308 Barrel Length vs Muzzle Velocity


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bobhaverford
January 16, 2006, 09:30 PM
I've just read Boston's Gun Bible and he basically dismisses the Armalite as useless insofar as battle rifles are concerned. But I just love the ergonomics of the of the AR as well as the accuracy.

I'm going to buy the DPMS and am trying to decide which barrel length to get. The 24" weighs in at 11.28 pounds. The 16 inch carbine weighs in at approx 8.5 pounds. I love the idea of the carbine weight, but want to know how much I'd be giving up in muzzle velocity.

They also have an 18" barrel which weighs in at 9.5 pounds and may represent a good compromise. What would the muzzle velocity from this gun be?

I'm going to use the gun for plinking, target shooting and SHTF scenarios. Thanks in advance for any help.

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mndfusion
January 16, 2006, 09:41 PM
Go for the middle lenght...

rangerruck
January 16, 2006, 10:58 PM
well to get max equit burn and push out of the powder you actually need a bbl of about 39 inches. so the diff between 18 and 24 is really a non starter, unless you are doing some serious target/sniper work at 800 meters or more.

rangerruck
January 16, 2006, 11:00 PM
one more thing, if you are going to spend that kind of $$$ get a krebs custom. uses ak style piston rods, not just gas trap. much cleaner and more reliable. or a robinson armament.

bobhaverford
January 16, 2006, 11:56 PM
Just found this information from the following web site: http://www.chuckhawks.com/rifle_barrel.htm

The 2001 Edition of the Shooter's Bible states, in the introduction to the Centerfire Rifle Ballistics section, "Barrel length affects velocity, and at various rates depending on the load. As a rule, figure 50 fps per inch of barrel, plus or minus, if your barrel is longer or shorter than 22 inches." But they do not say what category of load to which this 50 fps average pertains.

Jack O'Connor wrote in The Rifle Book that, "The barrel shorter than standard has a velocity loss which averages about 25 foot-seconds for every inch cut off the barrel. Likewise, there is a velocity gain with a longer barrel." He went on to illustrate this using a .30-06 rifle shooting 180 grain bullets as an example, so his estimate was obviously for rifles in that general performance class.

Other authorities have tried to take into account the different velocity ranges within which modern cartridges operate. The Remington Catalog 2003 includes a "Centerfire Rifle Velocity Vs. Barrel Length" table that shows the following velocity changes for barrels shorter or longer than the test barrel length:

MV 2000-2500 fps, the approximate change in MV per 1" change in barrel length is 10 fps.
MV 2500-3000 fps, the approximate change in MV per 1" change in barrel length is 20 fps.
MV 3000-3500 fps, the approximate change in MV per 1" change in barrel length is 30 fps.
MV 3500-4000 fps, the approximate change in MV per 1" change in barrel length is 40 fps.

The 45th Edition of the Lyman Reloading Handbook also has a table showing Center Fire Rifle Velocity Vs. Barrel Length. Their figures apply to barrels between 20 and 26 inches in length and agree with the Remington figures. The Lyman table shows the following approximate velocity changes:

For rifles with muzzle velocities in the 1000-2000 fps range, the change in velocity for each 1" change in barrel length is 5 fps.
For rifles with muzzle velocities in the 2001-2500 fps range, the change in velocity for each 1" change in barrel length is 10 fps.
For rifles with muzzle velocities in the 2501-3000 fps range, the change in velocity for each 1" change in barrel length is 20 fps.
For rifles with muzzle velocities in the 3001-3500 fps range, the change in velocity for each 1" change in barrel length is 30 fps.
For rifles with muzzle velocities in the 3501-4000 fps range, the change in velocity for each 1" change in barrel length is 40 fps.

The 43rd edition of the Lyman reloading Handbook gave some concrete examples of velocity loss for specific calibers and loads. The Lyman technicians chronographed some high velocity cartridges in rifles with barrels ranging in length from 26" down to 22" with the following results:

The average loss for the .243 Win./100 grain bullet was 29 fps per inch.
The average loss for the .264 Win. Mag./140 grain bullet was 32 fps per inch.
The average loss for the .300 H&H Mag./220 grain bullet was 25 fps per inch.

For standard high intensity cartridges in the same test, the Lyman technicians chronographed the cartridges in barrel lengths ranging in length from 24" down to 20" with the following results:

The average loss for the .270 Win./130 grain bullet was 37 fps per inch.
The average loss for the .270 Win./150 grain bullet was 32 fps per inch.
The average loss for the .300 Sav./180 grain bullet was 17 fps per inch.
The average loss for the .30-06/180 grain bullet was 15 fps per inch.
The average loss for the .35 Rem./200 grain bullet was 11 fps per inch.

After a bunch of disclaimers, the Lyman people concluded, "The rule of thumb is that high speed, high pressure cartridges shed more speed in short barrels than do the low speed, large bore types." It's funny, but that is what I had suspected all along!

Joelk
January 17, 2006, 09:11 AM
My 16 inch AR10 chronographs about 90 to 100 fps slower than my 20 inch AR10. I don't have a 24 inch to compare yet.

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