Did you ever study gas laws in high school chemistry or physics? If you did you might remember PVT - Pressure Volume Temperature. This is not a guess or a perception, it is a basic law of nature. There is no more proof necessary if you understand basic physics.
Reducing the volume without reducing mass = more pressure. Period.
Seating a bullet deeper reduces volume. Burning powder produces gas, which is the mass. The same mass in a reduced volume = more pressure.
Look at 45acp load data. Did you ever notice that sometimes different bullets of the same weight can have different start and max loads of the same powder? The reason is that a different profile bullet may be seated deeper in the case, which reduces volume and increases pressure. The load must be adjusted to compensate and prevent overpressure.
I would like to see your source that says seating a bullet deeper will have no effect on pressure.
About regurgitation - the worst is trying to spit out your own feet.
In a bottle neck cartridge, seating depth has almost no effect on pressure, unless the bullet is seated in the lands. The OP was farther off the lands than he intended. If "long" would have put him into the rifling, then he reduced pressure. If both settings were off the lands, then it made very little difference.
A pistol case is filled with very fast powder. case volume matters a lot, because the powder burns so quickly.
A rifle case has slow powder (and the 339 Lap would have been slooow powder) The burning takes place over a couple of inches. The first pressure is from the primer. In a full case this will generate about 10,000 PSI. You can see this little event on the transducer pressure curve.
Then the powder ignites, pressure builds and at around 20,000 psi the neck expands and releases the bullet. If the bullet is jammed in the rifling, it does not move much until the pressure builds higher. If the bullet is off the rifling, it moves and keeps going when it hits the rifling.
The more it is off the rifling, the faster it is going when it hits rifling and the less impact the event has on pressure.
I have fired 1000's of rounds in the lab, using a pressure gun. Each shot records pressure and velocity. Not just peak, but the entire event. The events are recorded on the computer and can be printed out.
This curve is a 308 Win. 155 gr Sierra MK bullet and about 45 gr of 4895. Note how vertical the pressure curve is. Our discussion is about what happens in that brief period.
If the bullet is off the lands, it offers less resistance to the pressure build up. If it is jammed in the rifling, it is a pipe bomb until something lets go (the bullet).
In a pistol case of fast powder, the curve is even more vertical, so the case volume at the instant of firing has more significance.
OP. Your posting raises two interesting issues:
Seat depth and pressure. This is only an issue when loaded to max pressure. If you started with the START load (as you should) you can start with any OAL that you want.
The load data in the book, including the OAL, refers to YOUR rifle. It is a guide to developing a load for your rifle. It is not a recipe book. Some rifles are very different than others. The amount of throat in the chamber varies a lot from rifle to rifle and has a huge impact on peak pressure.
When you start loading for a new rifle, the first thing you do is load a round very long, chamber it, and see how much rifling mark there is, and then seat it deeper and deeper until it no longer hits the rifling. then make a note of that length. (This length will vary by bullet make and weight) I load my first batch of ammo at that length +20 thou. I load 5 of each powder charge, starting with the START load, and in 0.3 or 0.5 increments, all the way to the book Max. These are shot to see what seems to group best. I am prepared to bring some ammo home, if I run into pressure.
The next time I go out, I will try the best load and plus and minus 0.3 gr, to fine tune, and I will have the same test loaded with the bullets into the rifling. I usually find off the rifling works best, but sometimes a bullet will like to be jammed.
