i used the 140gr bthp last year had the same problem shooting from the trailer we use to shoot from. but when i went and did pron shooting, they shot great. i wondering if i should go 10thou off the lands like i did with the 140bthp rounds tried 30thou off the land and they were all over the place so not sure.
"trailer we use to shoot from" - as in a thing that moves, and has springs between your rifle and the ground? yeah thats highly likely to cause issues. try shooting off something big, flat, solid and heavy, as in it doesnt move when you kick it heavy..... but also realise that your shooting position does influence how the rifle/ammo works -
@lowlight did some videos on this, eg prooving that there was a measurable difference in muzzle velocity between bench and prone.
as for distance off the lands..... a specific measurement given by someone else is like playing the lotto with someone elses lucky numbers, might work, might not. also, the lands move a tiny bit with every shot, watch erik cortinas video about "chasing the lands is stupid"
try a process like this
load a bunch of rounds, for instance 50
5 at maximum mag length
5 10 thou shorter
5 more ten thou shorter than that
5 ten thou shorter than that and so on until you have enough for 10 five round groups.
set up a target with 10 aiming marks. 1 for each length.
fire 1 round from each length at its intended aiming mark. - you dont shoot 1 group then the next (idea here is to average things like barrel dirtyness/wear, shooter fatigue etc over all the groups) try and fire each shot with as close to the same wind condition as you can.
then 1 more..... etc until you have fired your 10 five round groups
one group will be better than all the others, you can either pick that length and run with it, or you can repeat the process with that lenght as the center length, and use 1 or 2 thou intervals in length.