Hey Dan,
I am in Llano County, central Texas. I use the redding dies, and will be happy to give you some brass I have formed from .223 to test in your chamber. I have subsonics and lots of other goodies. I have been using 2400 for subsonics, cause I had some on hand. Start with about 9-10 grains, and adjust for speed.
This has some numbers, and loads:
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Sierra 300 PDF