Effects 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 touches the lands at firing. I believe it and don't do it, but what happens for COLs in between the two extremes?

Take as an example, I load my .308Win 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 touching the lands at 2.950 inches. FYI, I don't shoot at this COL.

I hope these questions return some informative answers and possibly recommendations for websites that treat this subject fully.

Thanks,

menotomyhunter

[Dillohide]

I accidently loaded spire points in my 30-30 NEF the first time that touched the lands ... loading the spire point to the crimp colar like a normal 30-30 bullet puts it against the lands. I didn't notice undo pressure signs but accuracy was bad. Pushing the bullet in past the crimp colar and off the lands improved accuracy tremendously. Unfortunately there is not a lot of data for spire points in the 30-30 so that mistake is easy enough to make.

[trotterlg]

As the OAL increases the chamber pressure decreases, until you are very close to the lands where it will increase.  More room in the case = less pressure (unless you take it to an extreme and load a very reduced load).  Larry

[menotomyhunter]

trotterlg

Is .020" off the lands OK to avoid the higher pressure of touching the lands? Or put another way, how close is close enough to avoid the high pressure condition?

mh

[Paul5388]

MH,

Putting the bullet into the rifling is like using an extra firm crimp.  It retards bullet movement, which raises pressure.  .020" off the rifling should be more than sufficient to avoid a pressure increase.

[pascalp]

Using in-land in spite of crimp or neck tension to get a consistent pressure shot after shot is good and safe IF the load is specifically and carefully built.

I use cast bullet, and usually try to built my reloads with bullet kissing rifling. I try to have both; bullet in-land and at least "1 caliber" length in the neck.
Obviously lead bullet doesn't offer the same resitance than FMJ when engraving.