I have a good load with 180 gr TSX @ 3.620 oal. I wanted to utilize some 200Gr. Barnes LRX and seated @ 3.645. I was lucky to have a hit the paper. 8" high and to the right. Next shot was 8" high to the left and my 3rd shot was 2" low. I was dumbfounded. Checked everything to make sure I had nothing loose anywhere and Shot some of my 180 gr TSX's. 3 shots and they were all touching. Only thing I can come up with is that the LRX are tipped and I need to seat them further out. I can go 3.815 and magazine works as well as dumping a unspent round. Should I start that long and work back or pick a point and work up to that length? Still not near the lands.

That lrx is terrifically long...I'd wonder if its stabilizing in your twist rate.
That said I had a fussy 300 wby ul that wouldn't shoot till i seated the bullets really deep.
Its a crap shoot...when pondering seatng depth. Maybe try a few recipes with. 050" depth difference and see if something shows promise...then work from the promising depth.

It is 1 in 10, which the 200gr LRX says it must be a 1 in 10 or faster. They are long as heck. I will do just that and load 3 rounds, each group of 3 with different oal and see if one gets me home. Did you have such a wide grouping with your 300 WBY until you found the right depth? Thanks for the help!

I have to seat them fairly deep for my 300 RUM, which is built on a Model 70 action. 3.585 +/- or so.

My ultralight was AWFUL till I seated really deep. I've never seen a gun respond to seating depth the way that did. I was barely moa of paper plate at 200 and I tried EVERY slow powder and decent "huntable" bullet near magazine length.
When i went to ogive barely out of the case mouth it started shooting under moa.
Last time I shot it it gave me a 1.5" group at 300 yds w/a 150 tsx. It also shoots 180 tsx and 200 accubond quite well.