Jaimie,
The biggest pressure change with bullet contact that I've seen measured is this one from RSI (copied here with permission from Jim Ristow):
Quote:
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Originally Posted by RSI
Here's a classic good load for 6PPC. The only difference between all these traces is the first 3 were seated right on the lands. The last 4 were 30 thousands of an inch off the lands. Note the substantially reduced pressure and overall reduction in energy.
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That's about 20% change. It will correspond to about 7% to 10% difference in powder charge depending on the load and rifle. I like to use the tried and true 10% reduction normally associated with load workups when starting with a bullet in the lands. Still another option is soft-seating. Mid Tompkins told a class I took that he relies on this method. The neck is sized so that it holds the bullet, but you can still move it with your fingers. It is seated out long, then loaded singly and closing the bold then finishes seating the bullets. The only drawback to this loading method occurs if you have to eject an unfired round for any reason. Often the bullet sticks in the throat and is pulled out as you extract the case, so you have to be careful to be muzzle-up and to capture the case to avoid spilling powder all throughout your action and not forget to use a cleaning rod to knock the bullet out before your try to chamber another. Obviously that knocks this method out at ranges that don't allow muzzles to point into the air for reasons of fall-zone hazard.
The Berger recommendation is for their secant ogive bullets. Tangent ogive bullets are more forgiving of seating depth variation. This is the reason for the newer Berger Hybrid design, which has a tangent ogive where the ogive meets the bullet bearing surface, that switches over to secant profile as the ogive starts to narrow.
Whether or not bullet tilt will matter to your gun much is yet another variable. Some guns, especially those with tight match chambers, do not seem to be as adversely impacted by bullet tilt or exact jump to the lands as looser commercial and military chambers are. Tight bores can straighten them further. I put up a diagram of measured results of bullet tilt in another thread for a tight benchrest chamber verses a military '03 NM chamber. It represents tilted bullets with the high sides indexed at 90° intervals around the chamber. I'll put it below as an FYI. Note that if you measure your high sides and orient them all to the same direction in the chamber, you cut the total error in half.
If you are interested in the effect of bullet tilt on your gun, I would copy Harold Vaughn's method and intentionally bend about 0.004" of runout into some loaded rounds by using a neck-size hole in your bench or other convenient place. Abbatiello found that with the .30-06 NM loads, that was about as much tilt as made any difference. Greater tilt than 0.0040"-0.0045" had no additional effect on group size. The bore seemed to straighten any additional tilt out, so it didn't stay with the bullet through the bore.
As you've observed, many long-seated rounds are strictly for single-loading an not for magazine feeding.
As to chronographs an pressure, there are indirect ways to estimate pressure from a chronograph reading, but nothing other than direct measurement will give you a certain peak value. The average pressure behind the bullet during its time in the barrel is directly proportional to the kinetic energy of the bullet at the muzzle, which is proportional to the square of the bullet velocity. However, average pressure is determined by a combination of peak pressure and the much lower muzzle pressure. Where muzzle pressure tends to vary close to directly with powder charge, peak pressure varies exponentially with powder charge, so it is not only higher by what is typically around a factor of five or six, but any change in velocity will reflect a greater change in peak pressure than in muzzle pressure. How much greater depends on where you are in the exponential curve.
I can give you formulas for working that out from your velocity plus input from published measured pressure and velocity data for the same powder, but it's a bit of a trial. I have an incomplete Excel file for doing this. Remind me by PM in a couple of weeks and I'll try to finish it.
Meanwhile, the main value of the chronograph to the average shooter, as already mentioned, are proving the velocity consistency of your load for longer ranges. You can also use it to make charge adjustments needed to return to your original development load velocity when you get a different powder lot, change cases or primers, or get a different make of bullets that are the same weight. You can also check the effects of changing temperature conditions, barrel warming, etcetera.