Barell length for 270 and 308

[kb]

What is the optimum barrel length for each caliber?  I am considering possibly getting a 308, but already have a 270 (great caliber by the way).  What is the length of barrel needed to fully burn all of the powder, before de-acceleration begins?
 
For instance a 22 rimfire will achieve max velocity in a 17 inch barrel, before it begins to slow down.
 
Thanks  
 
kb

[Slamfire]

I believe the factory ballistic table are compiled from 24" barrels. Anything shorter would cause a slight 35 to 50 fps loss in muzzle velocity. Shorter barrels make a louder noise also. The difference is too slight to worry about though, 200 fps is significant, and different rifle will vary by nearly that amount.

[Gregory]

I think a 270 would be better served with a 24" barrel since your burning close to 60 gr of powder.  In the 308, where you are burning maybe 45 gr of powder  I feel 22" is adequate.  
Some manufacturers go as short as 18-20" on some carbine models and some as long as 26" on magnums.  For most centerfire round bolt action rifles I'd like to see 23" become the "standard" barrel length.  It is a nice compromise between the 22" and 24" lengths.

[nomosendero]

kb
If I understand you correctly, you are tying the "optimum barrel length"
with the length where ALL the powder is burned & velocity starts dropping.
While I agree that you can do this with all the popular rimfires & small
centerfires like the Hornet, because the barrel will still be a reasonable length, this reasoning does not carry over logically to the bigger cartridges. If you want absolutely ALL of the velocity possible, the barrel length would be quite long, longer than reason would allow even in a 308!

I think in the case of the .308 & 270 it is logical to think in terms of
diminishing returns, in other words, at what barrel length does it become
impractical to add another inch because your velocity increase will be
small.
 
In the case of the 308, we have discussed this on a different thread, but
most of the velocity can be achieved in 20-22". I like 20" because this is
a short action round & it makes for a compact gun to do the things that I
do with a 308 to begin with & I give up very little performance. If you shoot targets 600-1,000 yards then I would want 26" or more, so really with these cartridges your choice must be application driven!

In the case of the 270, many people like the 22" & good velocities can be
achieved, but I believe this cartridge is better served with a 24" tube. If
you wanted a "bean field" or stand rifle, a 26" would serve you well but
the 24" is a great all-round choice. I know one very good hunter & shooter
who built a 29" heavy 270 for target & long range hunting, but it did not
gain that much over a 26" tube.

With some cartridges if used for long range work, like the 300 RUM, a 28"
tube makes alot of sense to realize the benefits of the round, even though
it makes the gun a little more specialized.

So, we must look at each cartridge & more importantly the application for
each gun, determine through the research that someone else has already done the velocity gain you can expect "on average" with each additional
inch of barrel & strike a logical balance.
Just my thoughts:  Rod Smith

[jro45]

My 270 Wincheater has a 22" barrel and like was already said it loses about 50 some fps. So in reality when I shoot a 130gr bullet at 3150 fps it is only going  around 3070 or something like that.

[Lone Star]

Virtually all powder in normal-pressured rifle loads burns in the first few inches ahead of the chamber; even a 10" barrel will burn it all (silhouette shooters, firing .308s, etc, know this). The longer barrel provides more time for the gas to push against the bullet base - this is what accelerates the bullet to higher speeds.  Eventually the gas loses enough velocity that it no longer accelerates the bullet, but that is a long way from the breech.  1000 yard shooters using 29" .308s still see a minor gain in velocity at that length.

Data from Rifleshooter Magzine, September 2003 for the .270 with 130-grain bullet and H4350 powder:

27"......26"....25"....24".....23"....22"....21"
3115..3093..3070..3054..3035..3027..3001

The difference between a 22" and 26" barrel is just 66 fps - no one would ever notice this tiny difference in the field.

For the .308: G&A 9/87 with 180-grain bullet and IMR4320 powder:

25"......24"....23"....22"....21"....20"....19".....18"....17".....16"
2730..2700..2680..2660..2640..2620..2580..2560..2535..2480

Again not much difference, certainly not enough to be noticed in the field.

[Lawdog]

Stay with a 24" barrel in both the .270 and .308.  You will loose a little velocity but you will also gain in muzzle blast.  The shorter the barrel the louder the bang.  Lawdog
 

[kb]

What about accuracy?  For a hunting rifle, what barrel length?

[nomosendero]

Due to barrel harmonics & other factors that affect accuracy, no one can
say what the most accurate barrel length is & I hope no one tries.

[Gregory]

Due to barrel harmonics & other factors that affect accuracy, no one can
say what the most accurate barrel length is & I hope no one tries.

 
Don't benchrest shooters use relatively short very heavy barrels to enhance accuracy?

[nomosendero]

Gregory
Yes they do because a shorter barrel is stiffer & can make a very small
difference in your group & they have a weight limitation or else you could
add a longer tube but heavier & be back in the same accuracy. Benchrest
is normally in the shorter yardages but when you look at 1000 yard shooters, then the barrels are usually 26" or longer, the small added velocity gives them a long range edge, though slight. The Tactical/Sniper
308's will vary from 20-26", depending on manufactuter, etc. I just read
an article about the Ed Brown Marine Sniper in the new Guns Mag. & it is
24", so like I said before, it is far from cut and dried.

[Lone Star]

What about accuracy?  For a hunting rifle, what barrel length?

It doesn't matter really.  The quality of the barrel and chambering job are more important.  Silhouette shooters proved over 20 years ago that short barrels (10"-15") can be just as accurate as long ones.  Do not even consider potential accuracy in your choice of barrel length...if you want more accuracy, get a better quality barrel.

[nomosendero]

Lone Star

I agree 100% concerning the barrel quality, it is the most important part
of building an accurate rifle. Additionally, I view a rifle as a system with a
quality barrel, light & crisp trigger, a stable stock bedded properly, strong
& straight mount/rings & true receiver along with a quality scope & loads
working together to produce the desired results & deciplined practice for
the shooter!!

[Bearwolf31]

I have both calibers in 22" sporter barrels. The 270 gets the nod on being slightly more accurate than my 308. The great thing about them is that I use the same powder (44 & 45gr IMR4064) in both of them. Keeps it nice and simple that way.

[jro45]

I have this 30/06 and it was shooting with a 22" barrel 2860 fps [ 150gr bullet ] so in time I shot this barrel out and got another one put on. This new barrel is 26" long and I get 2960 fps with it. So with 4" more lenght
I get 100 more fps.

[Lawdog]

I have this 30/06 and it was shooting with a 22" barrel 2860 fps [ 150gr bullet ] so in time I shot this barrel out and got another one put on. This new barrel is 26" long and I get 2960 fps with it. So with 4" more lenght
I get 100 more fps.

That's about an average gain.  Lawdog