The barrel isn’t the only thing that affects accuracy, but the barrel can affect accuracy primarily in three ways: barrel weight, barrel length, and barrel twist.cmgee67 wrote:So how exactly does a barrel determine the accuracy of the bullet. If someone says the barrel is innacurate how does that cause the bullet to vear off track? Is it the rifling? Gas? Imperfection in the bullet? Shooter? I’m sure all of that comes into play but I’m just curious.
BARREL WEIGHT: Given two barrels of equal length, the heavier barrel will be stiffer. A stiffer barrel dampens barrel harmonics - the frequency and amplitude of barrel vibration as the shot is fired. The greater the amplitude of the vibrations at the muzzle, the greater the “spread” will be at the point of impact. The heavier barrel vibrates with less amplitude and frequency at the barrel, thus the spread at POI is smaller, which equals better accuracy.
BARREL LENGTH: Up to some “ideal” length, the longer the barrel, the greater the muzzle velocity as the powder is able to achieve a complete burn while in the bore. The greater the velocity, the higher the bullet’s ballistic coefficient (BC). The higher the bullet’s BC, the more stable it will be in flight, and the longer it will remain stable over the length of its trajectory. Stable bullets are more likely to behave the same, one shot to the next, which increases consistency of performance at the POI. At some point, the barrel is TOO long, which is when the powder has finished burning inside the bore and the gasses are no longer expanding at sufficient rate to continue accelerating the bullet. Every inch of bore from end of combustion to muzzle creates friction which begins to slow the bullet down....which decreases its BC, etc., etc. The ideal length of barrel for cartridges like .308, .30-06, .270, etc., in a bolt action rifle, appears to be between 22” to 26”. That’s why you see most hunting rifles in those common hunting calibers with barrels in that range of lengths. HOWEVER: that kind of barrel length is not optimum in a battle rifle, which is why modern battle rifles like ARs, SR25/M110s, SCAR 16s and 17s, etc, tend to have barrel lengths of 20” or less.....mostly around 16”-18”. In that case, greater range (and for the most part, greater accuracy) is being given up in exchange for portability, CQB requirements, weight, etc.
BARREL TWIST: Generally speaking, there is a range of twist rates - measured as the length of bore required for the rifling to complete 1 full turn - which is ideal for any given caliber. For instance, in .223/5.56, twists of 1:12”, 1:9”, 1:8”, and 1:7” are used, with the latter 3 being the most common. A “tighter” twist completes one full turn in a shorter length of bore. So a 1:7 twist is “tighter” than a 1:9 twist. As a general rule for any given caliber, the tighter the twist, the better it will be at stabilizing heavy-for-caliber bullets, and the longer the twist, the better it will stabilize light-for-caliber bullets. If a light bullet is fired at a high velocity through a too-tight twist, that bullet can be spun fast enough to cause it to come apart in flight, or to cause it to spin itself off of the desired trajectory.....in other words, your fastball becomes a curve. So if you shoot 45 grain .224 bullets through a 1:7 twist with a MV of 3300 fps, you’ll likely lose accuracy. But if you shoot a 75 grain bullet through that same barrel, you’ll see a marked improvement in accuracy.
Pick a barrel of optimum weight (the heaviest you can realistically tote around in the field), optimum length (the longest you can realistically tote in the field that allows optimum powder burn without slowing the bullet down), and the optimum twist for the ammo you will be shooting, and that will give you the best accuracy you can expect for a hunting rifle. For example, a .308 bolt action, shooting bullets in the 150 to 165 grain range, through a 22” barrel with a 1:12 twist will be fine. That will give you the velocities you’d like to see for optimum terminal ballistics.
For a target rifle you’ll be shooting either from a bench, or from the prone on a rest like a bipod, a heavy, longer barrel of tighter twist will let you shoot heavier bullets at higher velocities over longer ranges. For instance, a .308 bolt action, shooting bullets in the 168 to 208 grain range, through a 24” to 26” barrel with a 1:10 twist will be fine. That would give you enough to heavy bullets further while retaining enough velocity to keep their BC up over longer ranges.
Mind you these are all generalities, and each rifle is an individual weapon with its own characteristics. Here’s another “general” rule: Bolt actions tend as a class to be more accurate than semiautos. The reason is mostly because the bolt gun will achieve consistent lockup from shot to shot, but the superior rigidity of action in a bolt gun comes into play, as well as the likelihood that your mass-produced milspec barrel for an AR is just not made to the same tolerances as the barrel in say a good Remington 700 or Savage 10. That does not mean that you can’t build a semiauto that will outshoot a bolt gun. My son used to have a AR15 varmint rifle that would put 4 77 grain OTM into .25” at 100 yards. That’s crazy good accuracy, and better than any of our bolt actions, which are very accurate rifles in their own right. But as a general thing, I would take a bolt action over a semiauto of the same caliber most of the time, if accuracy were my primary objective. But even among semiautos as a class, the above factors of barrel weight, barrel length, and barrel twist still apply, just as they do in a bolt gun.
And exactly NONE of this has anything to do with the accuracy of the bullet itself, which is a separate issue. For instance, in .308, while will a 168 grain Sierra Matchking perform better than a 165 grain Federal Fusion bullet? (Hint: look for the word “match”.....) A crappy barrel won’t make a match bullet fly accurately, and a great barrel can’t make a crappy bullet fly accurately. But a good barrel with a good bullet is gold, and a great barrel with a great bullet is magic.
I hope that helps.