Home Forums Hunting & Shooting Ask The Gunwriters Berger Bullets OAL vs Pressure.

OAL vs Chamber pressure in a Bottle Necked rifle round has been discussed here many times. The general consensus is that the longer the OAL the higher the pressure and the shorter the OAL the less the pressure.

Is there something special or different with the Berger bullets that would cause the opposite to be true as far as pressure and OAL is concerned?

I found this on Berger's web site and it is in complete contrast to the above OAL vs Pressure relationship.

Is it their bullets that make the difference or are they just "wrong" in general.

The primary effect of loading a cartridge long is that it leaves more internal volume inside the cartridge. This extra internal volume has a well known effect; for a given powder charge, there will be less pressure and less velocity produced because of the extra empty space. Another way to look at this is you have to use more powder to achieve the same pressure and velocity when the bullet is seated out long. In fact, the extra powder you can add to a cartridge with the bullet seated long will allow you to achieve greater velocity at the same pressure than a cartridge with a bullet seated short.
When you think about it, it makes good sense. After all, when you seat the bullet out longer and leave more internal case volume for powder, you�re effectively making the cartridge into a bigger cartridge by increasing the size of the combustion chamber. Figure 1 illustrates the extra volume that�s available for powder when the bullet is seated out long.

http://www.bergerbullets.com/effect...and-cartridge-base-to-ogive-cbto-part-1/

I'm not sure anybody said the longer the OAL the higher the pressure. Just possibly somebody said the closer to the lands the higher the pressure; the more the running start - prime example freebore as in Weatherby rifles or 9x23 pistols (with SAAMI spec chambers) - the lower the pressure all else equal or Ceteris paribus or near enough.

I sure wouldn't want a lot of bullet setback in anything from .357 Sig pistol to say a a gas gun that needed more neck tension. See e.g. Brownell's book on Pressure Factors as published by Wolfe when they did longer pieces and reprinted since for a discussion of pressure factors some acting to increase pressure and some acting to reduce pressure.

It is certainly true that from before the days of Colonel Cooper's Lion Scout
"Lion Scout" was a 350 Rem Mag built on a ZKK to allow longer loading than the Remington Model 600 Super Scout. And lo these many years there have been a myriad of 7x57 laments about how a short action requires short seating and so handicaps the cartridge and a long action is longer than the 7x57 needs. There are many examples of people using an '06 length action or otherwise long throating a 7X57 to pickup a very little of extra internal volume to add a smidgen of powder.

In any event Berger covers the matter at the location cited.


Given the 4:1 rule for extra powder I'm not inclined to think a slight extra capacity matters nor am I inclined to long throat anything but a Weatherby to increase capacity but some people do. If I didn't like the 7X57 in a short action I'd go anything from 7mm-08 to whatever and .280 in an '06 length action.

Just possibly a given Berger bullet or another make of extra long extra streamlined bullet will have a greater OAL but it's a long sharp nose extending down the barrel and the bullet may not carry full diameter so far forward as less streamlined bullets and so the extra streamlined bullet can have a greater OAL and still have a run to the rifling. Lots of possibilities bound up in making things otherwise equal or not.

Typically, a Berger of the same weight will have a shorter bearing surface so there is a little bit of built in reprieve from pressure there.

Are these not the same thing? If I load longer am I not also closer to the lands?

A couple years ago I asked this of the Gunwriters.
does reducing OAL in a bottle-necked rifle round increase pressure, decrease pressure or have little or no affect?

This is the reply posted by Mule Deer.

It decreases peak pressure, for two reasons. The longer "jump" of the bullet to the rifling results in a lower peak pressure, since the bullet engraves more easily the faster it's going when it hits the rifling.

Also involved is the "progressive" burning of almost all modern rifle powders. This means the pressure increases relatively slowly from the time of ignition. Thus peak pressure occurs when the bullet beyond the barrel throat, with very slow-burning powders as much as 3-4 inches.

Handgun powders are much faster-burning, and even regressive, meaning pressure peaks when they're first ignited. Thus seating them deeper allows more time for initial pressure to build.

This effect is slightly exaggerated in revolvers. The initial, quick pressure rise is relieved somewhat when the bullet passes the cylinder gap, allowing some gas to escape. When bullets are seated deeper it takes them longer to pass the cylinder gap, giving pressure more time to rise.

http://www.24hourcampfire.com/ubbthreads/ubbthreads.php/topics/5474598/1

His comments and these charts contradict what Berger is saying about OAL, case capacity and pressure.





Then there is this on Absolute Chamber Pressure in Center Fire Rifles.

http://deepblue.lib.umich.edu/bitstream/handle/2027.42/3866/bac6873.0001.001.pdf?sequence=5

Nothing there that seems inaccurate near as I can tell.


I think this is what they are talking about....(?).Let's say for example that you start out with a 168 gr Berger ( or 160 AB,whatever) in a 7 Rem Mag Rem 700 with bullets seated to 3.290. You max out at 2900-2950 fps velocity.

But your magazine will allow a longer OAL. You go in and extend the throat so that bullets can now be seated to about 3.5"+ . Your bullet seating in relation to the lands is the same but you have created a bit of space in the case by seating the bullets out longer,so can increase charges a bit and pick up a bit more velocity and you now find you max out at 3050 or so with the same bullet.

The rule about seating long vs seating short still applies but you have changed the OAL by extending the throat forward.If you seat deeper, you will drop pressure, and increase pressure as you seat closer and further from the lands,same as with standard 3.290 OAL, but you have now changed the length of the throat, essentially adding a bit of case capacity.

I have seen this many times with the 7 Rem Mag and the 300 Win Mag by installing a H&H-length box,extending he throat, and seating bullets to 3.5"+.

Like Dennis said the Bergers have less bearing surface, but the same principal will apply with a 160 gr AB, Partition or any other,so it is not a situation created by anything to do with the Berger in particular although you might find the Berger take s a slightly higher max charge.

From there what you get for velocity depends on the rest of the barrel and may vary even as much from rifle to rifle but that's another subject.

I don't think so, I think they are talking about differing OAL in the same rifle with the same chamber.

http://www.bergerbullets.com/effect...and-cartridge-base-to-ogive-cbto-part-1/

So was I. They are not wrong.Maybe Litz explains it better.

Here is a quote taken from the article:


"The primary effect of loading a cartridge long is that it leaves more internal volume inside the cartridge. This extra internal volume has a well known effect; for a given powder charge, there will be less pressure and less velocity produced because of the extra empty space. Another way to look at this is you have to use more powder to achieve the same pressure and velocity when the bullet is seated out long. In fact, the extra powder you can add to a cartridge with the bullet seated long will allow you to achieve greater velocity at the same pressure than a cartridge with a bullet seated short...".


The illustration of the two cartridges with different bullet seating explains it better than words.


Litz goes on to say that sticking to SAAMI OAL can inhibit performance sometimes,something recognized by guys like John Wooters and Bob Hagel years ago using heavy bullets in magnum cartridges. Maybe even more applicable today with the very long, high BC bullets that allow guys to reach really far.He is explaining one way to optimize cartridge performance.

From Mule Deer.

It decreases peak pressure, for two reasons. The longer "jump" of the bullet to the rifling results in a lower peak pressure, since the bullet engraves more easily the faster it's going when it hits the rifling.

Also involved is the "progressive" burning of almost all modern rifle powders. This means the pressure increases relatively slowly from the time of ignition. Thus peak pressure occurs when the bullet beyond the barrel throat, with very slow-burning powders as much as 3-4 inches.

Handgun powders are much faster-burning, and even regressive, meaning pressure peaks when they're first ignited. Thus seating them deeper allows more time for initial pressure to build.

This effect is slightly exaggerated in revolvers. The initial, quick pressure rise is relieved somewhat when the bullet passes the cylinder gap, allowing some gas to escape. When bullets are seated deeper it takes them longer to pass the cylinder gap, giving pressure more time to rise.

Steve what is your point? That Brian Litz is wrong? I am not sure I understand.

He and John are saying the same thing.With a given load, the longer the jump to the lands, the lower the pressures.The closer to the lands, the higher the pressure.

What Litz is saying, is that you can optimize the velocity with long, heavy bullets if you abandon the SAAMI spec throat, and throat the rifle so that it will accept bullets seated further out of the case. This, in essence creates a bit more case capacity ,and allows you to increase the powder charge a bit to optimize your velocity at the same pressure level.

It's really pretty simple and has been known a long time.

Rather than read about it, buy a rifle and try it out yourself by working with a standard throat, long and short seatin; then extend the throat and see what the results are

The confusion here is that the two discussion points; longer OAL and further from the lands can't happen simultaneously in the same rifle. When working up loads in a given rifle you are trading one for the other. Which one is more influential I do not know.