http://www.longrangehunting.com/forums/f28/0-008-off-versus-into-lands-162-amax-55730/
The above Thread contains my single experience wherein bullet seating depth caused a dramatic change in group size. Note that I did increase the powder charge slightly in the load with the bullet seated off the lands in the effort to maintain MV consistent with the load that had the bullet jammed into the lands. This was a 28" long lightweight barrel. Fact is, all of my rifles have lighter contour barrels than the majority of the rifles I've observed on display and discussed on this Forum. Simply because my hunting is backpack style - largely due to the terrain where the game is located and the utter lack of roads in Alaska. So I hike considerable distances in and back out.
Are the majority of your rifles in the 11 lb and heavier class (bare rifle - no scope, rings, bipod, etc.)? My primary hunting rifles are between 6 3/8 lb and 8 3/4 lb. I have a couple in the 10 lb range that rarely get hauled out on the backpack hunts. I'm convinced extra load development work is required to get these lighter profile barrels to shoot to their potential. More barrel movement requires honing in on a very stable/steady point within the barrel's motion. When I read of other's load development processes and efforts - or the comparative lack thereof - that quickly yield good accuracy, they are most often shooting rifles with a minimum bare weight exceeding 11 lbs.
So I toss out the barrel weight/contour idea, wondering if that could explain some of the difference in our experiences with bullet seating depth and its affect on accuracy. I'm not at all convinced that the bullet seating depth-caused affect on accuracy is independent of the MV changes caused by the differing seating depths. I have a firm suspicion that modified seating depths and powder charge weights both affecting group size in that common manner - by altering MV.
The way to test this out would be to maintain constant muzzle velocity while varying seating depths, to see if the varied seating depths then result in significant changes in group size. Powder charges would have to be tweaked and all loads chronographed. But I don't have the ammo, money, time, or barrel life to launch this experiment. Let alone that burning desire. So I use methods that seem to work in both a cost & time-effective manner - with my rifles. That's powder selection and powder charge weight, prior to seating depths. Primer brand, and magnum LR versus standard LR has been hit or miss with me. I can't say that I've seen any consistent accuracy affecting trends. I have seen some affects on ES & SD while testing over tandem and triplicate chronograph setups, but all in all it seems to be very rifle and cartridge specific, IME.
My rifles have generally been unique when it comes to load development. What worked with one has no greater than 50/50 odds of working on another. The most one can do is employ some systematic method of load development, keep a good record of the resultant flow of data, and go in the direction of improvement in group sizes. I find it relatively easy to identify and discard some load combinations that are a lost cause, and not deserving any further time and effort. I find it harder to figure out which components and combinations have the best potential to produce an acceptable and satisfying end result. Long range accuracy, while maintaining acceptably low ES & SD, and acceptably low MV variation with temperature swings. It's pretty darn satisfying when it all comes together.
Perhaps other members will provide their experiences, observations, and conclusions.