I guess nobody ever found load data for the 175gr Sub-X and 300BLK? I've loaded 190 Sub-X's in 300BLK and .308. I found some 30-30 data but there is a good bit of difference in case capacity between the 30-30 and 300BLK. But, if looking for supsonic velocity, I might be able to work down from the minimum 30-30 charge. I may see what I can do with that today.
I've got some 150gr Sierra Pro Hunter bullets. I wonder what that would look like supersonic in the 300BLK?
Looking through the Sierra load data for 300AAC it looks like I can get about 1,750fps with H4198. Not the most exciting finding but AA1680 will get me to 1,950 fps (book value), which is a little better. If I test it'll be with a bolt gun not AR though.