I have a question for the experts here, since I certainly am not one. Is there any relationship between 'barrel length' and 'accuracy' when comparing AR's? My question has to do with a choice of a 16" vs. a 20" inch barrel vs. a 24". It almost seems to make intrinsic sense that the longer the barrel, the more velocity the bullet will have after it leaves the barrel and that, in theory, all other factors being equal, the more accurate it should be.

However, I have spoken to some that claim that this is an "old" concept because with the improvement in modern-day powder, almost all the powder has been burned after after about 16 inches, and that there really is no significant accuracy difference between a 16 inch barrel and a 20 inch or 24 inch barrel--again, all other factors being equal.

Any expert opinions here?

BJ

Quoted: I have a question for the experts here, since I certainly am not one. Is there any relationship between 'barrel length' and 'accuracy' when comparing AR's? My question has to do with a choice of a 16" vs. a 20" inch barrel vs. a 24". It almost seems to make intrinsic sense that the longer the barrel, the more velocity the bullet will have after it leaves the barrel and that, in theory, all other factors being equal, the more accurate it should be. However, I have spoken to some that claim that this is an "old" concept because with the improvement in modern-day powder, almost all the powder has been burned after after about 16 inches, and that there really is no significant accuracy difference between a 16 inch barrel and a 20 inch or 24 inch barrel--again, all other factors being equal. Any expert opinions here? BJ

Not more accurate.

Velocity does not equate to accuracy.

A higher velocity bullet reaches its target faster so it drops less and is less affected by wind to a degree, but does not make it inherently more accurate.

I think with a 300yd long barrel the really long shots would be a piece of cake. Stinks for CQB though.

Barrel length does not affect "accuracy" or "precision".  It does affect velocity, and that translates into a flatter trajectory requiring less adjustment on the shooter's part for bullet drop and windage.  Longer barrels and added velocity just increase the shooter's margin for error when taking long shots rather than making the rifle a more precise instrument.  On some rifles the length of the sight radius is greater with a longer barrel, making precise sight alignment easier.  The quality of the finish of the bore and crown, as well as the quality and specifications of ammuntion, determine how accurate a rifle is.  

What about the statement regarding the improvement in modern day powder, propellant, etc.--ie. the stuff that fills the cartridge? Has there really been a real improvent in this that somehow will make modern cartridges 'better' than the ones in the past (eg. versus 20 years ago)?

BJ

Quoted: What about the statement regarding the improvement in modern day powder, propellant, etc.--ie. the stuff that fills the cartridge? Has there really been a real improvent in this that somehow will make modern cartridges 'better' than the ones in the past (eg. versus 20 years ago)? BJ

20 years ago, not much. 60 years ago , yes.

but still with a .223 and good powder you don't burn all of it in a 16" barrel or even a 24". And again you are only gaining velocity.  

As for powder, changes are being made all the time, but the net effect is not too significant.  New formulas and blends are put out for the factory and handloaders as new bullet weights and new calibers come out, but consistency has been fairly static.  Also, even if the powder were to burn completely in 16" of  barrel the pressure in the barrel would not drop to zero at that point and additional barrel length would still add to velocity due to the residual pressure.

Not accuracy, but better terminal effects at range for barrel length.   All things being equal, if you need to reach out an touch someone rather than paper at 600 yards, go with the longer barrel.

The longer sight radius may also lead some to believe the longer barrels are more accurate, they are just easier to sight.

My 0.02, Cheers,

Michael

A shorter barrel length can manifest itself as less accurate due to a greater variation in muzzle velocity.  On some chrono measurements I made on shorter 308 guns, not only did the velocity drop, but the velocity spread (extreme spread and standard deviation) widened.  This will show up as a wider shot pattern.  The only question is if you are shooting at a great enough range to matter.

If the really short barrels were the key to accuracy, then our tanks, naval artillery, and ground artillery would be running around with the shortest barrels possible.

Quoted: If the really short barrels were the key to accuracy, then our tanks, naval artillery, and ground artillery would be running around with the shortest barrels possible.

Except that they would lose so much velocity (and range) as to be useless, regardless of accuracy.  You still have to accelerate the projectile to an effective velocity before accuracy begins to become an issue.  A 2500 lb. shell fired from a 16" bore naval gun with a 36" barrel would be....  Well, you can probably imagine.  

So, to summarize....for a .308 caliber, would the 'average' shooter notice any difference in accuracy at say 500 yds between a 16 inch and a 20 inch barrel??

BJ

For all practical purposes on an AR if you have to use it for defense the same holds true today as it did 60 ears ago95% of combat infantry engagements take place at under 100 yds and of that 75% take place at well under 50 yds.So on an AR say shortest legaly possible will still have the accuracy and velocity to get the job done.

Quoted: Quoted: If the really short barrels were the key to accuracy, then our tanks, naval artillery, and ground artillery would be running around with the shortest barrels possible. Except that they would lose so much velocity (and range) as to be useless, regardless of accuracy.  You still have to accelerate the projectile to an effective velocity before accuracy begins to become an issue.  A 2500 lb. shell fired from a 16" bore naval gun with a 36" barrel would be....  Well, you can probably imagine.

My point exactly.

You should choose the right sized barrel for your AR-15 based on your needs. A quality 24" barrel shouldn't hurt accuracy anymore than a quality 20" or 16" barrel. The 24" barrel is worth about 100-200 fps more than a 20" barrel (both being 1 in 9 twist).

Quoted: So, to summarize....for a .308 caliber, would the 'average' shooter notice any difference in accuracy at say 500 yds between a 16 inch and a 20 inch barrel?? BJ

He would not notice a difference in accuracy, per se, but due to velocity differences he would notice that gravity and wind were bigger factors at 500 meters if he shot both barrel lengths back to back with the same ammunition.  Bear in mind that a bullet and powder combination that is efficient (consistent) in a 20" bbl. may not be so in a bbl. that is 4" shorter due to powder burn rates, etc.  One might have higher SD's in terms of muzzle velocity due to changes in barrel length, and that could affect accuracy, but there are a lot of moving parts involved in this.  Barrel length is not really a factor.