MV on 308 20" Barrel

[brotus2]

I am considering buying a Remington 700 SPS Varmint with 26" heavy contour barrel in 308. I was talking with a knowledgeable shooter at long range (1000 Yards) and he said all I need is a 20" barrel. According to him, the powder charge has totally burned by the time bullet has reached 20-22 inches in the barrel and by the time it exits at 26 inches the bullet has actually slowed down.

Is he correct?????

 

[cw308]

I have a 700 LTR 308 20" Brl. The 20" is more accurate up to 500 yds. longer ranges I would go with the 700P 26" Remington rifles have 1-12 twist & alot of free bore . would not go heaver than 168 gr HPBT bullet

 

[jmr40]

No he is not correct. The powder burns well before 20", but pressure still builds behind the bullet, and the bullet is still accelerating. I suppose you could reach a barrel length where the bullet starts to slow down, but you'd need a lot more than 26"

Your friend has misunderstood a few things. Some cartridges really need a lot more barrel than others. The 308 is very efficient and has what I call a "sweet spot" of between 20-22". You will still see gains in veocity if you go past 20", but for each inch you go longer you will see less and less gains. Going shorter will see greater and greater loses.

I personally see about 18-22" as being about perfect for the 308. If you are going to be shooting at 1,000 yards or more then the tiny bit of extra velocity a 26" barrel would give over 20"( around 100-150 fps would be my guess) might just be enough to matter. At closer or mid ranges I think the more compact barrel length more than offsets the small gains in velocity.

 

[mrawesome22]

I am considering buying a Remington 700 SPS Varmint with 26" heavy contour barrel in 308. I was talking with a knowledgeable shooter at long range (1000 Yards) and he said all I need is a 20" barrel. According to him, the powder charge has totally burned by the time bullet has reached 20-22 inches in the barrel and by the time it exits at 26 inches the bullet has actually slowed down.

Is he correct?????

Depends on the powder used.

 

[Stealth01]

Various mathematical equations were developed to relate muzzle velocity to barrel length, but one of the simplest relationships was developed by Homer S. Powley. He defined the relationship between muzzle velocity and barrel length, as one giving muzzle velocity (v) as a function of charge weight C, bullet weight B and expansion ratio (R). Here the expansion ratio is defined as the ratio of the barrel volume plus cartridge volume (total volume of the gun) to the cartridge volume. The equation relating to these factors is represented by:

v = K[C(1- R^-.25)/ (B + C/3)]^.5

Where, v is in f.p.s., C and B are in grains and K is a constant that depends on chamber pressure and other factors in the gun. The expansion ratio is dimensionless. From this equation, it can be seen that for a given gun, with a given powder charge and bullet weight, the muzzle velocity is dependent only on the expansion ratio. By cutting off the barrel the barrel volume is reduced, thereby reducing the expansion ratio. The relationship can be represented by:

F = [(1-R2^-.25)/(1-R1^-.25)]^.5

Where, F is the correction factor to correct the muzzle velocity at expansion ratio R1 to that of the reduced expansion ratio R2 (shorter barrel). For example, a rifle chambered for the .223 Rem. cartridge, has a 24" barrel, and fires a 50 gr. bullet at 3,080 f.p.s. when loaded with 25.1 gr. of IMR 3031 powder. The expansion ratio is 8.5. What is the muzzle velocity if the barrel is shortened to 22"? The new expansion ratio is 7.8. Plugging these values in the equation we get:

F = [(1-7.8^-.25)/(1-8.5^-.25)]^.5
= 0.9846
v = 0.9846 X 3,080
= 3,032 f.p.s.

The new velocity for the 22" barrel is 3,032 f.p.s. The velocity loss for removal of two inches of barrel is 48 f.p.s. (3,080 - 3,032).

And there you have it! There will be a test...

 

[44 AMP]

Also, while the mathmatics are a good guideline, there is a real world factor involved as well. And that is that some barrels are "faster" or "slower" than the median and will show different velocities from the published specs. 100fps is not unheard of.

SO, a given 20" barrel might be faster than a given 22" barrel (by a small amount), or more usually it might not be. Generaly the longer the barrel, the higher the velocity, but one does reach a point of diminishing returns.

 

[PawPaw]

The only time I've actually seen a bullet slow down in a rifle barrel was when I was shooting .38 special wadcutter loads through my Marlin 1894C. I was playing around with the chrony one afternoon and decided to try my target .38 special load through the carbine. I was using 2.7 grains of Bullseye under a 148 HBWC, a normally accurate load in my 4" revolvers that gives me 650 fps from those handguns. Through the Marlin, the chrony showed 606 fps average for 5 shots. I quit playing with it for fear of sticking a bullet in the barrel, but thought that it was an interesting experiment.