IMHO RL22 is my "go to" powder for .300 win mag with H4350 my second choice. It may be some other powder will work better but if one of those is not showing some promise, I figure something else is wrong.
You mention X bullets. You said "always shot well in factory loads." Clarify: in this specific gun, or others? If you've already had success with X bullets in factory ammo with this gun, then by all means keep fussing with powders, maybe change primers, adjust overall length, and so on. But if they have not worked in this gun, and you can't get them to work with a reasonable amount of effort / expenditure of components, I'd change bullets. There are just a lot of rifles out there which won't shoot X bullets well no matter what you use to put 'em in motion.
If you want to stick with a homogeneous metal bullet, give the Nosler E-Tip a try. They seem to shoot well more often than the X-es do .. at least, in my limited experience.
Another possibility would be to change X styles from a flat base to boat tail or vice versa or change weights. Some guns are quite finicky about bullet base shape and most are particular to some degree about muzzle velocity so changing weights can move you into or out of an accuracy node for your specific barrel.
G' luck!
Tom
