Try Hornady 150gn Spire SPFB they are .312 400m max I used 42gns Varget (ADI 2208 I'm in Aus) but suggest you start at 39 and work up also I left about 1/32"of the cannular showing then crimped it this was in No.4 2 groover and a 1916 Lithgowicon No1 Mk III worked very well in mine shot 98.11/100.20 @200m sitting in the No.4 so they work but again I stress to you work up slowly and watch for pressure signs as you have a different rifle than mine even though the same lineage.
My suggestion is make an empty dummy round to what I described and try it through the action if that works well, then you are 1/10th the way there.