OAL and Pressure

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zimmineee

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Ok, I'm on my information gathering quest before I take the step to reloading. I have always understood that the shorter the OAL (the deeper the bullet is seated in the cartridge) the higher the pressures? Am I right with this assumption? The only reason I ask is I just read an article on a webpage that seems to state the opposite. They state that increasing the OAL increases chamber pressure? I'm a little confused? Here is the link:


http://www.larrywillis.com/OAL.html

:confused:
 
If you get very close to the lands the pressure curve is different than if the bullet has a bit of a running start so to speak.

This analogy from the web site you referenced explains the phenomenon well: "To understand why chamber pressure is increased, think about this example of momentum. A car can easily drive over a curb with a little running start; but when you are parked right up against the same curb, and you step on the gas - it requires a lot more force to start moving. "
 
Bet bet is to stick with at least the minimum OAL's in the manual. Then you have nothing to worry about.
The distance the bullet has to move forward to the rifling lands affects maximum pressure. A big jump to the lands brings pressure down, a bullet jammed into the lands causes pressure spikes. This is influenced by bullet design, OAL, and chamber design/reaming.
 
There is absolutely no reason to ever put a hunting load together so the bullet touches the lands or is into them a bit. This is used by bench rest and long range shooters on occasion when it yields the best results, accuracy-wise. Pressures are elevated because it is harder to "engrave" the bullet under these conditions. Normal hunting loads should be off the lands a bit to avoid unpleasant occurrances (extract loaded round, bullet stays in throat, powder spills all over in action!) If they are off the lands even .010" or .015", the pressure will drop down considerably. Magazine length comes into consideration quite often, as well, with many magazines being too short to allow any semblance of close proximity to the lands. If the bullet has a fair distance to jump to the lands, as Slash5 said, it tends to lower pressures. Regards, Eagleye
 
one of my manuals states that it is very dangerous to seat 9mm bullets any shorter than the minimum COL " as this will increase pressure to dangerous levels".
both ends of the spectrum... too short and too long are both considered bad.
 
One way to think of it is that freebore acts very much like more room in the cartridge case. Pressure climbs when bullet first gets to the lands.
 
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