X39 is all I dabbled in . I did run a 200 gr paper patch over IMR 4350 1600 fps at mag length and 1700 single fed .

I don't remember what I had under the 155s , probably H322 or Reloader 10x . Having used them both in 223 62s and 6.8 with 120 jacketed up to 140 cast either should work well with 110-150 gr in the 300 .
4198 will walk a line at the edge of too slow and just just barely enough/not quite enough to cycle it will probably give the best/highest velocity though .
I've been pleasantly surprised by H322 in a pretty wide selection of cartridges with good results.

H110/296 was developed in parallel with and for the 30 carb . You'll want the mag/GI primer and a moderate crimp if you use this one .