Ask Foghorn: How Much Does Barrel Length Matter?

James asks:

Barrel length – 20″, 24″ and 26″ seem to be the off-the-shelf offerings. 26″ seems impractical for all but long-range target practice, so that leaves 20″ and 24″. What are the reasons to choose one over the other? I know velocity and range are factors, but does a 24″ barrel really give that much more performance over a 20″?

Internal ballistics was never my strong suit, but I think I can give this a stab. Gimme a second to break out my old physics textbooks…

A barrel has two functions in a firearm.

  1. Make the bullet fly straight.
  2. Make the bullet fly fast.

The first one is pretty obvious —  a good barrel is the difference between 1 MoA and 1 MoP (Minute of Person) groups. It not only needs to grip the bullet, but it needs to hold it tightly and evenly as it moves through the barrel. Just like the ill-maintained PKM in “The 9th Company,” if the barrel isn’t precisely straight then the round is going to be very quickly off course. And if the barrel doesn’t grip the bullet tightly and make it engage the rifling then the bullet will tumble just like a smoothbore musket. CAI had an issue not too long ago where they issued 5.56 barrels on 5.45 guns, and the .1 mm difference was enough to make the guns uselessly inaccurate.

The second function is less obvious, but the longer a bullet is in the barrel the faster it will fly (and the faster it flies the farther it will go). The expanding gases from the gunpowder are only able to act upon the bullet while it is in the barrel and pushing it along, and the second the bullet leaves the barrel it starts dropping and slowing down. If you take this to the extreme (say, a 2 mile barrel with a 5.56 round) eventually the friction of moving down the barrel will be greater than the force of the expanding gasses and the bullet will stop, but barrels become impractical to move and aim long before they reach that point. Which brings us to the main point of this article…

In most cases, longer barrels mean better ballistics. Gunsmiths learned very early on that longer barrels usually make the bullets fly straighter (even before the advent of rifling) and length also means faster bullets. But the longer barrels also made the guns heavy and difficult to move around, especially through the woods. In other words, at some point you get what is called a “diminishing rate of return” — you stop seeing the same level of benefits from additional barrel length. Finding the right barrel length is all about determining the point at which an additional inch of barrel is not worth the extra weight and inconvenience compared to the additional velocity it provides.

This is a graph of the data produced by accuratereloading.com (the same people from whom I stole the lead image), who took a perfectly good 22 inch rifle and kept chopping it down inch by inch and recording the muzzle velocity at each barrel length. It’s very slight, but you can visually detect a change in the “slope” of the line comparing the segment between 10 and 11 inches to the segment between 21 and 22 inches despite the increase in barrel length being identical. It’s obvious that the increase in muzzle velocity from 21 to 22 inches was less than the increase in muzzle velocity between 10 and 11 inches, indicating that there is in fact a diminishing rate of return and the gun was approaching the point where additional barrel length would either have no effect or start decreasing muzzle velocity.

Accuracy is much harder to quantify, but similar to muzzle velocity there comes a point where more barrel doesn’t necessarily mean more accuracy. There’s also crazy stuff like barrel harmonics to think about, but in general the 22″-26″ range for .308 is in that “Goldilocks” region for accuracy (provided the barrel is straight and tight and yadda yadda yadda).

Choosing which barrel is best for you is a trade-off between velocity and accuracy on the one hand and portability on the other. For you, who indicated that you were looking for a more “tactical” .308, the 22″ barrel should fit the bill perfectly. The difference between 22″ and 24″ is only going to be around 50-100 FPS, but you will definitely feel the difference in weight and bulkiness with the longer barrel. I went with a 20″ barrel on my Weatherby Vanguard, and having shot it for a while I think the ideal barrel length for any .308 (in my opinion) is right around 22″ — it’s still portable enough to be maneuverable at that length but it’s also long enough to be accurate at long range. In other words, my gun is too short and I regret it so learn from my mistakes.

A bigger factor in final accuracy of barrels is the “barrel profile,” which is the thickness and weight of the barrel. There’s an Ask Foghorn in the works for that one too, so stay tuned.

For more information ChuckHawks.com has an interesting article about changes in muzzle velocity as a factor of barrel length, but the information is based on testing for which they give very few details. Nevertheless it made for some interesting reading material.

If you have a topic you want to see covered in a future “Ask Foghorn” segment, email guntruth@me.com.

To browse previous Ask Foghorn segments visit http://www.thetruthaboutguns.com/category/askfoghorn/.

comments

  1. avatar GS650G says:

    Reloading data I have for shot shells suggests that after 14 inches the gases have created maximum pressure against the load, everything else is just pipe. Shot shells are a whole lot different than rifled bullets though, and the wads affect the results a lot.

  2. avatar Tom says:

    A lot depends on the cartridge. 22LR cartridges really only need a Ruger 10-22 length of barrel.
    Differences in barrel length have an impact with rifles, handguns even more so.

    1. avatar Joe Grine says:

      I’ve heard that for .22 LR rifles, barrels longer than 16 inches or so can actually cause a reduction in muzzle velocity. Not sure if thats true or not – can anybody confirm?

  3. avatar Pro.0s says:

    “A barrel has two functions in a firearm.

    Make the bullet fly straight.
    Make the bullet fly fast.”

    Disagree with the 1st function. Bullets don’t fly straight. They yaw in flight and travel in a parabolic trajectory. The latter is limited by function of time to target and therefore velocity and BC. Good barrel, bullet, case and powder combination produce consistent results. Larger the deviation, the more inconsistent the results and larger the group.

    1. avatar Graybeard says:

      Technically, you are correct, however when trying to provide an understandable answer to a specific question without overwhelming the questioner with information that, in the end, might just end up confusing them, a bit of simplification is called for.

      Nick was summarizing what the function of the barrel is supposed to be, not writing a technical paper on what a barrel does, in a way calculated to give the questioner the basic concepts without overwhelming him with technical niceties that might just confuse him.

      Which is why he is so good at writing these things.

  4. avatar Pete says:

    “A bigger factor in final accuracy of barrels is the “barrel profile,” which is the thickness and weight of the barrel. ”

    Saw an article in one of my gun mags a few months back that looked at the accuracy of various barrel lengths and profiles. They found that for consistent, multi-shot long-range accuracy, a short (18″ or so), stiff (i.e. thick) barrel tended to be more accurate than a longer, slimmer one. Had a lot to do with barrel harmonics and differential heating on longer shot strings. The short barrels did lose velocity compared to the longer ones (maybe 100-200fps for a .308). There is a sniper rifle manufacturer that is making short, thick barrel rifles to take advantage of the better accuracy and more compact size. The other downside is weight – these rifles tend to fall in the 10-12 pound range with a scope.

    1. avatar "Dr."Dave says:

      I read the same. And I know people who have achieved amazing accuracy results with a thick, 18” fluted barrel that was free floated.

      More important than length, methinks, is the stability of the barrel. You want something that doesn’t have any external stress on it (i.e, free floating), and is stiff enough and mounted securely into the receiver. Something with a good bedding and a free floated stock will certainty out shoot a similar rifle with a longer barrel that isn’t has an action that’s insufficiently bedded and a barrel that isn’t free floating.

      My PTR-91 has a similar profile barrel and is accurate far beyond what I expected for a semi-auto .308.

  5. avatar Kirk says:

    Heh…he said, “my gun is too short.” Heh. Sorry: the juvenile grabbed the wheel.This helps a lot as I’m deciding this very issue on a SCAR 17.

  6. avatar James says:

    Excellent! I did see somewhere else (I forget where by now) that a 22″ barrel was just about optimal for a .308, and that anything more than that was just making life harder on yourself.

    The only thing is that, from what I see looking at .308s in my price range (RRA, LWRC, DPMS, Bushmaster…), aside from the carbine offerings which I’m not entirely interested in, 20″ seems to be the standard, then barrel lengths jump out to 24″ and 26″.

    I suppose that if I’m going to be picky (and who shouldn’t when looking to shell out a couple grand), I could just get my .308 a piece at a time and build it myself. But that would lead to the question of compatibility…

  7. avatar Joe Grine says:

    Hey Nick:

    With regard to the CIA AK74 /Tantal barrels you reference, I’ve always been of the (non-scientific / just a hunch) opinion that the problem was not incorrect barrel caliber, but rather that CIA simply got the twist rate wrong. The Russian military 5.45 x 39 round (7N6) is a very long 53 gr. bullet and contains some sort of penetrator core. It really needs a 1:8 twist rate (or tighter) to stabilize in flight. The commercial stuff from Wolf is usually 60 or 70 grains, making it even more difficult to stablize. I’m curious: has there been anything definitive published that identifies the cause of the CIA barrel problem and backs it up with evidence, or is your statement just based on internet chatter / folk-lore?

    1. avatar Nick Leghorn says:

      One of my friends who owned one of the unfortunate firearms decided to measure the bore, just for fun. It came out to be (IIRC) .219, which is right for 5.56 not 5.45.

      Over/under stabilization will either lead to bullet disintegration or tumbling at distance, but the only thing I know of that would make something keyhole at 20 yards is an improperly large bore diameter.

      1. avatar Joe Grine says:

        OK, Good to know. I know from personal experience that the wrong barrel twist rate will cause massive keyholing – even at short ranges. I own a HK93 that was manufactured in the early 1980s. Most 5.56 x 45 military rifles of that era,including teh M-16 and the HK 93, have a 1:12 twist rate, because the standard NATO round at that time contained a 55 grain bullet. I was at the range one day trying out a new IOR Valdada scope for the HK 93, and I was getting 20-25 inch groups at 50 yards – with obvious pizza slice keyholes. At some point, it dawned on me that the keyholes were due to the fact that I was shooting standard NATO Radway Green 62 grain (SS109, which is identical to U.S. designated M855). Someone at the range was nice enough to let me buy a box of 55 grain fodder off of them, and the groups immediately returned to dime-size at 50 yards.

  8. avatar Drew says:

    Magnum rifles especially benefit from longer barrel length. With .264 WinMag for instance, a 24″ barrel (as opposed to 26″) drops the velocity down into .270 range. Doesn’t matter as much with .308 but I feel .260 Remington also benefits greatly from a 24″ barrel.

    1. avatar Nick Leghorn says:

      Didn’t have a large enough dataset for the barrel lengths I was looking for. Call me when they publish results for 10-30 inch barrels.

  9. avatar Gerald says:

    Just wanted to say a quick “thank you”. I’ve been enjoying catching up on the “Ask Foghorn” articles. Great explanations and writing!

    1. avatar Lance says:

      +1 on GPS Defense

      I got to take a tour of the place and it was amazing. TTAG should definitely send a member of the AZ contingent to one of it’s classes.

      1. avatar Patrick Carrube says:

        I got dibs!

        1. avatar Nick Leghorn says:

          Damn! Too slow…

  10. avatar Ando says:

    Shorter barrels are more accurate to a point. Deflection increases in proportion to the cube of length. Practically speaking this means a 26″ barrel is less then half as stiff as a 20″ barrel. This is not to say that long barrels can’t be accurate. This is one important factor in thin contour hunting barrels, but it is one important factor out of many. With all other factors staying equal, a shorter barrel will be more accurate.

  11. avatar frankgon4 says:

    I think what was left out was twist rate of the barrel. Most .308 come in 1 in 12 twist. Mine is 1 in 10 twist rate with 24 inch barrel.
    Different twist rates have varying affects on bullets.

  12. avatar Dean Weingarten says:

    Barrels 26 inch and longer are definitly not impractical. Millions of people used barrels longer than 26 inches for generations and did well with them in the woods and in combat. Longer barrels have several advantages:

    1. With magnum cartidges, you gain significant velocity.

    2. With iron sights, you gain significant practical accuracy.

    3. With subsonic bullets, you reduce the sound signature significantly.

  13. avatar Uncle Lar says:

    I have always believed that with modern faster powders a short stiff barreled rifle could be tweaked to equal or surpass it’s longer leaner cousins both in accuracy and in velocity.
    On a somewhat related note, I recall stories from back in the black powder days where the gunmaker would fire a new muzzle loader over fresh snow or failing that a white sheet with increasing loads. When the gun started throwing unburned powder you backed off a couple grains and declared that the maximum load. The idea being to match the powder charge with the bullet and barrel length.

  14. avatar Dave J says:

    yeh, I dont care so much about “straight” but I do care about consistent….harmonics and all that. What is considered the optimum barrel length for the 300BLK? I am considering having a TC Contender barrel chambered in the round for a young hunter’s first deer rifle.
    thanks in advance Nick

    1. avatar Nick Leghorn says:

      I seem to remember that Kevin (AAC founder) designed the cartridge such that all of the powder is burned and used after 9″ of barrel. 16″ is what I use and it seems about perfect for me.

  15. avatar Dave J says:

    Copy, I dont wish to pursue the whole SBR stamp thing, therefore 16″ is what the Feds will currently permit in the TC format with a butt-stock. Thanks again.

  16. avatar Rich M says:

    You may be interested in a book by Harold Vaughn – retired aeroballistics engineer from Sandia National Lab. “Rifle Accuracy Facts” is based on actual experimentation. It gets a bit technical in places. One of the things I found interesting is the fact that a sporter contour barrel can be very accurate, provided the harmonics are right. So, it is not quite as simple as “short and rigid” makes for accuracy.

  17. avatar CARLOS BONNETT says:

    What would be the correct length for my rifle barrel for 50 yards target shooting?

  18. avatar Brent lee says:

    A sporter barrel can be just as accurate, for 1-2 shots. Then it strings.

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