Barrel Length and Barrel Volume

[jtmoose]

How much does barrel volume effect barrel length and velocity variation? I was considering building a short barrel (<20") magnum. Everything I've read comments about barrel length affecting velocity but I assume that a larger bore would reduce the effects of a shorter barrel.

For example, a 28" .243 has a little less barrel volume than an 18" .308 barrel. I would assume that if a 26" barrel for a .257 Weatherby Magnum (same volume also) is adequate, an 18" 300wsm would burn similarly.

Am I missing something, is it only about length and not volume? Is this also why the larger calibers are "inherently more accurate"? Does their larger barrel volume burn more efficiently just like a fatter case does?

 

[FearNoWind]

I think you'll find that surface drag has more influence on the velocity than does volume.

 

[jtmoose]

At what length does surface drag over come charge pressure?

 

[Engineering101]

jtmoose

I don't know much about the subject but I have learned a couple of things which I'll mention. Larger calibers are inherently LESS accurate. Compare a 223 Rem to a 30-06. A good one of the former will beat a good one of the later.

A longer barrel in a given chambering will give higher muzzle velocity because it gives more time for the powder to burn. A short barrel magnum therefore would not take advantage of the fact it is a magnum as more of the powder would travel out the end of the barrel unburnt. If you use a faster burning powder in a short barreled magnum to account for the short barrel, you have to reduce the amount of powder or end up overpressure which also defeats the purpose of the magnum.

I suggest you get yourself a copy of QuickLoad and do some what ifs before you get too carried away.

 

[jtmoose]

Why is the 308 considered so accurate? I thought it might have to do with the caliber.

How much powder goes unburnt? Does a wsm not offer any velocity increase over a 308? Or, what length do they approach equal velocity?

 

[tbrice23]

jtmoose

I don't know much about the subject but I have learned a couple of things which I'll mention. Larger calibers are inherently LESS accurate. Compare a 223 Rem to a 30-06. A good one of the former will beat a good one of the later.

A longer barrel in a given chambering will give higher muzzle velocity because it gives more time for the powder to burn. A short barrel magnum therefore would not take advantage of the fact it is a magnum as more of the powder would travel out the end of the barrel unburnt. If you use a faster burning powder in a short barreled magnum to account for the short barrel, you have to reduce the amount of powder or end up overpressure which also defeats the purpose of the magnum.

I suggest you get yourself a copy of QuickLoad and do some what ifs before you get too carried away.

Nevermind

 

[elkaholic]

There are several factors that probably enter in as far as bore and efficiency, but in general, you are correct that a larger bore with the same case capacity as a smaller bore, will be more efficient. i.e. cutting 2 inches off of a 264 WM will lose a lot more velocity that the same off of a 338 WM, all else equal. As far as larger bores being more accurate, I think that has already been covered......Rich

 

[FearNoWind]

Why is the 308 considered so accurate? I thought it might have to do with the caliber.

The accuracy of any rifle is dependant on many factors. It's generally a combination of caliber, load, barrel rate of twist, barrel length relative to bullet surface (drag), etc. The .308 is not accurate simply because of it's caliber

How much powder goes unburnt? Does a wsm not offer any velocity increase over a 308? Or, what length do they approach equal velocity?

The amount of powder that remains (if any) would depend on the amount of powder contained in the load and how much of it was able to ignite in the firing process. The velocity of the WSM may or may not exceed the velocity of the .308 - it's all relative to the load and other factors previously listed.

I'd recommend you make an investment in this set of books.

http://www.amazon.com/Bryan-Litz/e/B00J216MMI/ref=sr_tc_2_0?qid=1454304469&sr=8-2-ent

 

[FEENIX]

The accuracy of any rifle is dependant on many factors. It's generally a combination of caliber, load, barrel rate of twist, barrel length relative to bullet surface (drag), etc. The .308 is not accurate simply because of it's caliber.

Well said! In addition, the biggest factor is the human factor ... the NUT behind the trigger.

I keep things simple and don't sweat it; efficiency and effectiveness of caliber/chambering X is realized when game X is successfully harvested humanely.

 

[MagnumManiac]

How much does barrel volume effect barrel length and velocity variation? I was considering building a short barrel (<20") magnum. Everything I've read comments about barrel length affecting velocity but I assume that a larger bore would reduce the effects of a shorter barrel.

For example, a 28" .243 has a little less barrel volume than an 18" .308 barrel. I would assume that if a 26" barrel for a .257 Weatherby Magnum (same volume also) is adequate, an 18" 300wsm would burn similarly.

Am I missing something, is it only about length and not volume? Is this also why the larger calibers are "inherently more accurate"? Does their larger barrel volume burn more efficiently just like a fatter case does?

What you are describing is expansion ratio, and bore cross sectional area.
Here's an example you should be able to see in any loading manual.
We have 3 chamberings using the same 30-06 parent case; 30-06, 338-06 and 35 Whelen.
As bore sectional area increases, if we take the 30-06 as the start point with a 200gr bullet at 2680fps in a 24" barrel, the 338-06 shoots the same bullet weight faster by 100fps and the 35 Whelen shoots the same weight just over 100fps faster again.
The reason for this is simple, the greater area there is on the base of the bullet, the more area there is to push on, the same reason big cubic inch engines, or those with large cylinders and short strokes, produce more torque.
The reason longer barrels produce higher velocity is due to the extra area the gas can expand into.

Cheers.
gun)

 

[jtmoose]

Do I understand it correctly that:

1.) Using a heavier bullet keeps the powder burning in the barrel longer because the bullet doesn't get out of the way as fast.

2.) A fast powder with too heavy a bullet will overpressure because it cannot expand the chamber far enough fast enough.

3.) Slow powder would do better with a longer barrel and heavier projectile to give it enough burn time.

4.) A fast powder would be better with a lighter projectile and a barrel length providing the means to a long enough burn time.