Need help with 338 federal Sub Sonic Loads

Aug 13, 2013
Spring TX
Hey all. I just got a 12” 8 twist 338 federal barrel for my scar 17.
Idea is, I want subs to cycle out of it. First loads were 300 grn scenars with 15.0 grns of trailboss. Bolt barely cracked open with gas adjustment screw wide open.

My question is, can I use a slower burning powder to help get some retention and pressure to the gas block.