308 Subsonic RedDot/Clays

I’ve been working on reloading .308 subsonic rounds with Red Dot and Clays powder. I’m looking to get around 975fps-1000fps with 170gr-190gr projectiles. When shooting a string of 5 rounds (same powder charge, primer, seating depth, etc.) I am seeing the velocity drop on every round using a MagnetoSpeed V3. I read on another forum that when using TrailBoss the same thing occurs due to the lack of trace lube in the powder. I’ve looked all over and can’t find anything about that happening with a shotgun powder. Any help on to why this is happening? Also sometimes my first shot is 100fps or so lower than the next 4 shots and sometimes all 5 shots hold steady and I can get an SD below 10 Specs: .308 22” 1-10 twist Alpha brass Fed GM215M primers Hornady MATCH 168gr BTHP Sierra ProHunter 170gr FN (flat base) Sierra ProHunter 180gr RN (flat base) Hornady SUB-X 190gr (boat tail)