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.
- Make the bullet fly straight.
- 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.
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