6.5grendel TGK 120g

has anyone loaded these in a 6.5grendel (howa mini)?
I'm having problems getting it to group well.. No problems using 120gr Sierra Prohunters and XBR
velocities good and done a limited ladder 28.2-28.8gr xbr (using the bolt action Sierra load data; max 29.3gr...previous loads in the 120-123gr all shoot well with 28.2gr without pressure.
28.8gr grouped well in vertical with horizontal stringing (0.6moa vert but 1.5moa horizontal)... I've read that horizontal stringing is normally shooter error, but definitely felt steady and no pulling on that group.... 4shots, clean barrel etc... does anyone have experience on these bullets in terms of preferred seating depth?

I know the 277 140 grain isn’t fussy.

Just load and shoot 10. Then you'll know. A 4shot group doesn't tell you enough to "read" any "stringing" - it's just chance.