Greetings,
1/2" bars with Hornady bushings work great. Unfortunately, the range of the bushings is limited and not very useful for precision bullseye shooting.
I have been machining bushings for the Star "O", "OO", and 1/2" bars using the volume method presented by Richard Lee. The bushings are marked with the volume in cubic centimeters (cc) and with the aid of a speadsheet you can quickly select the bushing you need.
Attached are some PDF files of the "O", "OO", and Pacific/Hornady (1/2") bushings.
I can supply the original excel files so you can customize them to your powder lots.