Just as swampratt said, just a flattish / small top hat primer is not necessarily a sign of over pressure. if you are pushing your shoulders back a little to much, this will also happen. i usually go off bolt or ejector marks.
I have not been as lucky to find a larger spread in accuracy...
it sounds like for what you are doing a balance beam or the frankford would be fine. you mainly need one to get your powder measure dialed in and then for occasional sanity checks... I used the frankford for a while to weigh bullets. it seemed pretty accurate. it always registered...
i run 325 Hornady FTX with lil gun, starline brass, and Winchester large pistol primers. started at 26 and I think i am up t0 29.3 or 29.6 with no pressure signs. I have the wilson combat upper/barrel..
I think 27.5-28.5gr would shoot 5 shot groups into one (BIG) ragged hole at 100. I think...