Yes you can better your accuracy by adjusting type/charge weight of powder and with your bullet seating depth. Adjust one factor until you see an improvement in accuracy then adjust another factor until the accuracy improves. These two adjustments will give you the most results. Trying different brass and primers will yield minimal improvements.

Also, belted magnums can be tricky when setting up your sizing dies. If you set the shoulder back to much when sizing, you can have case head separation. I've seen someone blow out the cases on the second loading because he set the shoulder back too much. I tried to explain this to him but he did not seem to comprehend the concept of headspace.