Switching from factory 180 Core-Lokt, to 150 BT.

Tried IMR 4350, and IMR 4064, working up and down with each. 4 to 6 inch groups with all combinations.

Suggestions?

What you're dealing with here (IMHO) is barrel harmonics. If the guy doesn't want to try ANY OTHER BULLET then you have to get creative with different powders and seating depths. You might try taking a bullet comparator and find the distance to the lands of some of the factory bullets the gun "likes" and then start playing with powder charges from there. If that doesn't work you might try seating depths that actually give a "big" jump to the lands like .050" or .100". It sounds like he has a very fussy gun and and there will probably be a lot of "trials and tribulations" until the "secret" recipe can be found. Factory loads are probably using powders faster than 4350 (except those light magnums) so those might be the powders to concentrate on. One last thing. I have a bar and have found it real critical to testing for it to make sure I use a sandbag that lets the forarm slide "or jump" easily when you are shooting. I used to use an "owls ear" front bag that kind of gripped the forearm and that really hindered the accuracy somehow.

Not a biggie, and if my memory serves correctly, Nosler lists them both as using the same load data.