really concerned with. What I see as we shorten barrels is an increase in standard deviation, leading to vertical stringing with short barrels at long range. When the barrel is very long, we burn a relatively larger amount of the powder, so a small amount of unconsumed fuel is only a small percentage of the whole charge. When the barrel is shorter, we can't burn it all any more, and the less we burn, the unburnt portion becomes a greater percentage of the whole. I see loads providing an 8 to 10 fps standard deviation in a 24" barrel show 15-20 fps in a 16" gun.
     

    danb

    dont be a dumbass
    Feb 24, 2013
    22,704
    google is your friend, I am not.
    ...and on the exact area we are cutting.

    An inch off an already short barrel is worse than an inch off an already long barrel.

    The difference between a 24" .308 and a 26" is barely worth carrying the extra steel, and almost no one with a standard chamber (long throated 190s notwithstanding) shoots a 28" .308 - there's just not enough return on it.

    The difference between a 20" barrel and an 18" barrel, or an 18" compared to a 16", is relatively greater, especially with long range loads.

    While the reduced velocity can be an problem, that isn't the issue I am really concerned with. What I see as we shorten barrels is an increase in standard deviation, leading to vertical stringing with short barrels at long range. When the barrel is very long, we burn a relatively larger amount of the powder, so a small amount of unconsumed fuel is only a small percentage of the whole charge. When the barrel is shorter, we can't burn it all any more, and the less we burn, the unburnt portion becomes a greater percentage of the whole. I see loads providing an 8 to 10 fps standard deviation in a 24" barrel show 15-20 fps in a 16" gun.

    yeah, that is a good and very interesting point.

    To honest though, if you were to observe vertical stringing in my shooting, 100% chance its my inconsistent hold and I need more practice.
     

    TheOriginalMexicanBob

    Ultimate Member
    Jul 2, 2017
    32,179
    Sun City West, AZ
    Barrel harmonics play a role as well. My understanding the accuracy issues with the Mini-14 were found to be barrel harmonics. The harmonics change with length and barrel thickness and not always in the ways one might believe.
     

    Boats

    Beer, Bikes n Boomsticks
    Mar 13, 2012
    4,073
    Howeird County
    TTAG covered this with an actual scientific test.


    https://www.thetruthaboutguns.com/2...el-length-muzzle-velocity-and-accuracy/lready covered, the accuracy difference of long vs short barrels is a myth.

    So did rifle shooter


    https://rifleshooter.com/2014/12/30...ato-barrel-length-versus-velocity-28-to-16-5/

    The difference is velocity, which in long range shooting means the bullet hits transonic velocity sooner, which is the biggest influencing factor on long range accuracy. However, even on the extremes, the difference on velocity between a 28"barrel and a 16.5" barrel is about 300 fps.

    Honestly, working up the right load that works best with your barrel and action is more important than agonizing over barrel length.

    From a personal standpoint, I have found that Australian Outback .308 nets me sub-moa at 200 yds out of a 16.5"aAR10 and is still super Sonic at 700yds. It even does 1"groups @ 100 out of a Springer Socom II, which previously struggled to do 4" at the same range with XM20C
     

    OLM-Medic

    Banned
    BANNED!!!
    May 5, 2010
    6,588
    I'm not a fan of longer barrels.

    A lot of people have shown me that 16" , 14.5" and sometimes even 12.5" .223 barrels can still do fine at internediate range. I regret getting a 18" .223 barrel at all.

    It does cause less velocity, which means more drop and more wind influence, and thus more narrow margin of error in theory, but not by a lot.

    It also will effect the range at which your bullet turns trans sonic (which means barely supersonic), which is the most unstable part of flight.

    I wouldn't worry about any of this unless you're going longer ranges, like >600y.
     
    Everyone knows Mini-14 is a tack driving all powerful sniper rifle as seen in movies and video games.

    Well, duh...
     

    Attachments

    • a team.jpg
      a team.jpg
      9 KB · Views: 190

    moose&squirrel

    Banned
    BANNED!!!
    Aug 25, 2018
    253
    The term "Accuracy" is often interchanged with "Precision".

    A longer barrel may prove more accurate, in that holding a consistent point of aim may be easier due to increased mass and/or sight radius.

    A shorter barrel is often more precise, because for any given diameter or profile, making the barrel shorter makes it stiffer and thus less reactive.

    You can see evidence of this by noticing most benchrest rifles have shorter barrels that are stiffer and more precise, while most position rifles are longer and heavier so the shooter can control them better and/or to provide a longer sight radius, both helping to achieve better accuracy.

    That said, I have seen several 16" .308s shoot sub-MOA groups out to 1,200 yards. It is mostly a function of barrel quality, but the shooter really has to be focused to shoot a short rifle from field positions with repeatable precision.

    One aspect of long range accuracy one will give up with a shorter barrel is that the reduction in muzzle velocity means longer flight time and more sensitivity to conditions. On a known distance course, extra drop isn't a big issue. Conversely, on an unknown distance course, when shooting your own range estimates, increased drop places greater emphasis on your ability to correctly range targets and tends to compound your errors. Wind drift will be greater on any type course and giving up velocity means your drift will be worse and your calls have to be better.

    1200 yards? I doubt I could see a target at 1200 yards out of Mount Palomar's telescope! :)
     

    E.Shell

    Ultimate Member
    Feb 5, 2007
    10,245
    Mid-Merlind
    TTAG covered this with an actual scientific test.

    https://www.thetruthaboutguns.com/2...el-length-muzzle-velocity-and-accuracy/lready covered, the accuracy difference of long vs short barrels is a myth.
    Bad link, maybe dropped or moved...

    The title of the article does beg the question: Are we actually talking about "accuracy" or "precision"?

    If we are talking about "precision", then no, barrel (and installation) quality is the prevailing factor and length is a minor factor. I had said this in my post above. In benchrest where PRECISION is the goal and precision is measured in thousandths of an inch and not MOA, the trend went to shorter, more rigid barrels 20 years ago. You won't find one on the line right now more than about 21 or 22". Those guys really do know a bunch about precision.

    If we are talking about "accuracy", a completely different concept than "precision", then in order to believe there is no difference in barrel length, we would have to believe that, with equal quality barrels, it is just as easy to shoot good groups unsupported with a 16" barrel as it would be with a 26" barrel. I would reject this notion. The 26" barrel provides greater mass out front and is far less reactive to handling issues. If we use iron sights, then the longer barrel provides an even greater ACCURACY advantage by having a longer sight radius. Don't see many high power competitors sporting 16" HBARs, although they are much easier to carry...

    So again, what are you talking about, accuracy or precision?
    Yeah, I saw that when it was published... I didn't really know how to take the data, but if you pick it apart, there is some useful info.

    On one hand, I'd like to just toss it out, because 5 round groups of non-match ammo tells us almost nothing and we cannot access the raw data, only some dubious averages. When we have milsurp type ammo, or ammo emulating ball ammo, we normally have very poor velocity control because velocity control isn't the critical parameter. Feeding through auto and semi-auto rifles and going BANG every time is vastly more important, just for starters. Durability, temperature stability, waterproofing...anything but precise velocity control, but hey, let's use this this crap as a test control anyway, LOL.

    IMO, the only ammo worth looking at in that test is the FGMM 168. I have found FGMM 168s to reliably deliver 7-9 fps SD numbers through full length rifles and that should be close enough to provide meaningful results.

    On the other hand, the sparse and poorly presented data actually does prove one of my points too.

    FWIW, I currently have a LabRadar chronograph and also have a Magneto-Speed that I used before that, and before the Magneto-Speed I used a borrowed Ohler 35p quite a bit. I have also had a couple of the junk chronographs. We have chronographed many rounds of ammo through hundreds of rifles, both in load development and in ballistic analysis. The statements I had made above are grounded in facts and field evidence.

    The missing raw test data is extremely important here, as evidenced by the otherwise unexplained jumps in velocity. It is simply not logical to accept that between 20" and 19" we lost 33 fps, but only lost 9 fps between 19" and 18", but THEN, we lost 42 fps between 18" and 17". CLEARLY, the data is flawed, and in at least two ways. That's OK though, we'll play it out with YOUR data source anyway.

    First flaw is that the 5 round sample size is insufficient and greater samples will help to smooth out these obvious aberrations. Seriously, for another hundred bucks worth of ammo, we could have really seen something valid.

    The next flaw, as I had mentioned above, ES and SD numbers start to get BIG when the barrel gets too short and the erratic test results are suspect. Big velocity deviations mean that some rounds depart pretty far from the median value. When we start getting rounds that jump away from the average, the average is skewed and of little scientific value in this context. All it takes is one or two rounds from the rather limited 5 round sample to move the average away from the median value, so when we see ridiculous jumps in velocity, as shown in the test results, we are seeing the effects of increased deviation. This sorta proves my point regarding increasing deviation in shorter barrels.

    Question: How is this, albeit limited, information not showing us excessive deviation?

    Looking at velocity loss per inch, but trying to make these numbers make sense is a bit of a stretch, but we can try.

    If we took any three consecutive numbers and smoothed the average - like if we take the 20 to 19, the 19 to 18, then the 18 to 17, we would have a SLIGHTLY better picture of what was going on, and we have an average loss of 29 fps per inch in this barrel length span. If we go to the 28 to 27, then the 27 to 26, then the 26 to 25, we get an average of 16 fps per inch in this barrel length span. This sorta proves my point about losing increasing amounts of velocity as we get to the short end.

    Question: How can we say that velocity loss is linear throughout a barrel length range of 16" to 26" when there is a demonstrable difference?

    Thank you for linking the data we could use to demonstrate what's happening.

    If you were to read my posts above, you will see that this is EXACTLY what I had said was going on. This would take far less analysis and be much easier to see were the raw data published instead of relying on averages based on statistically marginal sample sizes.
    The difference is velocity, which in long range shooting means the bullet hits transonic velocity sooner, which is the biggest influencing factor on long range accuracy.
    While crossing the transonic boundary does upset some bullets and is generally considered detrimental to precision (and consequently accuracy), the effects are often overrated. We shoot FGMM 175s to 1,200 yards and many 16" 7.62/.308 rifles have no trouble keeping sub-MOA groups. According to the ballistic calcs, we went transonic (generally accepted to be about 1,125 fps (https://en.wikipedia.org/wiki/Transonic)) back at 850 yards. I would suggest that in theory, there is no difference between theory and field testing, but field testing often proves otherwise.

    If we did exactly the same thing with the Sierra 168s (as loaded by FGMM) we would typically destabilize shortly after going transonic and we would be lucky to hit a 12 x 16" target at 900 AT ALL.
    However, even on the extremes, the difference on velocity between a 28"barrel and a 16.5" barrel is about 300 fps.
    Which is like changing cartridges. ADD 300 fps to a .308 and you have a .300 WinMag. Take 300 away and it's a .30-30...
    Honestly, working up the right load that works best with your barrel and action is more important than agonizing over barrel length.
    Good advice if one already has the barrel. When choosing barrel length, it's nice to have correct information to balance one's needs between a light handy rifle and one capable of enhanced long range performance.
     

    lazarus

    Ultimate Member
    Jun 23, 2015
    13,678
    The term "Accuracy" is often interchanged with "Precision".

    A longer barrel may prove more accurate, in that holding a consistent point of aim may be easier due to increased mass and/or sight radius.

    A shorter barrel is often more precise, because for any given diameter or profile, making the barrel shorter makes it stiffer and thus less reactive.

    You can see evidence of this by noticing most benchrest rifles have shorter barrels that are stiffer and more precise, while most position rifles are longer and heavier so the shooter can control them better and/or to provide a longer sight radius, both helping to achieve better accuracy.

    That said, I have seen several 16" .308s shoot sub-MOA groups out to 1,200 yards. It is mostly a function of barrel quality, but the shooter really has to be focused to shoot a short rifle from field positions with repeatable precision.

    One aspect of long range accuracy one will give up with a shorter barrel is that the reduction in muzzle velocity means longer flight time and more sensitivity to conditions. On a known distance course, extra drop isn't a big issue. Conversely, on an unknown distance course, when shooting your own range estimates, increased drop places greater emphasis on your ability to correctly range targets and tends to compound your errors. Wind drift will be greater on any type course and giving up velocity means your drift will be worse and your calls have to be better.

    Your last. Now, for target shooting with known ranges or a laser range finder there is really only wind effect that influences precision. However for practical shooting a longer barrel, with higher velocity, means the round is flatter reducing the impact of error from the shooter on estimating range quickly (and possibly inaccurately) as well as wind drift.

    The difference in practical accuracy between 16” and 18” is likely to be minimal even at reasonably long ranges.

    I am building an 18” AR-10 instead of 16” for many reasons though. The 50fps extra muzzle velocity is just that tiny bit of extra energy at the target, slight bit flatter shooting, slight but less wind drift, slightly easier to hold steady off hand and field positions and a fair amount reduced muzzle blast (IMHO there is a noticeable difference between a 16” and 18” .308 barrel for blast)

    I’d go 20” for more of all of that, but as this is going to be a hunting rifle, there I feel like I am compromising more on weight and difficulty maneuvering in brush outweighing the other benefits I am gaining. Why I went with a 18” for my 6.5 grendel AR-15
     

    Users who are viewing this thread

    Forum statistics

    Threads
    274,930
    Messages
    7,259,487
    Members
    33,350
    Latest member
    Rotorboater

    Latest threads

    Top Bottom