[Unclenick]

It's complicated. In a rifle, getting a jacketed bullet close to the lands often sees pressure rise 20% or so, and getting it seated too deep also raises pressure by using up too much powder space, so there is a pressure minimum inbetween. A handgun shooting lead bullets is a little different. The lubricated lead bullets slip out of the cases easily. So while seating too deeply can still raise pressure theoretically, in many short case handgun cartridges the bullets are unseated by primers before the powder pressure really begins to build. Especially with lead. This means your control over pressure by seating depth is less reliable than in the rifle, as the gun may be building pressure with the bullet out in the throat somewhere, regardless of where you seat it. However, it's unsafe to count on that happening, so I advise not seating shorter than the manual says.

As to seating lead bullets long, they don't requires as much force to be push into the rifling as jacketed bullets do, so I've never seen a pressure issue arise from seating them into a throat. The thing you gain is that the bullet starts into the throat straight. The harder jacketed bullets seem to straighten themselves up on their way into the bore under pressure, but with lead bullets, if they start into the throat at a slight angle under pressure, they just swage into the bore at that same angle, making their mass slightly eccentric. The slower speed of chambering doesn't seem to do the same damage primer and propellant pressures do in this regard.

That is what causes the accuracy issue. It's especially bad in guns whose fit is so loose that the chambers are long enough that a standard length case can't reach the end of the chamber before the rim is stopped by the extractor hook. This is called headspacing on the extractor, and it is really common. One gunsmith said he estimated 70% of 1911's he sees are doing this. In this situation the gun fires with the cartridge pivoting on the extractor hook over to the hook side of the chamber. It doesn't seem to bother jacketed bullets, but leading and inaccuracy happen when lead bullet loads are handled this way.