1970's Chapuis Double Express in 9.3x74R

[lkydvl]

Loading jacketed for the above is giving me some trouble. Chapuis states with 90% certainty it was regulated with Norma 285 Oryx.
I have not been able to load a 285 and get it to shoot any semblance of a group. At 100 yds I am in the 4-5 inch group size. Loading 64 grs of 4350 gives me flattened primers. I am reluctant to try a heavier charge. What powder would you suggest I try for what appears to be a need for higher velocity?
My cast loads of 285 FN regulate well at a load of 38 grs of 5744 for one shot from each barrel. The next two shots are 6 inches left and slightly higher tho group size is the same.

 

[375er]

No personal experience with it but I have read about using H380 or W760 in the cartridge.

 

[Rem700stw]

I have a Chapuis 9.3x74 SxS with a set of 20 gauge barrels of similar vintage. My load is 58 gr IMR 4350 with 286 Swift A-Frame. Used it on giraffe last year.