I'll have to check. I may have 4227 and ...... maybe Red Dot (not sure). Do you have load data for them for a 212 grain bullet, or close?
9.5-10gn of 4227 should work.
This was posted on my thread.
https://www.texasguntalk.com/threads/subsonic-300bo.65793/#post-1183103
It may not cycle though. 4227 has worked in 2 AR's for me though.