tac...
i myself dont leave airspace in 577 / 450 rounds at all , weather i use betram / kynock or much larger capacity cbc cases which can swallow 120gns of bp.
i tend to use around 60gns of swiss and 3 gns of kapok.
another forum i frequent is the the british military forum where many experienced shooters shoot martinis with 85gns and air as the filler....
please check it out.....https://www.tapatalk.com/groups/...r+space#p18340