I had the same problem and I've got some decent glass. I struggled to consistently group within a 15 killzone at 10m - this was prone, rested on a beanbag (not in a comp btw).
The problem for me was parallax error. I put a scope enhancer on my scope and was putting pellet on pellet straight away - oddly enough, slightly left of the centre of the killzone? (I have a theory about that now, but didn't have a clue about it at the time).
Cheers, hope you enjoyed the test, I thought it would be fun to have a play with some code to display a zoomed in sight picture on a photograph. The message I guess I'm trying to convey with this test is that bracketing can be extremely accurate, but... It's not easy to do, it's a skill you'd need to learn and practice.
Mildot's are extremely handy for holdover, and windage too - it's not all about bracketing.
The 30/30 ret on the other hand was designed specifically for bracketing (a deer's shoulder).