I've lately wondered about the relationship between bullet seating depth and accuracy. I've always started seating my rounds 0.015" from the rifling with 5 round each of max book load, and one grain down each for two more sets of 5. If I didn't get less than 1" groups I changed to 0.01" and 0.02" and start all over again. I have not yet experienced a pressure problem even though I'm not starting at the traditional 10% below max. I suspect it is because the bullets are seated much further out than the manuals provide in their data. I have a .270, for example, that easily shoots less than 1" groups with 57 grains of H414, while the latest Hogdon book lists max at 53.5 gr. The bullet is a Hornady 130gr SST and the cannelure looks like it is WAY above the mouth of the case.
My wondering is: is seating depth close to rifling the accuracy factor or is it more simply just a pressure factor based on seating depth? For example, if I seat that 130gr bullet 0.2" (what I can find in factory rounds) and reduce the powder charge to book max or less, will I get the same velocity (and pressure) and accuracy as when I have the bullet seated with much more powder and a shallow seating depth?