Hi Wayne.
I used both .5% and 1% antimony cores in my .30's with my preference being 1%. With the proper core seating punch, both would produce a perfectly concentric bleed by ring of .005-.006 with
no core flashing up the walls.
Core build up on the punch(es) was minimal with both the .5% and 1%....the 1% was noticeably less, however. A couple of swipes with some 0000 steel wool is all it took to clean the punch.
When I made my own cores, 1% was my preference. When I started getting them from Charlie Hood, he only did the .5% cores, so that's where I ended up. I'd tested my own cores with .5% and 1% so I knew there was no difference in accuracy so making the switch to Charlie's cores was an easy transition.
I did some 2% cores and they shot great, also. The core seating process with a manual core seater requires more effort depending on your core seating press, etc, etc. Randy Robinett's air-over core seater sure made me smile when I used it.