Per rimfire, the 0.002" effect on the powder space is good for about 1 fps. But the other thing it affects is the gas bypass around the bullet, which QL and GRT have no model to account for. In general, for gas bypass, the closer you are to the lands, the bigger the percent effect a small variation will have.
Bart is correct that the shoulder-to-bullet-throat-contact-point on the ogive is what actually determines bullet jump. The firing pin drives the case forward so its shoulder stops on the shoulder of the chamber, then the bullet is in its final position. If the cartridge does not have a crimped primer, the primer will drive it forward, too. Someone firing primed empty cases found just firing primers would set his case shoulders back a couple of thousandths, but I forget the details. You can try the experiment yourself. If you can measure the effect, you probably want to seat to allow for it, and you will want to use cases with the same load history.
I built a gauge for measuring this long ago, but about three years later Redding came out with their Instant Indicator which does this and will do a couple of other things as well. Just choose the neck bushing for the Ogive at about 0.001-0.002" under bullet OD for secant ogive bullets and use a bore diameter insert for tangent ogive bullets and you will have the point of throat contact registered in most instances. The Instant Indicator is cartridge-specific to get the shoulder contact right, so be prepared to shell out some money if you are interested in using one for more than one chambering.
You can also make that measurement with the Hornady tool, first measuring the head-to-shoulder dimension with a case comparator insert, then making the head to bullet ogive measurement, noting both and subtracting the one from the other, and using this difference as the basis for matching your loaded rounds. You can calibrate for each insert by zeroing it with the jaws closed on it before you put a cartridge in. However, I recommend you replace the Hornady bullet comparator insert with one of the Sinclair stainless steel inserts, as the latter are cut to mimic a throat and contact the bullet further down and at a more representative location than the aluminum Hornady insert does.
All the things Bart outlined about primers are important, but I will also mention seating them to reconsolidate about 0.003" deeper than where the anvil touches down in the bottom of the primer pocket. This is very hard seating. It pushes the anvil that far into the mix (what Allan Jones calls "setting the bridge", meaning the quantity of priming mix bridging the gap between the tip of the anvil and the inside bottom of the primer cup). Old McDonnell-Douglas reports for the military on primers for firearms and pyrotechnics (flares, ejector seat charges, etc.) indicated 0.002" to 0.006" of reconsolidation is best, but a later version changes that to 0.002" to 0.004" based on testing by Naval Ordnance at Indian Head. This all comports well with Dan Hackett's account of the importance of it, though he didn't go there by measuring, rather he just seated primers hard:
"There is some debate about how deeply primers should be seated. I don’t pretend to have all the answers about this, but I have experimented with seating primers to different depths and seeing what happens on the chronograph and target paper, and so far I’ve obtained my best results seating them hard, pushing them in past the point where the anvil can be felt hitting the bottom of the pocket. Doing this, I can almost always get velocity standard deviations of less than 10 feet per second, even with magnum cartridges and long-bodied standards on the ’06 case, and I haven’t been able to accomplish that seating primers to lesser depths."
Dan Hackett
Precision Shooting Reloading Guide, Precision Shooting Inc., Pub. (R.I.P.), Manchester, CT, 1995, p. 271.
I will add that with some powders (particularly spherical) I have found primer flash hole deburring also to be a help, so if you aren't using brass by Lapua, or Norma, or ADG, or one of the other premium brass makers who are careful to avoid leaving burrs, I would go after the burrs.
