Effect of COL?

[menotomyhunter]

What happens to chamber pressure as a cartridge's overall length (COL) is varied from the COL recommended by the bullet's manufacturer, taken as a minimum, up to the length that touches the lands? I know the traditional body of knowledge says that the pressure jumps significantly when the bullet is set to touch the lands. I believe it and don't do it, but what happens for COLs between the two extremes? How is pressure affected?

Take as an example, I load my .308Win NEF Survivor with 150gr Remington CoreLokt PSPs. The SAAMI max. recommended length for .308Win is 2.810 inches. Typical reloading manuals place the COL around 2.700 inches depending on the exact bullet configuration. In my Survivor, I can seat this bullet out to touch the lands at 2.950 inches. FYI, I don't shoot at this COL.

As an aside, I've never seen an electronic recording of the chamber pressure versus time from the instant a rifle trigger is pulled until the bullet exits the barrel. I have seen representations of such graphs by writers, but not the real thing. I have also heard that a bullet stops one or more times before it exits the barrel! Anyone know anything about this?

Thanks,

menotomyhunter

[PaulS]

What happens to chamber pressure as a cartridge's overall length (COL) is varied from the COL recommended by the bullet's manufacturer, taken as a minimum, up to the length that touches the lands? I know the traditional body of knowledge says that the pressure jumps significantly when the bullet is set to touch the lands. I believe it and don't do it, but what happens for COLs between the two extremes? How is pressure affected?

Take as an example, I load my .308Win NEF Survivor with 150gr Remington CoreLokt PSPs. The SAAMI max. recommended length for .308Win is 2.810 inches. Typical reloading manuals place the COL around 2.700 inches depending on the exact bullet configuration. In my Survivor, I can seat this bullet out to touch the lands at 2.950 inches. FYI, I don't shoot at this COL.

As an aside, I've never seen an electronic recording of the chamber pressure versus time from the instant a rifle trigger is pulled until the bullet exits the barrel. I have seen representations of such graphs by writers, but not the real thing. I have also heard that a bullet stops one or more times before it exits the barrel! Anyone know anything about this?

Thanks,

menotomyhunter

menotomyhunter,
The more airspace that exists in a cartridge when loaded the lower the pressure will be (rule of thumb). When the bullet has more resistance to its forward travel the pressure in the chamber rises to overcome the resistance. If you provide the resistance to the bullet by seating it on the lands then it must generate enough pressure to overcome that so the pressure goes up. How much depends on the depth of the rifling, the angle and finish of the taper into the rifling. Each chamber is different and so the actual pressure effects will be different.
Very few shooters will ever see the actual trace of a chamber pressure test. The printouts are a bit more common and copies of those printouts can be found in many articles about the subject. Be assured that once the bullet starts on its trip down the barrel if it should stop you are going to have a problem with putting the parts of that barrel back together. The bullet never stops as it travels down the barrel after being fired with a loaded case behind it. if you fire a bullet with only a primer it will stop after entering the bore but if you have loaded the case properly it only accelerates as it goes down the barrel.

[fishdog52]

Weatherby got away with some hot loads for years by liberal use of freebore, which is another way of looking at your concept.  The trick is to balance the effect of this between accuracy and impact on your pressure curve.
Starting your bullet in contact with the lands will generally shorten the curve, and increase the pressure, may, or may not be a bad thing, but needs to be recognized.  Most of the benchrest people try to keep bullet jump to a minimum.
Weatherby also had a habit of using very strong actions in his rifles to help keep us out of trouble.
My limit on my hunting guns has generally been the magazine, with the chamber typically able to handle a longer COL.