Rifle accuracy is governed by barrel vibrations and the long and short of it is if you can't get your barrel vibrations "tuned" to the sweet spot of barrel harmonics you will not shoot tight groups. Things as simple as not having the correct torque on your trigger guard screws can screw up your accuracy when tuning your hand loads.
Did you ever noticed the steps cut in a military Mauser barrel, the steps are at the vibration node points of the barrel and used to control barrel vibrations.
(don't try putting Viagra in with your powder it doesn't work)
Ball powders are harder to ignite due to the deterrent coatings used, they are double base powders and these coating are used to reduce the peak flame temperature and they sometimes need a hotter spark to get them burning.
"Sometimes" the seating depth has a effect BUT in most cases your magazine is the limiting factor in bullet seating length unless you are single feeding your cartridges.
I would try different powders and find one that shoots well and then try and fine tune your load from there with seating depths etc.
The groups below were an accident, I was fire forming new cases with the bullets jambed hard into the rifling and varying the loads slightly just checking for pressure signs on the primers on my bolt action .223.
Bottom line, sometimes you work your tail off looking for the best load and sometimes it just falls in your lap when your goofing off.