Re: Seating primers
I too have pointed out the problem of placing a stop on a priming device... even when using uniformed primerpockets, due to the fact that the measurements we are controlling in most cases is the face of the primer to the front of the rim, not the critical dimension, anvil to bottom of the primer pocket. I think feel is the way to go when seating primers due simply to the variances in primer height, primer pocket depth (if not uniformed) and rim thicknesses. However, I firmly believe that you should gauge primer pocket expansion with a measuring tool, not a primer.
As to consistancy of primer seating tension mattering to a load... When I develop a load, I load 3-5 cases to neck splits, untill I see that there is too much primerpocket expansion at a given level, or I decide that the load will be mild enough on brass to last a long time AKA I get bored. I do this for several reasons. It lets me see if the load likes FL sizing or neck sizing, I can see how much the brass grows through firing and sizing, I can get a feel for what pressure im at by primer pocket expansion, and what the pressure is doing to the brass in general. In doing this, I can relate both precision, and POI of the load with both used brass, and new. At different times, and different loads in different rifles, ive seen some that don't change at all, some that shoot better with many times fired brass, and some that are at their best the first go round. That being said, ive batched some of the many fired cases with my regular brass after they survived enough loadings in instances that the poi stays the same, and ive never notticed that tight primer pockets shoot to a different POI than fresh brass. Thats kinda a round-about way to answer your question, but no, I haven't found it to matter. Seat the primer to the bottom of the pocket, measure expansion on a few cases each time you load a batch, and be happy!